Posted on by Matt Williams

Who Was Galileo Galilei?

Portrait of Galileo Galilei by Giusto Sustermans (1636). Credit: nmm.ac.uk

When it comes to scientists who revolutionized the way we think of the universe, few names stand out like Galileo Galilei. A noted inventor, physicist, engineer and astronomer, Galileo was one of the greatest contributors to the Scientific Revolution. He build telescopes, designed a compass for surveying and military use, created a revolutionary pumping system, and developed physical laws that were the precursors of Newton’s law of Universal Gravitation and Einstein’s Theory of Relativity.

But it was within the field of astronomy that Galileo made his most enduring impact. Using telescopes of his own design, he discovered Sunspots, the largest moons of Jupiter, surveyed The Moon, and demonstrated the validity of Copernicus’ heliocentric model of the universe. In so doing, he helped to revolutionize our understanding of the cosmos, our place in it, and helped to usher in an age where scientific reasoning trumped religious dogma.

Early Life:

Galileo was born in Pisa, Italy, in 1564, into a noble but poor family. He was the first of six children of Vincenzo Galilei and Giulia Ammannati, who’s father also had three children out of wedlock. Galileo was named after an ancestor, Galileo Bonaiuti (1370 – 1450), a noted physician, university teacher and politician who lived in Florence.

His father, a famous lutenist, composer and music theorist, had a great impact on Galileo; transmitting not only his talent for music, but skepticism of authority, the value of experimentation, and the value of measures of time and rhythm to achieve success.

The Camaldolese Monastery at Vallombrosa, 35 km southeast of Florence, where Galileo was educated from to. Credit: nobility.org
The Camaldolese Monastery at Vallombrosa, 35 km southeast of Florence, where Galileo was educated until 1581. Credit: nobility.org

In 1572, when Galileo Galilei was eight, his family moved to Florence, leaving Galileo with his uncle Muzio Tedaldi (related to his mother through marriage) for two years.When he reached the age of ten, Galileo left Pisa to join his family in Florence and was tutored by Jacopo Borghini -a mathematician and professor from the university of Pisa.

Once he was old enough to be educated in a monastery, his parents sent him to the Camaldolese Monastery at Vallombrosa, located 35 km southeast of Florence. The Order was independent from the Benedictines, and combined the solitary life of the hermit with the strict life of a monk. Galileo apparently found this life attractive and intending to join the Order, but his father insisted that he study at the University of Pisa to become a doctor.

Education:

While at Pisa, Galileo began studying medicine, but his interest in the sciences quickly became evident. In 1581, he noticed a swinging chandelier, and became fascinated by the timing of its movements. To him, it became clear that the amount of time, regardless of how far it was swinging, was comparable to the beating of his heart.

When he returned home, he set up two pendulums of equal length, swinging one with a large sweep and the other with a small sweep, and found that they kept time together. These observations became the basis of his later work with pendulums to keep time – work which would also be picked up almost a century later when Christiaan Huygens designed the first officially-recognized pendulum clock.

Galileo Demonstrating the New Astronomical Theories at the University of Padua by Félix Parra (1873). Credit: munal.gob.mx
Galileo Demonstrating the New Astronomical Theories at the University of Padua, by Félix Parra (1873). Credit: munal.gob.mx

Shortly thereafter, Galileo accidentally attended a lecture on geometry, and talked his reluctant father into letting his study mathematics and natural philosophy instead of medicine. From that point onward, he began a steady processes of inventing, largely for the sake of appeasing his father’s desire for him to make money to pay off his siblings expenses (particularly those of his younger brother, Michelagnolo).

In 1589, Galileo was appointed to the chair of mathematics at the University of Pisa. In 1591, his father died, and he was entrusted with the care of his younger siblings. Being Professor of Mathematics at Pisa was not well paid, so Galileo lobbied for a more lucrative post. In 1592, this led to his appointment to the position of Professor of Mathematics at the University of Padua, where he taught Euclid’s geometry, mechanics, and astronomy until 1610.

During this period, Galileo made significant discoveries in both pure fundamental science as well as practical applied science. His multiple interests included the study of astrology, which at the time was a discipline tied to the studies of mathematics and astronomy. It was also while teaching the standard (geocentric) model of the universe that his interest in astronomy and the Copernican theory began to take off.

Telescopes:

In 1609, Galileo received a letter telling him about a spyglass that a Dutchman had shown in Venice. Using his own technical skills as a mathematician and as a craftsman, Galileo began to make a series of telescopes whose optical performance was much better than that of the Dutch instrument.

Galileo Galilei's telescope with his handwritten note specifying the magnifying power of the lens, at an exhibition at The Franklin Institute in Philadelphia. Credit: AP Photo/Matt Rourke
Galileo Galilei’s telescope with his handwritten note specifying the magnifying power of the lens, at an exhibition at The Franklin Institute in Philadelphia. Credit: AP Photo/Matt Rourke

As he would later write in his 1610 tract Sidereus Nuncius (“The Starry Messenger”):

“About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby. Of this truly remarkable effect several experiences were related, to which some persons believed while other denied them. A few days later the report was confirmed by a letter I received from a Frenchman in Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to investigate means by which I might arrive at the invention of a similar instrument. This I did soon afterwards, my basis being the doctrine of refraction.”

His first telescope – which he constructed between June and July of 1609 – was made from available lenses and had a three-powered spyglass. To improve upon this, Galileo learned how to grind and polish his own lenses. By August, he had created an eight-powered telescope, which he presented to the Venetian Senate.

By the following October or November, he managed to improve upon this with the creation a twenty-powered telescope. Galileo saw a great deal of commercial and military applications of his instrument(which he called a perspicillum) for ships at sea. However, in 1610, he began turning his telescope to the heavens and made his most profound discoveries.

Galileo Galilei showing the Doge of Venice how to use the telescope by Giuseppe Bertini (1858). Credit: gabrielevanin.it
Galileo Galilei showing the Doge of Venice how to use the telescope, by Giuseppe Bertini (1858). Credit: gabrielevanin.it

Achievements in Astronomy:

Using his telescope, Galileo began his career in astronomy by gazing at the Moon, where he discerned patterns of uneven and waning light. While not the first astronomer to do this, Galileo artistic’s training and knowledge of chiaroscuro – the use of strong contrasts between light and dark – allowed him to correctly deduce that these light patterns were the result of changes in elevation. Hence, Galileo was the first astronomer to discover lunar mountains and craters.

In The Starry Messenger, he also made topographical charts, estimating the heights of these mountains. In so doing, he challenged centuries of Aristotelian dogma that claimed that Moon, like the other planets, was a perfect, translucent sphere. By identifying that it had imperfections, in the forms of surface features, he began advancing the notion that the planets were similar to Earth.

Galileo also recorded his observations about the Milky Way in the Starry Messenger, which was previously believed to be nebulous. Instead, Galileo found that it was a multitude of stars packed so densely together that it appeared from a distance to look like clouds. He also reported that whereas the telescope resolved the planets into discs, the stars appeared as mere blazes of light, essentially unaltered in appearance by the telescope – thus suggesting that they were much farther away than previously thought.

Using his telescopes, Galileo also became one the first European astronomer to observe and study sunspots. Though there are records of previous instances of naked eye observations – such as in China (ca. 28 BCE), Anaxagoras in 467 BCE, and by Kepler in 1607 – they were not identifies as being imperfections on the surface of the Sun. In many cases, such as Kepler’s, it was thought that the spots were transits of Mercury.

In addition, there is also controversy over who was the first to observe sunspots during the 17th century using a telescope. Whereas Galileo is believed to have observed them in 1610, he did not publish about them and only began speaking to astronomers in Rome about them by the following year. In that time, German astronomer Christoph Scheiner had been reportedly observing them using a helioscope of his own design.

At around the same time, the Frisian astronomers Johannes and David Fabricius published a description of sunspots in June 1611. Johannes book, De Maculis in Sole Observatis (“On the Spots Observed in the Sun”) was published in autumn of 1611, thus securing credit for him and his father.

In any case, it was Galileo who properly identified sunspots as imperfections on the surface of the Sun, rather than being satellites of the Sun –  an explanation that Scheiner, a Jesuit missionary, advanced in order to preserve his beliefs in the perfection of the Sun.

Using a technique of projecting the Sun’s image through the telescope onto a piece of paper, Galileo deduced that sunspots were, in fact, on the surface of the Sun or in its atmosphere. This presented another challenge to the Aristotelian and Ptolemaic view of the heavens, since it demonstrated that the Sun itself had imperfections.

On January 7th, 1610, Galileo pointed his telescope towards Jupiter and observed what he described in Nuncius as “three fixed stars, totally invisible by their smallness” that were all close to Jupiter and in line with its equator. Observations on subsequent nights showed that the positions of these “stars” had changed relative to Jupiter, and in a way that was not consistent with them being part of the background stars.

Galilean Family Portrait
The Galilean moons, shown to scale – Io (top right), Europa (upper left), Ganymede (right) and Callisto (bottom left). Credit: NASA/JPL

By January 10th, he noted that one had disappeared, which he attributed to it being hidden behind Jupiter. From this, he concluded that the stars were in fact orbiting Jupiter, and they were satellites of it. By January 13th, he discovered a fourth, and named them the Medicean stars, in honor of his future patron, Cosimo II de’ Medici, Grand Duke of Tuscany, and his three brothers.

Later astronomers, however, renamed them the Galilean Moons in honour of their discoverer. By the 20th century, these satellites would come to be known by their current names – Io, Europa, Ganymede, and Callisto – which had been suggested by 17th century German astronomer Simon Marius, apparently at the behest of Johannes Kepler.

Galileo’s observations of these satellites proved to be another major controversy. For the first time, a planet other than Earth was shown to have satellites orbiting it, which constituted yet another nail in the coffin of the geocentric model of the universe. His observations were independently confirmed afterwards, and Galileo continued to observe the satellites them and even obtained remarkably accurate estimates for their periods by 1611.

Heliocentrism:

Galileo’s greatest contribution to astronomy came in the form of his advancement of the Copernican model of the universe (i.e. heliocentrism). This began in 1610 with his publication of Sidereus Nuncius, which brought the issue of celestial imperfections before a wider audience. His work on sunspots and his observation of the Galilean Moons furthered this, revealing yet more inconsistencies in the currently accepted view of the heavens.

Cardinal Bellarmine had written in 1615 that the Copernican system could not be defended without "a true physical demonstration that the sun does not circle the earth but the earth circles the sun". Galileo considered his theory of the tides to provide the required physical proof of the motion of the earth. This theory was so important to him that he originally intended to entitle his Dialogue on the Two Chief World Systems the Dialogue on the Ebb and Flow of the Sea. For Galileo, the tides were caused by the sloshing back and forth of water in the seas as a point on the Earth's surface sped up and slowed down because of the Earth's rotation on its axis and revolution around the Sun. He circulated his first account of the tides in 1616, addressed to Cardinal Orsini. His theory gave the first insight into the importance of the shapes of ocean basins in the size and timing of tides; he correctly accounted, for instance, for the negligible tides halfway along the Adriatic Sea compared to those at the ends. As a general account of the cause of tides, however, his theory was a failure. If this theory were correct, there would be only one high tide per day. Galileo and his contemporaries were aware of this inadequacy because there are two daily high tides at Venice instead of one, about twelve hours apart. Galileo dismissed this anomaly as the result of several secondary causes including the shape of the sea, its depth, and other factors. Against the assertion that Galileo was deceptive in making these arguments, Albert Einstein expressed the opinion that Galileo developed his "fascinating arguments" and accepted them uncritically out of a desire for physical proof of the motion of the Earth. Galileo dismissed the idea, held by his contemporary Johannes Kepler, that the moon caused the tides. He also refused to accept Kepler's elliptical orbits of the planets, considering the circle the "perfect" shape for planetary orbits.Cardinal Bellarmine had written in 1615 that the Copernican system could not be defended without "a true physical demonstration that the sun does not circle the earth but the earth circles the sun". Galileo considered his theory of the tides to provide the required physical proof of the motion of the earth. This theory was so important to him that he originally intended to entitle his Dialogue on the Two Chief World Systems the Dialogue on the Ebb and Flow of the Sea. For Galileo, the tides were caused by the sloshing back and forth of water in the seas as a point on the Earth's surface sped up and slowed down because of the Earth's rotation on its axis and revolution around the Sun. He circulated his first account of the tides in 1616, addressed to Cardinal Orsini. His theory gave the first insight into the importance of the shapes of ocean basins in the size and timing of tides; he correctly accounted, for instance, for the negligible tides halfway along the Adriatic Sea compared to those at the ends. As a general account of the cause of tides, however, his theory was a failure. If this theory were correct, there would be only one high tide per day. Galileo and his contemporaries were aware of this inadequacy because there are two daily high tides at Venice instead of one, about twelve hours apart. Galileo dismissed this anomaly as the result of several secondary causes including the shape of the sea, its depth, and other factors. Against the assertion that Galileo was deceptive in making these arguments, Albert Einstein expressed the opinion that Galileo developed his "fascinating arguments" and accepted them uncritically out of a desire for physical proof of the motion of the Earth. Galileo dismissed the idea, held by his contemporary Johannes Kepler, that the moon caused the tides. He also refused to accept Kepler's elliptical orbits of the planets, considering the circle the "perfect" shape for planetary orbits.
Galileo’s Sidereus Nuncius (“Starry Messenger”), published in 1610, laid out his observations of the moon’s surface, which included mountains and impact craters. Credit: brunelleschi.imss.fi.it

Other astronomical observations also led Galileo to champion the Copernican model over the traditional Aristotelian-Ptolemaic (aka. geocentric) view. From September 1610 onward, Galileo began observing Venus, noting that it exhibited a full set of phases similar to that of the Moon. The only explanation for this was that Venus was periodically between the Sun and Earth; while at other times, it was on the opposite side of the Sun.

According to the geocentric model of the universe, this should have been impossible, as Venus’ orbit placed it closer to Earth than the Sun – where it could only exhibit crescent and new phases. However, Galileo’s observations of it going through crescent, gibbous, full and new phases was consistent with the Copernican model, which established that Venus orbited the Sun within the Earth’s orbit.

These and other observations made the Ptolemaic model of the universe untenable. Thus, by the early 17th century, the great majority of astronomers began to convert to one of the various geo-heliocentric planetary models – such as the Tychonic, Capellan and Extended Capellan models. These all had the virtue of explaining problems in the geocentric model without engaging in the “heretical” notion that Earth revolved around the Sun.

In 1632, Galileo addressed the “Great Debate” in his treatise Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems), in which he advocated the heliocentric model over the geocentric. Using his own telescopic observations, modern physics and rigorous logic, Galileo’s arguments effectively undermined the basis of Aristotle and Ptolemy’s system for a growing and receptive audience.

Frontispiece and title page of the Dialogue, 1632. Credit: moro.imss.fi.it
Frontispiece and title page of the Dialogue, 1632. Credit: moro.imss.fi.it

In the meantime, Johannes Kepler correctly identified the sources of tides on Earth – something which Galileo had become interesting in himself. But whereas Galileo attributed the ebb and flow of tides to the rotation of the Earth, Kepler ascribed this behavior to the influence of the Moon.

Combined with his accurate tables on the elliptical orbits of the planets (something Galileo rejected), the Copernican model was effectively proven. From the middle of the seventeenth century onward, there were few astronomers who were not Copernicans.

The Inquisition and House Arrest:

As a devout Catholic, Galileo often defended the heliocentric model of the universe using Scripture. In 1616, he wrote a letter to the Grand Duchess Christina, in which he argued for a non-literal interpretation of the Bible and espoused his belief in the heliocentric universe as a physical reality:

“I hold that the Sun is located at the center of the revolutions of the heavenly orbs and does not change place, and that the Earth rotates on itself and moves around it. Moreover … I confirm this view not only by refuting Ptolemy’s and Aristotle’s arguments, but also by producing many for the other side, especially some pertaining to physical effects whose causes perhaps cannot be determined in any other way, and other astronomical discoveries; these discoveries clearly confute the Ptolemaic system, and they agree admirably with this other position and confirm it.“

More importantly, he argued that the Bible is written in the language of the common person who is not an expert in astronomy. Scripture, he argued, teaches us how to go to heaven, not how the heavens go.

Galileo facing the Roman Inquisition. by Cristiano Banti's (1857). Credit: law.umkc.edu
Galileo facing the Roman Inquisition. by Cristiano Banti’s (1857). Credit: law.umkc.edu

Initially, the Copernican model of the universe was not seen as an issue by the Roman Catholic Church or it’s most important interpreter of Scripture at the time – Cardinal Robert Bellarmine. However, in the wake of the Counter-Reformation, which began in 1545 in response to the Reformation, a more stringent attitude began to emerge towards anything seen as a challenge to papal authority.

Eventually, matters came to a head in 1615 when Pope Paul V (1552 – 1621) ordered that the Sacred Congregation of the Index (an Inquisition body charged with banning writings deemed “heretical”) make a ruling on Copernicanism. They condemned the teachings of Copernicus, and Galileo (who had not been personally involved in the trial) was forbidden to hold Copernican views.

However, things changed with the election of Cardinal Maffeo Barberini (Pope Urban VIII) in 1623. As a friend and admirer of Galileo’s, Barberini opposed the condemnation of Galileo, and gave formal authorization and papal permission for the publication of Dialogue Concerning the Two Chief World Systems.

However, Barberini stipulated that Galileo provide arguments for and against heliocentrism in the book, that he be careful not to advocate heliocentrism, and that his own views on the matter be included in Galileo’s book. Unfortunately, Galileo’s book proved to be a solid endorsement of heliocentrism and offended the Pope personally.

Portrait, attributed to Murillo, of Galileo gazing at the words "E pur si muove" (not legible in this image) scratched on the wall of his prison cell. Credit:
Portrait of Galileo gazing at the words “E pur si muove” scratched on the wall of his prison cell, attributed to Bartolomé Esteban Murillo (1618-1682). Credit: Wikipedia Commons

In it, the character of Simplicio, the defender of the Aristotelian geocentric view, is portrayed as an error-prone simpleton. To make matter worse, Galileo had the character Simplicio enunciate the views of Barberini at the close of the book, making it appear as though Pope Urban VIII himself was a simpleton and hence the subject of ridicule.

As a result, Galileo was brought before the Inquisition in February of 1633 and ordered to renounce his views. Whereas Galileo steadfastly defended his position and insisted on his innocence, he was eventually threatened with torture and declared guilty. The sentence of the Inquisition, delivered on June 22nd, contained three parts – that Galileo renounce Copernicanism, that he be placed under house arrest, and that the Dialogue be banned.

According to popular legend, after recanting his theory publicly that the Earth moved around the Sun, Galileo allegedly muttered the rebellious phrase: “E pur si muove” (“And yet it moves” in Latin). After a period of living with his friend, the Archbishop of Siena, Galileo returned to his villa at Arcetri (near Florence in 1634), where he spent the remainder of his life under house arrest.

Other Accomplishments:

In addition to his revolutionary work in astronomy and optics, Galileo is also credited with the invention of many scientific instruments and theories. Much of the devices he created were for the specific purpose of earning money to pay for his sibling’s expenses. However, they would also prove to have a profound impact in the fields of mechanics, engineering, navigation, surveying, and warfare.

Galileo's La Billancetta, in which he describes a method for hydrostatic balance. Credit: Museo Galileo
Galileo’s La Billancetta, in which he describes a method for hydrostatic balance. Credit: Museo Galileo

In 1586, at the age of 22, Galileo made his first groundbreaking invention. Inspired by the story of Archimedes and his “Eureka” moment, Galileo began looking into how jewelers weighed precious metals in air and then by displacement to determine their specific gravity. Working from this, he eventually theorized of a better method, which he described in a treatise entitled La Bilancetta (“The Little Balance”).

In this tract, he described an accurate balance for weighing things in air and water, in which the part of the arm on which the counter weight was hung was wrapped with metal wire. The amount by which the counterweight had to be moved when weighing in water could then be determined very accurately by counting the number of turns of the wire. In so doing, the proportion of metals like gold to silver in the object could be read off directly.

In 1592, when Galileo was a professor of mathematics at the University of Padua, he made frequent trips to the Arsenal – the inner harbor where Venetian ships were outfitted. The Arsenal had been a place of practical invention and innovation for centuries, and Galileo used the opportunity to study mechanical devices in detail.

In 1593, he was consulted on the placement of oars in galleys and submitted a report in which he treated the oar as a lever and correctly made the water the fulcrum. A year later the Venetian Senate awarded him a patent for a device for raising water that relied on a single horse for the operation. This became the basis of modern pumps.

A replica of the earliest surviving telescope attributed to Galileo Galilei, on display at the Griffith Observatory. Credit: Wikipedia Commons/Mike Dunn
A replica of the earliest surviving telescope attributed to Galileo Galilei, on display at the Griffith Observatory. Credit: Wikipedia Commons/Mike Dunn

To some, Galileo’s Pump was a merely an improvement on the Archimedes Screw, which was first developed in the third century BCE and patented in the Venetian Republic in 1567. However, there is no apparent evidence connecting Galileo’s invention to Archimedes’ earlier and less sophisticated design.

In ca. 1593, Galileo constructed his own version of a thermoscope, a forerunner of the thermometer, that relied on the expansion and contraction of air in a bulb to move water in an attached tube. Over time, he and his colleagues worked to develop a numerical scale that would measure the heat based on the expansion of the water inside the tube.

The cannon, which was first introduced to Europe in 1325, had become a mainstay of war by Galileo’s time. Having become more sophisticated and mobile, gunners needed instruments to help them coordinate and calculate their fire. As such, between 1595 and 1598, Galileo devised an improved geometric and military compass for use by gunners and surveyors.

During the 16th century, Aristotelian physics was still the predominant way of explaining the behavior of bodies near the Earth. For example, it was believed that heavy bodies sought their natural place of rest – i.e at the center of things. As a result, no means existed to explain the behavior of pendulums, where a heavy body suspended from a rope would swing back and forth and not seek rest in the middle.

The Sector, a military/geometric compass designed by Galileo Galilei. Credit:
The Sector, a military/geometric compass designed by Galileo Galilei. Credit: chsi.harvard.edu

Already, Galileo had conducted experiments that demonstrated that heavier bodies did not fall faster than lighter ones – another belief consistent with Aristotelian theory. In addition, he also demonstrated that objects thrown into the air travel in parabolic arcs. Based on this and his fascination with the back and forth motion of a suspended weight, he began to research pendulums in 1588.

In 1602, he explained his observations in a letter to a friend, in which he described the principle of isochronism. According to Galileo, this principle asserted that the time it takes for the pendulum to swing is not linked to the arc of the pendulum, but rather the pendulum’s length. Comparing two pendulum’s of similar length, Galileo demonstrated that they would swing at the same speed, despite being pulled at different lengths.

According to Vincenzo Vivian, one of Galileo’s contemporaries, it was in 1641 while under house arrest that Galileo created a design for a pendulum clock. Unfortunately, being blind at the time, he was unable to complete it before his death in 1642. As a result, Christiaan Huygens’ publication of Horologrium Oscillatorium in 1657 is recognized as the first recorded proposal for a pendulum clock.

Death and Legacy:

Galileo died on January 8th, 1642, at the age of 77, due to fever and heart palpitations that had taken a toll on his health. The Grand Duke of Tuscany, Ferdinando II, wished to bury him in the main body of the Basilica of Santa Croce, next to the tombs of his father and other ancestors, and to erect a marble mausoleum in his honor.

Tomb of Galileo Galilei in the Santa Croce Basilica in Florence, Italy. Credit: Wikipedia Commons/stanthejeep
Tomb of Galileo Galilei in the Santa Croce Basilica in Florence, Italy. Credit: Wikipedia Commons/stanthejeep

However, Pope Urban VIII objected on the basis that Galileo had been condemned by the Church, and his body was instead buried in a small room next to the novice’s chapel in the Basilica. However, after his death, the controversy surrounding his works and heliocentricm subsided, and the Inquisitions ban on his writing’s was lifted in 1718.

In 1737, his body was exhumed and reburied in the main body of the Basilica after a monument had been erected in his honor. During the exhumation, three fingers and a tooth were removed from his remains. One of these fingers, the middle finger from Galileo’s right hand, is currently on exhibition at the Museo Galileo in Florence, Italy.

In 1741, Pope Benedict XIV authorized the publication of an edition of Galileo’s complete scientific works which included a mildly censored version of the Dialogue. In 1758, the general prohibition against works advocating heliocentrism was removed from the Index of prohibited books, although the specific ban on uncensored versions of the Dialogue and Copernicus’s De Revolutionibus orbium coelestium (“On the Revolutions of the Heavenly Spheres“) remained.

All traces of official opposition to heliocentrism by the church disappeared in 1835 when works that espoused this view were finally dropped from the Index. And in 1939, Pope Pius XII described Galileo as being among the “most audacious heroes of research… not afraid of the stumbling blocks and the risks on the way, nor fearful of the funereal monuments”.

A bust of Galileo at the Galileo Museum in Florence, Italy. The museum is displaying recovered parts of his body. Credit Kathryn Cook for The New York Times
A bust of Galileo at the Museo Galileo in Florence, Italy, where recovered parts of his body and many of his possessions are on display. Credit: NYT/Kathryn Cook

On October 31st, 1992, Pope John Paul II expressed regret for how the Galileo affair was handled, and issued a declaration acknowledging the errors committed by the Catholic Church tribunal. The affair had finally been put to rest and Galileo exonerated, though certain unclear statements issued by Pope Benedict XVI have led to renewed controversy and interest in recent years.

Alas, when it comes to the birth of modern science and those who helped create it, Galileo’s contributions are arguably unmatched. According to Stephen Hawking and Albert Einstein, Galileo was the father of modern science, his discoveries and investigations doing more to dispel the prevailing mood of superstition and dogma than anyone else in his time.

These include the discovery of craters and mountains on the Moon, the discovery of the four largest moons of Jupiter (Io, Europa, Ganymede and Callisto), the existence and nature of Sunspots, and the phases of Venus. These discoveries, combined with his logical and energetic defense of the Copernican model, made a lasting impact on astronomy and forever changed the way people look at the universe.

Galileo’s theoretical and experimental work on the motions of bodies, along with the largely independent work of Kepler and René Descartes, was a precursor of the classical mechanics developed by Sir Isaac Newton. His work with pendulums and time-keeping also previewed the work of Christiaan Huygens and the development of the pendulum clock, the most accurate timepiece of its day.

The 25 Euro coin minted for the 2009 International Year of Astronomy, showing Galileo on the obverse. Credit: coinnews.net
The €25 coin minted for the 2009 International Year of Astronomy, showing Galileo on the obverse. Credit: coinnews.net

Galileo also put forward the basic principle of relativity, which states that the laws of physics are the same in any system that is moving at a constant speed in a straight line. This remains true, regardless of the system’s particular speed or direction, thus proving that there is no absolute motion or absolute rest. This principle provided the basic framework for Newton’s laws of motion and is central to Einstein’s special theory of relativity.

The United Nations chose 2009 to be the International Year of Astronomy, a global celebration of astronomy and its contributions to society and culture. The year 2009 was selected in part because it was the four-hundredth anniversary of Galileo first viewing the heavens with his a telescope he built himself.

A commemorative €25 coin was minted for the occasion, with the inset on the obverse side showing Galileo’s portrait and telescope, as well as one of his first drawings of the surface of the moon. In the silver circle that surrounds it, pictures of other telescopes – Isaac Newton’s Telescope, the observatory in Kremsmünster Abbey, a modern telescope, a radio telescope and a space telescope – are also shown.

Other scientific endeavors and principles are named after Galileo, including the NASA Galileo spacecraft, which was the first spacecraft to enter orbit around Jupiter. Operating from 1989 to 2003, the mission consisted of an orbiter that observed the Jovian system, and an atmospheric probe that made the first measurements of Jupiter’s atmosphere.

This mission found evidence of subsurface oceans on Europa, Ganymede and Callisto, and  revealed the intensity of volcanic activity on Io. In 2003, the spacecraft was crashed into Jupiter’s atmosphere to avoid contamination of any of Jupiter’s moons.

The European Space Agency (ESA) is also developing a global satellite navigation system named Galileo. And in classical mechanics, the transformation between inertial systems is known as “Galilean Transformation“, which is denoted by the non-SI unit of acceleration Gal (sometimes known as the Galileo). Asteroid 697 Galilea is also named in his honor.

Yes, the sciences and humanity as a whole owes a great dept to Galileo. And as time goes on, and space exploration continues, it is likely we will continue to repay that debt by naming future missions – and perhaps even features on the Galilean Moons, should we ever settle there – after him. Seems like a small recompense for ushering in the age of modern science, no?

Universe Today has many interesting articles on Galileo, include the Galilean moons, Galileo’s inventions, and Galileo’s telescope.

For more information, check out the the Galileo Project and Galileo’s biography.

Astronomy Cast has an episode on choosing and using a telescope, and one which deals with the Galileo Spacecraft.

Posted on by Matt Williams

Who was Christiaan Huygens?

Portrait of Christiaan Huygens, painted by von Bernard Vaillant in 1686. Credit: Hofwijck Museum, Voorburg/Public Domain

The 17th century was a very auspicious time for the sciences, with advancements being made in the fields of physics, mathematics, chemistry, and the natural sciences. But it was perhaps in the field of astronomy that the greatest achievements were made. In the space of a century, several planets and moons were observed for the first time, accurate models were made to predict the motions of the planets, and the law of universal gravitation was conceived.

In the midst of this, the name of Christiaan Huygens stands out among the rest. As one of the preeminent scientists of his time, he was pivotal in the development of clocks, mechanics and optics. And in the field of astronomy, he discovered Saturn’s Rings and its largest moon – Titan. Thanks to Huygens, subsequent generations of astronomers were inspired to explore the outer Solar System, leading to the discovery of other Cronian moons, Uranus, and Neptune in the following century.

Continue reading “Who was Christiaan Huygens?”

Posted on by Matt Williams

Uranus’ “Frankenstein Moon” Miranda

Color composite of the Uranian satellite Miranda, taken by Voyager 2 on Jan. 24, 1986, from a distance of 147,000 km (91,000 mi). Credit: NASA/JPL

Ever since the Voyager space probes ventured into the outer Solar System, scientists and astronomers have come to understand a great deal of this region of space. In addition to the four massive gas giants that call the outer Solar System home, a great deal has been learned about the many moons that circle them. And thanks to photographs and data obtained, human beings as a whole have come to understand just how strange and awe-inspiring our Solar System really is.

This is especially true of Miranda, the smallest and innermost of Uranus’ large moons – and some would say, the oddest-looking! Like the other major Uranian moons, its orbits close to its planet’s equator, is perpendicular to the Solar System’s ecliptic, and therefore has an extreme seasonal cycle. Combined with one of the most extreme and varied topographies in the Solar System, this makes Miranda an understandable source of interest!

Discovery and Naming:

Miranda was discovered on February 16th, 1948, by Gerard Kuiper using the McDonald Observatory‘s Otto Struve Telescope at the University of Texas in Austin. Its motion around Uranus was confirmed on March 1st of the same year, making it the first satellite of Uranus to be discovered in almost a century (the previous ones being Ariel and Umbriel, which were both discovered in 1851 by William Lassell).

A montage of Uranus's moons. Image credit: NASA
A montage of Uranus’s moons. Image credit: NASA/JPL

Consistent with the names of the other moons, Kuiper decided to the name the object “Miranda” after the character in Shakespeare’s The Tempest. This continued the tradition set down by John Herschel, who suggested that all the large moons of Uranus – Ariel, Umbriel, Titania and Oberon – be named after characters from either The Tempest or Alexander Pope’s The Rape of the Lock.

Size, Mass and Orbit:

With a mean radius of 235.8 ± 0.7 km and a mass of 6.59 ± 0.75 ×1019 kg, Miranda is 0.03697 Earths times the size of Earth and roughly 0.000011 as massive. Its modest size also makes it one of the smallest object in the Solar System to have achieved hydrostatic equilibrium, with only Saturn’s moon of Mimas being smaller.

Of Uranus’ five larger moons, Miranda is the closest, orbiting at an average distance (semi-major axis) of 129,390 km. It has a very minor eccentricity of 0.0013 and an inclination of 4.232° to Uranus’ equator. This is unusually high for a body so close to its parent planet – roughly ten times that of the other Uranian satellites.

Since there are no mean-motion resonances to explain this, it has been hypothesized that the moons occasionally pass through secondary resonances. At some point, this would have led Miranda into being locked in a temporary 3:1 resonance with Umbriel, and perhaps a 5:3 resonance with Ariel as well. This resonance would have altered the moon’s inclination, and also led to tidal heating in its interior (see below).

Size comparison of all the Solar Systems moons. Credit: The Planetary Society
Size comparison of all the Solar Systems moons. Credit: NASA/The Planetary Society

With an average orbital speed of 6.66 km/s, Miranda takes 1.4 days to complete a single orbit of Uranus. Its orbital period (also 34 hours) is synchronous with its rotational period, meaning that it is tidally-locked with Uranus and maintains one face towards it at all times. Given that it orbits around Uranus’ equator, which means its orbit is perpendicular to the Sun’s ecliptic, Uranus goes through an extreme seasonal cycle where the northern and southern hemispheres experience 42 years of lightness and darkness at a time.

Composition and Surface Structure:

Miranda’s mean density (1.2 g/cm3) makes it the least dense of the Uranian moons. It also suggests that Miranda is largely composed of water ice (at least 60%), with the remainder likely consisting of silicate rock and organic compounds in the interior. The surface of Miranda is also the most diverse and extreme of all moons in the Solar System, with features that appear to be jumbled together in a haphazard fashion.

This consists of huge fault canyons as deep as 20 km (12 mi), terraced layers, and the juxtaposition of old and young surfaces seemingly at random. This patchwork of broken terrain indicates that intense geological activity took place in Miranda’s past, which is believed to have been driven by tidal heating during the time when it was in orbital resonance with Umbriel (and perhaps Ariel).

This resonance would have increased orbital eccentricity, and along with varying tidal forces from Uranus, would have caused warming in Miranda’s interior and led to resurfacing. In addition, the incomplete differentiation of the moon, whereby rock and ice were distributed more uniformly, could have led to an upwelling of lighter material in some areas, thus leading to young and older regions existing side by side.

Miranda
Uranus’ moon Miranda, imaged by the Voyager 2 space probe on January 24th, 1986. Credit: NASA/JPL-Caltech

Another theory is that Miranda was shattered by a massive impact, the fragments of which reassembled to produce a fractured core. In this scenario – which some scientists believe could have happened as many as five times – the denser fragments would have sunk deep into the interior, with water ice and volatiles setting on top of them and mirroring their fractured shape.

Overall, scientists recognize five types of geological features on Miranda, which includes craters, coronae (large grooved features), regiones (geological regions), rupes (scarps or canyons) and sulci (parallel grooves).

Miranda’s cratered regions are differentiated between younger, lightly-cratered regions and older, more-heavily cratered ones. The lightly cratered regions include ridges and valleys, which are separated from the more heavily-cratered areas by sharp boundaries of mismatched features. The largest known craters are about 30 km (20 mi) in diameter, with others lying in the range of 5 to 10 km (3 to 6 mi).

Miranda has the largest known cliff in the Solar System, which is known as Verona Rupes (named after the setting of Shakespeare’s Romeo and Juliet). This rupes has a drop-off of over 5 km (3.1 mi) – making it 12 times as deep as the Grand Canyon. Scientists suspect that Miranda’s ridges and canyons represent extensional tilt blocks – a tectonic event where tectonic plates stretch apart, forming patterns of jagged terrain with steep drops.

. Credit: NASA/JPL
Image taken by the Voyager 2 probe during its close approach on January 24th, 1986, with a resolution of about 700 m (2300 ft). Credit: NASA/JPL

The most well known coronae exist in the southern hemisphere, with three giant ‘racetrack’-like grooved structures that measure at least 200 km (120 mi) wide and up to 20 km (12 mi) deep. These features, named Arden, Elsinore and Inverness – all locations in Shakespeare’s plays – may have formed via extensional processes at the tops of diapirs (aka. upwellings of warm ice).

Other features may be due to cryovolcanic eruptions of icy magma, which would have been driven by tidal flexing and heating in the past. With an albedo of 0.32, Miranda’s surface is nearly as bright as that of Ariel, the brightest of the larger Uranian moons. It’s slightly darker appearance is likely due to the presence of carbonaceous material within its surface ice.

Exploration:

Miranda’s apparent magnitude makes it invisible to many amateur telescopes. As a result, virtually all known information regarding its geology and geography was obtained during the only flyby of the Uranian system, which was made by Voyager 2 in 1986. During the flyby, Miranda’s southern hemisphere pointed towards the Sun (while the northern was shrouded in darkness), so only the southern hemisphere could be studied.

At this time, no future missions have been planned or are under consideration. But given Miranda’s “Frankenstein”-like appearance and the mysteries that still surround its history and geology, any future missions to study Uranus and its system of moons would be well-advised.

We have many interesting articles on Miranda and Uranus’ moons here at Universe Today. Here’s one about about why they call it the “Frankenstein Moon“, and one about Voyager 2‘s historic flyby. And here’s one that answers the question How Many Moons Does Uranus Have?

For more information, check out NASA’s Solar System Exploration page on Miranda.

Sources:

Posted on by Matt Williams

Uranus’ “Sprightly” Moon Ariel

Mosaic of the four highest-resolution images of Ariel taken by the Voyager 2 space probe during its 1986 flyby of Uranus. Credit: NASA/JPL

The outer Solar System has enough mysteries and potential discoveries to keep scientists busy for decades. Case in point, Uranus and it’s system of moons. Since the beginning of the Space Age, only one space probe has ever passed by this planet and its system of moons. And yet, that which has been gleaned from this one mission, and over a century and a half of Earth- (and space-) based observation, has been enough to pique the interest of many generations of scientists.

For instance, just about all detailed knowledge of Uranus’ 27 known moons – including the “sprightly” moon Ariel – has been derived from information obtained by the Voyager 2 probe. Nevertheless, this single flyby revealed that Ariel is composed of equal parts ice and rock, a cratered and geologically active surface, and a seasonal cycle that is both extreme and very unusual (at least by our standards!)

Discovery and Naming:

Ariel was discovered on October 24th, 1851, by English astronomer William Lassel, who also discovered the larger moon of Umbriel on the same day. While William Herschel, who discovered Uranus’ two largest moons of Oberon and Titania in 1787, claimed to have observed four other moons in Uranus’ orbit, those claims have since been concluded to be erroneous.

A montage of Uranus's moons. Image credit: NASA
A montage of Uranus’s major moons. Image credit: NASA

As with all of Uranus’ moons, Ariel was named after a character from Alexander Pope’s The Rape of the Lock and Shakespeare’s The Tempest. In this case, Ariel refers to a spirit of the air who initiates the great storm in The Tempest and a sylph who protects the female protagonist in The Rape of the Lock. The names of all four then-known satellites of Uranus were suggested by John Herschel in 1852 at the request of Lassell.

Size, Mass and Orbit:

With a mean radius of 578.9 ± 0.6 km and a mass of 1.353 ± 0.120 × 1021 kg, Ariel is equivalent in size to 0.0908 Earths and 0.000226 times as massive. Ariel’s orbit of Uranus is almost circular, with an average distance (semi-major axis) of 191,020 km – making it the second closest of Uranus’ five major moons (behind Miranda). It has a very small orbital eccentricity (0.0012) and is inclined very little relative to Uranus’ equator (0.260°).

With an average orbital velocity of 5.51 km/s, Ariel takes 2.52 days to complete a single orbit of Uranus. Like most moons in the outer Solar System, Ariel’s rotation is synchronous with its orbit. This means that the moon is tidally locked with Uranus, with one face constantly pointed towards the planet.

Ariel orbits and rotates within Uranus’ equatorial plane, which means it rotates perpendicular to the Sun. This means that its northern and southern hemispheres face either directly towards the Sun or away from it at the solstices, which results in an extreme seasonal cycle of permanent day or night for a period of 42 years.

Size comparison between Earth, the Moon, and Ariel. Credit: NASA/JPL/USGS/Tom Reding
Size comparison between Earth, the Moon, and Ariel. Credit: NASA/JPL/USGS/Tom Reding

Ariel’s orbit lies completely inside the Uranian magnetosphere, which means that its trailing hemisphere is regularly struck by magnetospheric plasma co-rotating with the planet. This bombardment is believed to be the cause of the darkening of the trailing hemispheres (see below), which has been observed for all Uranian moons (with the exception of Oberon).

Currently Ariel is not involved in any orbital resonance with other Uranian satellites. In the past, however, it may have been in a 5:3 resonance with Miranda, which could have been partially responsible for the heating of that moon, and 4:1 resonance with Titania, from which it later escaped.

Composition and Surface Features:

Ariel is the fourth largest of Uranus’ moons, but is believed to be the third most-massive. Its average density of 1.66 g/cm3 indicates that it is roughly composed of equal parts water ice and rock/carbonaceous material, including heavy organic compounds. Based on spectrographic analysis of the surface, the leading hemisphere of Ariel has been revealed to be richer in water ice than its trailing hemisphere.

The cause of this is currently unknown, but it may be related to bombardment by charged particles from Uranus’s magnetosphere, which is stronger on the trailing hemisphere. The interaction of energetic particles and water ice causes sublimation and the decomposition of methane trapped in the ice (as clathrate hydrate), darkening the methanogenic and other organic molecules and leaving behind a dark, carbon-rich residue (aka. tholins).

The highest-resolution Voyager 2 color image of Ariel. Canyons with floors covered by smooth plains are visible at lower right. The bright crater Laica is at lower left. Credit: NASA/JPL
The highest-resolution Voyager 2 color image of Ariel, showing canyons with floors covered by smooth plains (lower right) and the bright Laica crater (lower left). Credit: NASA/JPL

Based on its size, estimates of its ice/rock distribution, and the possibility of salt or ammonia in its interior, Ariel’s interior is thought to be differentiated between a rocky core and an icy mantle. If true, the radius of the core would account for 64% of the moon’s radius (372 km) and 52% of its mass. And while the presence of water ice and ammonia could mean Ariel harbors an interior ocean at it’s core-mantle boundary, the existence of such an ocean is considered unlikely.

Infrared spectroscopy has also identified concentrations of carbon dioxide (CO²) on Ariel’s surface, particularly on its trailing hemisphere. In fact, Ariel shows the highest concentrations of CO² on of any Uranian satellite, and was the first moon to have this compound discovered on its surface.

Though the precise reason for this is unknown, it is possible that it is produced from carbonates or organic material that have been exposed to Uranus’ magnetosphere or solar ultraviolet radiation – due to the asymmetry between the leading and trailing hemispheres. Another explanation is outgassing, where primordial CO² trapped in Ariel’s interior ice escaped thanks to past geological activity.

The observed surface of Ariel can be divided into three terrain types: cratered terrain, ridged terrain and plains. Other features include chasmata (canyons), fault scarps (cliffs), dorsa (ridges) and graben (troughs or trenches). Impact craters are the most common feature on Ariel, particularly in the south pole, which is the moon’s oldest and most geographically extensive region.

False-color map of Ariel. The prominent noncircular crater below and left of center is Yangoor. Part of it was erased during formation of ridged terrain via extensional tectonics. Credit: NASA/JPL/USGS
False-color map of Ariel, showing the prominent Yangoor crater (left of center) and patches of ridged terrain (far left). Credit: USGS

Compared to the other moons of Uranus, Ariel appears to be fairly evenly-cratered. The surface density of the craters, which is significantly lower than those of Oberon and Umbriel, suggest that they do not date to the early history of the Solar System. This means that Ariel must have been completely resurfaced at some point in its history, most likely in the past when the planet had a more eccentric orbit and was therefore more geologically active.

The largest crater observed on Ariel, Yangoor, is only 78 km across, and shows signs of subsequent deformation. All large craters on Ariel have flat floors and central peaks, and few are surrounded by bright ejecta deposits. Many craters are polygonal, indicating that their appearance was influenced by the crust’s preexisting structure. In the cratered plains there are a few large (about 100 km in diameter) light patches that may be degraded impact craters.

The cratered terrain is intersected by a network of scarps, canyons and narrow ridges, most of which occur in Ariel’s mid-southern latitudes. Known as chasmata, these canyons were probably graben that formed due to extensional faulting triggered by global tension stresses – which in turn are believed to have been caused by water and/or liquid ammonia freezing in the interior.

These chasmata are typically 15–50 km wide and are mainly oriented in an east- or northeasterly direction. The widest graben have grooves running along the crests of their convex floors (known as valles). The longest canyon is Kachina Chasma, which is over 620 km long.

was taken Jan. 24, 1986, from a distance of 130,000 km (80,000 mi). The complexity of Ariel's surface indicates that a variety of geologic processes have occurred. Credit: NASA/JPL
Image of Ariel, taken on Jan. 24, 1986, from a distance of 130,000 km (80,000 mi) showing the complexity of Ariel’s surface. Credit: NASA/JPL

The ridged terrain on Ariel, which is the second most-common type, consists of bands of ridges and troughs hundreds of kilometers long. These ridges are found bordering cratered terrain and cutting it into polygons. Within each band (25-70 km wide) individual ridges and troughs have been observed that are up to 200 km long and 10-35 km apart. Here too, these features are believed to be a modified form of graben or the result of geological stresses.

The youngest type of terrain observed on Ariel are its plains, which consists of relatively low-lying smooth areas. Due to the varying levels of cratering found in these areas, the plains are believed to have formed over a long period of time. They  are found on the floors of canyons and in a few irregular depressions in the middle of the cratered terrain.

The most likely origin for the plains is through cryovolcanism, since their geometry resembles that of shield volcanoes on Earth, and their topographic margins suggests the eruption of viscous liquid – possibly liquid ammonia. The canyons must therefore have formed at a time when endogenic resurfacing was still taking place on Ariel.

Uranus and Ariel
Ariel’s transit of Uranus, which was captured by the Hubble Space Telescope on July 26th, 2008. Credit: NASA, ESA, L. Sromovsky (University of Wisconsin, Madison), H. Hammel (Space Science Institute), and K. Rages (SETI)

Ariel is the most reflective of Uranus’s moons, with a Bond albedo of about 23%. The surface of Ariel is generally neutral in color, but there appears to be an asymmetry where the trailing hemisphere is slightly redder. The cause of this, is believed to be interaction between Ariel’s trailing hemisphere and radiation from Uranus’ magnetosphere and Solar ultraviolet radiation, which converts organic compounds in the ice into tholins.

Like all of Uranus’ major moons, Ariel is thought to have formed in the Uranunian accretion disc; which existed around Uranus for some time after its formation, or resulted from a large impact suffered by Uranus early in its history.

Exploration:

Due to its proximity to Uranus’ glare, Ariel is difficult to view by amateur astronomers. However, since the 19th century, Ariel has been observed many times by ground-based on space-based instruments. For example, on July 26th, 2006, the Hubble Space Telescope captured a rare transit made by Ariel of Uranus, which cast a shadow that could be seen on the Uranian cloud tops. Another transit, in 2008, was recorded by the European Southern Observatory.

It was not until the 1980s that images were obtained by the first and only orbiter to ever pass through the Uranus’ system. This was the Voyager 2 space probe, which photographed the moon during its January 1986 flyby.  The probe’s closest approach was at a distance of 127,000 km (79,000 mi) – significantly less than the distances to all other Uranian moons except Miranda.

Voyager 2. Credit: NASA
Artist’s impression of the Voyager 2 space probe. Credit: NASA

The images acquired covered only about 40% of the surface, but only 35% was captured with the quality required for geological mapping and crater counting. This was partly due to the fact that the flyby coincided with the southern summer solstice, where the southern hemisphere was pointed towards the Sun and the northern hemisphere was completely concealed by darkness.

No missions have taken place to study Uranus’ system of moons since and none are currently planned. However, the possibility of sending the Cassini spacecraft to Uranus was evaluated during its mission extension planning phase in April of 2008. It was determined that it would take about twenty years for Cassini to get to the Uranian system after departing Saturn. However, this proposal and the ultimate fate of the mission remain undecided at this time.

All in all, Uranus’ moon Ariel is in good company. Like it’s fellow Uranians, its axial tilt is almost the exact same as Uranus’, it is composed of almost equal parts ice and rock, it is geologically active, and its orbit leads to an extreme seasonal cycle. However, Ariel stands alone when its to its brightness and its youthful surface. Unfortunately, this bright and youthful appearance has not made it an easier to observe.

Alas, as with all Uranian moons, exploration of this moon is still in its infancy and there is much we do not know about it. One can only hope another deep-space mission, like a modified Cassini flyby, takes place in the coming years and finishes the job started by Voyager 2!

We have many interesting articles on Ariel and Uranus’ moons here at Universe Today. Here’s one about Ariel’s 2006 transit of Uranus, its 2008 transit, and one which answers the all-important question How Many Moons Does Uranus Have?

For more information, check out NASA’s Solar System Exploration page on Ariel, and The Planetary Society’s Voyager 2 Ariel image catalog.

Sources:

 

Posted on by Matt Williams

Saturn’s Moon Dione

Ringside With Dione
Saturn's moon Dione, with Saturn's rings visible in the background. Credit: NASA/JPL

Thanks to the Cassini mission, a great deal has been learned about Saturn’s system of moons (aka. the Cronian system) in the past decade. Thanks to the presence of an orbiter in the system, astronomers and space exploration enthusiasts have been treated to a seemingly endless stream of images and data, which in turn has enabled us to learn many interesting things about these moons’ appearances, surface features, composition, and history of formation.

This is certainly true of Saturn’s bright moon of Dione. In addition to being the 15th largest moon in the Solar System, and more massive than all known moons smaller than itself combined, it has much in common with other Cronian satellites – like Tethys, Iapetus and Rhea. This includes being mainly composed of ice, having a synchronous rotation with Saturn, and an unusual coloration between its leading and trailing hemispheres.

Discovery and Naming:

Dione was first observed by Italian astronomer Giovanni Domenico Cassini on in 1684 using a large aerial telescope he set up on the grounds of the Paris Observatory. Along with the moons of Iapetus, Rhea and Tethys – which he had discovered in 1671, 1672 and 1684, respectively – he named these moons Sidera Lodoicea (“Stars of Louis”, after his patron, King Louis XIV of France).

These names, however, did not catch on outside of France. By the end of the 17th century, astronomers instead fell into the habit of naming Saturn’s then-known moons as Titan and Saturn I through V, in order of their observed distance from the planet. Being the second most-distant (behind Tethys) Dione came to be known as Saturn II for over a century.

An engraving of the Paris Observatory during Cassini's time. Credit: Public Domain
An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain

The modern names were suggested in 1847 by John Herschel (the son of famed astronomer William Herschel), who suggested all the moons of Saturn be named after Titans – the sons and daughters of Cronos in the Greek mythology (the equivalent of the Roman Saturn).

In his 1847 publication, Results of Astronomical Observations made at the Cape of Good Hope, he suggested the name Dione, an ancient oracular Titaness who was the wife of Zeus and the mother of Aphrodite. Dione is featured in Homer’s The Iliad, and geological features – such as craters and cliffs – take their names from people and places in Virgil’s Aeneid.

Size, Mass and Orbit:

With a mean radius of 561.4 ± 0.4 km and a mass of about 1.0954 × 1021 kg, Dione is equivalent in size to 0.088 Earths and 0.000328 times as massive. It orbits Saturn at an average distance (semi-major axis) of 377,396 km, with a minor eccentricity of 0.0022 – ranging from 376,566 km at periapsis and 378,226 km at apoapsis.

Dione’s semi-major axis is about 2% less than that of the Moon. However, reflecting Saturn’s greater mass, Dione’s orbital period is one tenth that of the Moon (2.736915 days compared to 28). Dione is currently in a 1:2 mean-motion orbital resonance with Saturn’s moon Enceladus, completing one orbit of Saturn for every two orbits completed by Enceladus.

Size comparison between Earth, the Moon, and Saturn's moon Dione. Credit: NASA/JPL/Space Science Institute
Size comparison between Earth, the Moon, and Saturn’s moon Dione. Credit: NASA/JPL/Space Science Institute

This resonance maintains Enceladus’s orbital eccentricity (0.0047) and provides tidal flexing that powers Enceladus’ extensive geological activity (which in turn powers its cryovolcanic jets). Dione has two co-orbital (aka. trojan) moons: Helene and Polydeuces. They are located within Dione’s Lagrangian points, 60 degrees ahead of and behind it, respectively.

Composition and Surface Features:

With a mean density of 1.478 ± 0.003 g/cm³, Dione is composed mainly of water, with a small remainder likely consisting of a silicate rock core. Though somewhat smaller and denser than Rhea, Dione is otherwise very similar in terms of its varied terrain, albedo features, and the different between its leading and trailing hemisphere.

Overall, scientists recognize five classes of geological features on Dione – Chasmata (chasms), dorsa (ridges), fossae (long, narrow depressions), craters, and catenae (crater chains). Craters are the most common feature, as with many Cronian moons, and can be distinguished in terms of heavily cratered terrain, moderately cratered plains, and lightly cratered plains.

The heavily cratered terrain has numerous craters greater than 100 km (62 mi) in diameter, whereas the plains areas tend to have craters less than 30 km (19 mi) in diameter (with some areas being more heavily cratered than others).

This global map of Dione, a moon of Saturn, shows dark red in the trailing hemisphere, which is due to radiation and charged particles from Saturn's intense magnetic environment. Credit: NASA/JPL/Space Science Institute
Global map of Dione, showing dark red in the trailing hemisphere (left), which is due to radiation and charged particles from Saturn’s. Credit: NASA/JPL/Space Science Institute

Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains areas present on the leading hemisphere. This is the opposite of what many scientists expected, and suggests that during the period of Heavy Bombardment, Dione was tidally locked to Saturn in the opposite orientation.

Because Dione is relatively small, it is theorized that an impact large enough to cause a 35 km crater would have been sufficient to spin the satellite in the opposite direction. Because there are many craters larger than 35 km (22 mi), Dione could have been repeatedly spun during its early history. The pattern of cratering since then and the leading hemisphere’s bright albedo suggests that Dione has remained in its current orientation for several billion years.

Dione is also known for its differently colored leading and trailing hemispheres, which are similar to Tethys and Rhea. Whereas its leading hemisphere is bright, its trailing hemisphere is darker and redder in appearance. This is due to the leading hemisphere picking up material from Saturn’s E-Ring, which is fed by Enceladus’ cryovolcanic emissions.

Meanwhile, the trailing hemisphere interacts with radiation from Saturn’s magnetosphere, which causes organic elements contained within its surface ice to become dark and redder in appearance.

Dione's trailing hemisphere, showing the patches of "whispy terrain". Credit: NASA/JPL
Dione’s trailing hemisphere, pictured by the Cassini orbiter, which shows its patches of “wispy terrain”. Credit: NASA/JPL

Another prominent feature is Dione’s “wispy terrain“, which covers its trailing hemisphere and is composed entirely of high albedo material that is also thin enough as to not obscure the surface features beneath. The origin of these features are unknown, but an earlier hypothesis suggested that that Dione was geologically active shortly after its formation, a process which has since ceased.

During this time of geological activity, endogenic resurfacing could have pushed material from the interior to the surface, with streaks forming from eruptions along cracks that fell back to the surface as snow or ash. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.

This hypothesis was proven wrong by the Cassini probe flyby of December 13th, 2004, which produced close-up images. These revealed that the ‘wisps’ were, in fact, not ice deposits at all, but rather bright ice cliffs created by tectonic fractures (chasmata). During this flyby, Cassini also captured oblique images of the cliffs which showed that some of them are several hundred meters high.

Atmosphere:

Dione also has a very thin atmosphere of oxygen ions (O+²), which was first detected by the Cassini space probe in 2010. This atmosphere is so thin that scientists prefer to call it an exosphere rather than a tenuous atmosphere. The density of molecular oxygen ions determined from the Cassini plasma spectrometer data ranges from 0.01 to 0.09 per cm3 .

Crescent Dione from Cassini, October 11, 2005. The crater near the limb at top is Alcander, with larger crater Prytanis adjacent to its left. At lower right, several of the Palatine Chasmata fractures are visible, one of which can be seen bisecting the smaller craters Euryalus (right) and Nisus. NASA / Jet Propulsion Laboratory / Space Science Institute
Dione viewed by Cassini on October 11th, 2005, showing the Alcander crater (top) and the larger Prytanis crater to its left. Credit: NASA/JPL/SSI

Unfortunately, the prevalence of water molecules in the background (from Saturn’s E-Ring) obscured detection of water ice on the surface, so the source of oxygen remains unknown. However, photolysis is a possible cause (similar to what happens on Europa), where charged particles from Saturn’s radiation belt interact with water ice on the surface to create hydrogen and oxygen, the hydrogen being lost to space and the oxygen retained.

Exploration:

Dione was first imaged by the Voyager 1 and 2 space probes as they passed by Saturn on their way to the Outer Solar System in 1980 and 1981, respectively. Since that time, the only probe to conduct a flyby or close-up imaging of Dione has been the Cassini orbiter, which conducted five flybys of the moon between 2005 and 2015.

The first close flyby took place on October 11th, 2005, at a distance of 500 km (310 mi), followed by another on April 7th, 2010, (again at a distance of 500 km). A third flyby was performed on December 12th, 2011, and was the closest, at an distance of 99 km (62 mi). The fourth and fifth flybys took place on June 16th and August 17th, 2015, at a distance of 516 km (321 mi) and 474 km (295 mi), respectively.

In addition to obtaining images of Cassini’s cratered and differently-colored surface, the Cassini mission was also responsible for detecting the moon’s tenuous atmosphere (exosphere). Beyond that, Cassini also provided scientists with new evidence that Dione could be more geologically active than previously predicted.

Based on models constructed by NASA scientists, it is now believed that Dione’s core experiences tidal heating, which increases the closer it gets to Saturn. Because of this, scientists also believe that Dione may also have a liquid water ocean at its core-mantle boundary, thus joining moons like Enceladus, Europa and others in being potential environments where extra-terrestrial life could exist.

This, as well as Dione’s geological history and the nature of its surface (which could be what gives rise to its atmosphere) make Dione a suitable target for future research. Though no missions to study the moon are currently being planned, any mission to the Saturn system in the coming years would likely include a flyby or two!

We have many great articles on Dione and Saturn’s moons here at Universe Today. Here is one about Cassini’s first flyby, its closest flyby, it’s possible geological activity, its canyons, and its wispy terrain.

Universe Today also has an interview with Dr. Kevin Grazier, a member of the Cassini-Huygens mission.

Posted on by Matt Williams

Saturn’s “Yin-Yang” Moon Iapetus

The dark and light side of Iapetus. Credit: NASA/JPL/Space Science Institute

Thanks to the Cassini mission, a great many things have been learned about the Saturn system in recent years. In addition to information on Saturn’s atmosphere, rotation and its beautiful and extensive ring system, many revelations have been made about Saturn’s system of moons. For example, very little was known about the obscure moon of Iapetus – sometimes nicknamed Saturn’s “yin-yang” moon – before Cassini‘s arrival.

In addition to its mysterious, equatorial ridge, this moon also has a two-tone appearance that has historically made direct observation quite difficult. Due to its distance from Saturn, close-up observation with space probes has also been quite difficult too until very recently. However, what we have learned in the past few years about Iapetus has taught us that it is a world of stark contrasts, and not just in terms of its appearance.

Discovery and Naming:

Iapetus was discovered by Giovanni Domenico Cassini in April 1671. Along with Rhea, Tethys and Dione, Iapetus was one of four moons Cassini discovered between 1671 and 1672 – which together he named Sidera Lodoicea (“Stars of Louis“, after his patron, Louis XIV). After the discovery, astronomers fell into the habit of referring to them using Roman numerals, with Iapetus being Saturn V.

The name Iapetus was suggested by John Herschel, the son of William Herschel, in his 1847 treatise Results of Astronomical Observations made at the Cape of Good Hope. Like all of Saturn’s moons, the name Iapetus was taken from the Titans of Greek mythology – the sons and daughters of Cronus (the Greek equivalent of the Roman Saturn). Iapetus was the son of Uranus and Gaia and the father of Atlas, Prometheus, Epimetheus and Menoetius.

An engraving of the Paris Observatory during Cassini's time. Credit: Public Domain
An engraving of the Paris Observatory during Cassini’s time. Credit: Wikipedia Commons

Geological features on Iapetus are named after characters and places from the French epic poem The Song of Roland. Examples of names used include the craters Charlemagne and Baligant, and the northern and southern bright regions, Roncevaux Terra and Sargassio Terra. The one exception is Cassini Regio the dark region of Iapetus, named after the region’s discoverer, Giovanni Cassini.

Size, Mass and Orbit:

With a mean radius of 734.5 ± 2.8 km and a mass of about 1.806 × 1021 kg, Iapetus is 0.1155 times the size of Earth and 0.00030 times as massive. It orbits its parent planet at an average distance (semi major axis) of 3,560,820 km. With a noticeable eccentricity of 0.0286125, its orbit ranges in distance from 3,458,936 km at periapsis and 3,662,704 km at apoapsis.

With an average orbital speed of 3.26 km/s, Iapetus takes 79.32 days to complete an single orbit of Saturn. Despite being Saturn’s third-largest moon, Iapetus orbits much farther from Saturn than its next closest major satellite (Titan). It has also the most inclined orbital plane of any of the regular satellites – 17.28° to the ecliptic, 15.47° to Saturn’s equator, and 8.13° to the Laplace plane. Only the irregular outer satellites like Phoebe have more inclined orbits.

Size comparison of Earth, the Moon, and Iapetus. Credit: NASA/JPL/Tom Reding
Size comparison of Earth, the Moon, and Iapetus. Credit: NASA/JPL-Caltech/SSI/LPI/Tom Reding

Composition and Surface Features:

Like many of Saturn’s moons – particularly Tethys, Mimas and Rhea – Iapetus has a low density (1.088 ± 0.013 g/cm³) which indicates that it is composed primary of water ice and only about 20% rock. But unlike most of Saturn’s larger moons, its overall shape is neither spherical or ellipsoid, instead consisting of flattened poles and a bulging waistline.

Its large and unusually high equatorial ridge (see below) also contributes to its disproportionate shape. Because of this, Iapetus is the largest known moon to not have achieved hydrostatic equilibrium. Though rounded in appearance, its bulging appearance disqualifies it from being classified as spherical.

As is common with Cronian moons, Iapetus’ surface shows considerable signs of cratering. Recent images taken by the Cassini spacecraft have revealed multiple large impact basins, with at least five measuring over 350 km in diameter. The largest, Turgis, has a diameter of 580 km, with an extremely steep rim and a scarp about 15 km in height. It has also been concluded that Iapetus’ surface supports long-runout landslides (aka. sturzstroms), which could be due to ice sliding.

As already noted, another interesting feature on Iapetus is its famous equatorial ridge, which measures 1300 km in length, 20 km wide, 13 km high, and runs along the center of the Cassini Regio dark region. Though indications had been made as to the existence of a mountain chain in this region earlier, the ridge was observed directly for the first time when the Cassini spacecraft took its first images of Iapetus on December 31st, 2004.

But perhaps Iapetus’ best known feature is its two-tone coloration. This was first observed by Giovanni Cassini in the 17th century, who noted that he could only view Iapetus when it was on the west side of Saturn and never on the east. At the time, he correctly concluded that Iapetus was tidally-locked with Saturn, and that one side was darker than the other. This conclusion was later backed up by observations using ground-based telescopes.

The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra – which lies north of the equator – and Saragossa Terra, which is south of it. Today, it is understood that dark regions are carbonaceous, and likely contain organic compounds similar to the substances found in primitive meteorites or on the surfaces of comets – i.e. frozen cyano-compounds like hydrogen cyanide polymers.

The pattern of coloration is analogous to a spherical yin-yang symbol, hence the nickname “Saturn’s yin-yang moon.” The difference in coloration between the two Iapetian hemispheres is quite extreme. While the leading hemisphere is dark, with an albedo of 0.03–0.05 (and has a slight reddish-brown coloring), most of the trailing hemisphere and poles are almost as bright as Europa (albedo 0.5–0.6).

Enhanced-color map (27.6 MB). The leading hemisphere is at the right. NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute
Enhanced-color map of Iapetus, using data collected by the Cassini probe.  The leading hemisphere is at the right. Credit: NASA/JPL-Caltech/SSI/LPI

Thus, the apparent magnitude of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9. Theories as to its cause generally agree that the original dark material must have come from outside Iapetus, but that subsequent darkening is caused by the sublimation of ice from the warmer areas of Iapetus’s surface, causing volatile compounds to sublimate out and retreat to colder regions.

Because of its slow rotation of 79 days, Iapetus experiences enough of a temperature difference to facilitate this. Near the equator, heat absorption by the dark material results in a daytime temperatures in Cassini Regio of 129 K (-144.15 °C/-227.5 °F)  compared to 113 K (-160.15 °C/-256.3 °F) in the bright regions. The difference in temperature means that ice sublimates from Cassini Regio, then deposits in the colder bright areas and especially at the even colder poles.

Over geologic time scales, this would further darken Cassini Regio and brighten the rest of Iapetus, creating a runaway thermal feedback process of ever greater contrast in albedo, ending with all exposed ice being lost from Cassini Regio. This model is the generally accepted one because it explains the distribution of light and dark areas, the absence of shades of grey, and the thinness of the dark material covering Cassini Regio.

Three different false-color views of Saturn's moon Iapetus show the boundary of the global "color dichotomy" on the hemisphere of this moon facing away from Saturn. Credit: NASA/JPL/Space Science Institute
Three different false-color views of Saturn’s moon Iapetus, showing the boundary of the global “color dichotomy”. Credit: NASA/JPL/Space Science Institute

However, it is acknowledged that a separate process would be required to get this process thermal feedback started. It is therefore theorized that initially, dark material came from elsewhere, most likely some of Saturn’s small, retrograde moons. Material from these moons could have been blasted off either by micrometeoroids or a large impact.

This material would then have been darkened from exposure to sunlight, then swept up by the leading hemisphere of Iapetus. Once this process created a modest contrast in albedo (and hence, temperature) on Iapetus’ surface, the thermal feedback process would have come into play and exaggerated it further.

The greatest source of this material is believed to be Phoebe, the largest of Saturn’s outer moons. The discovery of a tenuous disk of material in the plane of (and just inside of) Phoebe’s orbit, which was announced on October 6th, 2009, supports this theory.

Exploration:

The first robotic spacecraft to explore Iapetus were the Voyager 1 and Voyager 2 probes, which passed through the Saturn system on their way to the outer Solar System in 1980 and 1981. Data from these missions provided scientists with the first indications of Iapetus’ mountains, which were thereafter informally referred to as the “Voyager Mountains”.

Saturn's moon Iapetus. Image credit: NASA/JPL/Space Science Institute.
Saturn’s moon Iapetus, captured by the Cassini space probe on New Year’s Eve 2004. Credit: NASA/JPL/Space Science Institute.

Only the Cassini orbiter has ever explored Saturn’s moon of Iapetus, which captured multiple images of the moon from moderate distances since 2004. For instance, on New Year’s Eve 2004, Cassini passed Iapetus at a distance of 122,647 kilometers (76,209 miles) and captured the four visible light images that were put together to form the view of its equatorial ridge jutting out to the side (shown above).

However, its great distance from Saturn makes close observation difficult. As a result, Cassini made only one targeted close flyby, which took place on September 10th, 2007 at a minimum range of 1227 km. It was during this flyby that data was obtained which indicated that thermal segregation is likely the primary force responsible for Iapetus’ dark hemisphere. No future missions are planned at this time.

Iapetus is a world of contrasts, and not just in terms of its color. In addition, it is a very small moon that still managed to be massive enough to achieve hydrostatic equilibrium. And despite being one of Saturn’s larger moons, it orbits at a distance usually reserved for smaller, irregular moons.

Coupled with the fact that scientists are still not sure why it has its unusual walnut-shape, Iapetus is likely to be a target for any research missions headed to study the Cronian moons in the coming years.

We have many great articles on Iapetus and Saturn’s moons here at Universe Today. Here is one about its famous ridge, its two-tone coloring, the ice avalanches it periodically experiences, and whether or not it consumed one of Saturn’s rings.

For more information, check out NASA’s View of the Solar System page on Iapetus, and the Cassini Solstice Mission’s page.

Posted on by Matt Williams

Saturn’s “Death-Star” Moon Mimas

A view of Mimas from the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute

Much has been learned about Saturn’s system of moons in recent decades, thanks to the Voyager missions and the more recent surveys conducted by the Cassini spaceprobe. Between its estimated 150 moons and moonlets (only 53 of which have been identified and named) there is no shortage of scientific curiosities, and enough mysteries to keep astronomers here on Earth busy for decades.

Consider Mimas, which is often referred to as Saturn’s “Death Star Moon” on a count of its unusual appearance. Much like Saturn’s moons Tethys and Rhea, Mimas’ peculiar characteristics represents something of a mystery. Not only is it almost entirely composed ice, it’s coloration and surface features reveal a great deal about the history of the Saturnian (aka. Cronian) system. On top of that, it may even house an interior, liquid-water ocean.

Discovery and Naming:

Saturn’s moon Mimas was discovered by William Herschel in 1789, more than 100 years after Saturn’s larger moons were discovered by Christian Huygens and Giovanni Cassini. As with all the seven then-known satellites of Saturn, Mimas’ name was suggested by William Herschel’s son John in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.

Mimas takes its name from one of the Titans of Greek mythology, who were the sons and daughters of Cronus (the Greek equivalent to Jupiter). Mimas was an offspring of Gaia, born from the blood of the castrated Uranus, who eventually died during the struggle with the Olympian Gods for control of the universe.

A replica of the telescope which William Herschel used to observe Uranus. Credit:
A replica of the telescope which William Herschel used to observe Uranus. Credit: Alun Salt/Wikimedia Commons

Size, Mass and Orbit:

With a mean radius of 198.2 ± 0.4 km and a mass of about 3.75 ×1019 kg, Mimas is equivalent in size to 0.0311 Earths and 0.0000063 times as massive. Orbiting Saturn at an average distance (semi-major axis) of 185,539 km, it is the innermost of Saturn’s larger moons, and the 8th moon orbiting Saturn. It’s orbit also has a minor eccentricity of 0.0196, ranging from 181,902 km at periapsis and 189,176 km at apoapsis.

With an estimated orbital velocity of 14.28 km/s, Mimas takes 0.942 days to complete a single orbit of Saturn. Like many of Saturn’s moons. Mimas rotation period is synchronous to its orbital period, which means it keeps one face constantly pointing towards the planet. Mimas is also in a 2:1 mean-motion resonance with the larger moon Tethys, and in a 2:3 resonance with the outer F Ring shepherd moonlet, Pandora.

Composition and Surface Features:

Mimas’ mean density of 1.1479 ± 0.007 g/cm³ is just slightly higher than that of water (1 g/cm³), which means that Mimas is mostly composed of water ice, with just a small amount of silicate rock. In this respect, Mimas is much like Tethys, Rhea, and Dione – moon’s of Saturn that are primarily composed of water ice.

Due to the tidal forces acting on it, Mimas is noticeably prolate – i.e. its longest axis is about 10% longer than the shortest, giving it its egg-shaped appearance. In fact, with a diameter of 396 km (246 mi), Mimas is just barely large and massive enough to achieve hydrostatic equilibrium (i.e. to become rounded in shape under the force of its own gravitation). Mimas is the smallest known astronomical body to have achieved this.

This mosaic, created from images taken by NASA's Cassini spacecraft during its closest flyby of Saturn's moon Mimas, looks straight at the moon's huge Herschel Crater Credit: NASA/JPL
Mosaic image of Mimas, created from images taken by NASA’s Cassini spacecraft, showing the Herschel crater in the center. Credit: NASA/JPL

Three types of geological features are officially recognized on Mimas: craters, chasmata (chasms) and catenae (crater chains). Of these, craters are the most common, and it is believed that many of them have existed since the beginning of the Solar System. Mimas surface is saturated with craters, with every part of the surface showing visible depressions, and newer impacts overwriting older ones.

Mimas’ most distinctive feature is the giant impact crater Herschel, named in honor of William Herschel (the discoverer of Uranus, its moons Oberon, and Titania, and the Cronian moons Enceladus and Mimas). This large crater gives Mimas the appearance of the “Death Star” from Star Wars. At 130 km (81 mi) in diameter, Herschel’s is almost a third of Mimas’ own diameter.

Its walls are approximately 5 km (3.1 mi) high, parts of its floor measure 10 km (6.2 mi) deep, and its central peak rises 6 km (3.7 mi) above the crater floor. If there were a crater of an equivalent scale on Earth, it would be over 4,000 km (2,500 mi) in diameter, which would make it wider than the continent of Australia.

The impact that made this crater must have nearly shattered Mimas, and is believed to have created the fractures on the opposite side of the moon by sending shock waves through Mimas’s body. In this respect, Mimas’ surface closely resembles that of Tethys, with its massive Odysseus crater on its western hemisphere and the concentric Ithaca chasma, which is believed to have formed as a result of the impact that created Odysseus.

Color map of Mimas, created using data provided by the Cassini spaceprobe. Credit: NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute
Color map of Mimas, created using data provided by the Cassini spaceprobe. Credit: NASA/JPL/Caltech/SSI/LPI

Mimas’ surface is also saturated with smaller impact craters, but no others are anywhere near the size of Herschel. The cratering is also not uniform, with most of the surface being covered with craters larger than 40 km (25 mi) in diameter. However, in the south polar region, there are generally no craters larger than 20 km (12 mi) in diameter.

Data obtained in 2014 from the Cassini spacecraft has also led to speculation about a possible interior ocean. Due to the planet’s libration (oscillation in its orbit), scientists believe that the planet’s interior is not uniform, which could be the result of a rocky interior or an interior ocean at the core-mantle boundary. This ocean would likely be maintained thanks to tidal flexing caused by Mimas’ orbital resonances with Tethys and Pandora.

A number of features in Saturn’s rings are also related to resonances with Mimas. Mimas is responsible for clearing the material from the Cassini Division, which is the gap between Saturn’s two widest rings – the A Ring and B Ring. The repeated pulls by Mimas on the Cassini Division particles, always in the same direction, forces them into new orbits outside the gap.

Particles in the Huygens Gap at the inner edge of the Cassini division are in a 2:1 resonance with Mimas. In other words, they orbit Saturn twice for each orbit competed by Mimas. The boundary between the C and B ring is meanwhile in a 3:1 resonance with Mimas; and recently, the G Ring was found to be in a 7:6 co-rotation eccentricity resonance with Mimas.

This figure illustrates the unexpected and bizarre pattern of daytime temperatures found on Saturn's small inner moon Mimas (396 kilometers, or 246 miles, in diameter). Credit: NASA/JPL/GSFC/SWRI/SSI
This figure illustrates the unexpected and bizarre pattern of daytime temperatures found on Saturn’s small inner moon Mimas. Credit: NASA/JPL/GSFC/SWRI/SSI

Exploration:

The first mission to study Mimas up close was Pioneer 11, which flew by Saturn in 1979 and made its closest approach on Sept. 1st, 1979, at a distance of 104,263 km. The Voyager 1 and 2 missions both flew by Mimas in 1980 and 1981, respectively, and snapped pictures of Saturn’s atmosphere, its rings, its system of moons. Images taken by Voyager 1 probe were the first ever of the Herschel crater.

Mimas has been imaged several times by the Cassini orbiter, which entered into orbit around Saturn in 2004. A close flyby occurred on February 13, 2010, when Cassini passed Mimas at a distance of 9,500 km (5,900 mi). In addition to providing multiple images of Mimas’ cratered surface, it also took measurements of Mimas’ orbit, which led to speculation about a possible interior ocean.

The Saturn system is truly a wonder. So many moons, so many mysteries, and so many chances to learn about the formation of the Solar System and how it came to be. One can only hope that future missions are able to probe some of the deeper ones, like what might be lurking beneath Mimas’ icy, imposing “Death Star” surface!

We’ve written many great articles about Mimas and Saturn’s moons here at Universe Today. Here’s one about the Herschel Crater, one about the first detailed look Cassini made, and one about it’s “Death Star” appearance.

Another great resource about Mimas is Solar Views, and you can get even more info from the Nine Planets.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

Posted on by Matt Williams

Saturn’s Moon Tethys

Saturn's moon Tethys, imaged by Cassini on April 14, 2012.

Thanks the Voyager missions and the more recent flybys conducted by the Cassini space probe, Saturn’s system of moons have become a major source of interest for scientists and astronomers. From water ice and interior oceans, to some interesting surface features caused by impact craters and geological forces, Saturn’s moons have proven to be a treasure trove of discoveries.

This is particularly true of Saturn’s moon Tethys, also known as a “Death Star Moon” (because of the massive crater that marks its surface). In addition to closely resembling the space station out of Star Wars lore, it boasts the largest valleys in the Solar System and is composed mainly of water ice. In addition, it has much in common with two of its Cronian peers, Mimas and Rhea, which also resemble a certain moon-size space station.

Discovery and Naming:
Originally discovered by Giovanni Cassini in 1684, Tethys is one of four moons discovered by the great Italian mathematician, astronomer, astrologer and engineer between the years of 1671 and 1684. These include Rhea and Iapetus, which he discovered in 1671-72; and Dione, which he discovered alongside Tethys.

Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. At the time of their discovery, he named the four new moons “Sider Lodoicea” (“the stars of Louis”) in honor of his patron, king Louis XIV of France.

An engraving of the Paris Observatory during Cassini's time. Credit: Public Domain
An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain

The modern names of all seven satellites of Saturn come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus). In his 1847 treatise Results of Astronomical Observations made at the Cape of Good Hope, he suggested that all should be named after the Titans – the brothers and sisters of Cronos – from Greek mythology.

Size, Mass and Orbit:
With a mean radius of 531.1 ± 0.6 km and a mass of 6.1745 ×1020 kg, Tethys is equivalent in size to 0.083 Earths and 0.000103 times as massive. Its size and mass also mean that it is the 16th-largest moon in the Solar System, and more massive than all known moons smaller than itself combined. At an average distance (semi-major axis) of 294,619 km, Tethys is the third furthest large moon from Saturn and the 13th most distant moon over all.

Tethys’ has virtually no orbital eccentricity, but it does have an orbital inclination of about 1°. This means that the moon is locked in an inclination resonance with Saturn’s moon Mimas, though this does not cause any noticeable orbital eccentricity or tidal heating. Tethys has two co-orbital moons, Telesto and Calypso, which orbit near Tethys’s Lagrange Points.

Diameter comparison of the Saturnian moon Tethys, Moon, and Earth. Credit: NASA/JPL/USGS/Tom Reding
Diameter comparison of the Saturnian moon Tethys, Moon, and Earth. Credit: NASA/JPL/USGS/Tom Reding

Tethys’ orbit lies deep inside the magnetosphere of Saturn, which means that the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.

Composition and Surface Features:
Tethys has a mean density of 0.984 ± 0.003 grams per cubic centimeter. Since water is 1 g/cm3, this means that Tethys is comprised almost entirely of water ice. In essence, if the moon were brought closer to the Sun, the vast majority of the moon would sublimate and evaporate away.

It is not currently known whether Tethys is differentiated into a rocky core and ice mantle. However, given the fact that rock accounts for less 6% of its mass, a differentiated Tethys would have a core that did not exceed 145 km in radius. On the other hand, Tethys’ shape – which resembles that of a triaxial ellipsoid – is consistent with it having a homogeneous interior (i.e. a mix of ice and rock).

This ice is also very reflective, which makes Tethys the second-brightest of the moons of Saturn, after Enceladus. There are two different regions of terrain on Tethys. One portion is ancient, with densely packed craters, while the other parts are darker and have less cratering. The surface is also marked by numerous large faults or graben.

The Odysseus Crater, a Credit: NASA/JPL/SSI
The Odysseus Crater, the 400 km surface feature that gives Tethys it’s “Death Star” appearance. Credit: NASA/JPL/SSI

The western hemisphere of Tethys is dominated by a huge, shallow crater called Odysseus. At 400 km across, it is the largest crater on the surface, and roughly 2/5th the size of Tethys itself. Due to its position, shape, and the fact that a section in the middle is raised, this crater is also responsible for lending the moon it’s “Death Star” appearance.

The largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long, making it the second longest valley in the Solar System. Named after the island of Ithaca in Greece, this valley runs approximately three-quarters of the way around Tethys’ circumference. It is also approximately concentric with Odysseus crater, which has led some astronomers to theorize that the two features might be related.

Scientists also think that Tethys was once internally active and that cryovolcanism led to endogenous resurfacing and surface renewal. This is due to the fact that a small part of the surface is covered by smooth plains, which are devoid of the craters and graben that cover much of the planet. The most likely explanation is that subsurface volcanoes deposited fresh material on the surface and smoothed out its features.

Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.
Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.

Like all other regular moons of Saturn, Tethys is believed to have formed from the Saturnian sub-nebula – a disk of gas and dust that surrounded Saturn soon after its formation. As this dust and gas coalesced, it formed Tethys and its two co-orbital moons: Telesto and Calypso. Hence why these two moons were  captured into Tethys’ Lagrangian points, with one orbiting ahead of Tethys and the other following behind.

Exploration:
Tethys has been approached by several space probes in the past, including Pioneer 11 (1979), Voyager 1 (1980) and Voyager 2 (1981). Although both Voyager spacecraft took images of the surface, only those taken by Voyager 2 were of high enough resolution to truly map the surface. While Voyager 1 managed to capture an image of Ithaca Chasma, it was the Voyager 2 mission that revealed much about the surface and imaged the Odysseus crater.

Tethys has also been photographed multiple times by the Cassini orbiter since 2004. By 2014, all of the images taken by Cassini allowed for a series of enhanced-color maps that detailed the surface of the entire planet (shown below). The color and brightness of Tethys’ surface have since become sources of interest to astronomers.

On the leading hemisphere of the moon, spacecraft have found a dark bluish band spanning 20° to the south and north from the equator. The band has an elliptical shape getting narrower as it approaches the trailing hemisphere, which is similar to the one found on Mimas.

This set of global, color mosaics of Saturn's moon Tethys was produced from images taken by NASA's Cassini spacecraft during its first ten years exploring the Saturn system. Credit: NASA / JPL-Caltech / Space Science Institute / Lunar and Planetary Institute
Global, color mosaics of Saturn’s moon Tethys, as produced from images taken by NASA’s Cassini spacecraft between 2004-2014. Credit: NASA/JPL-Caltech/Space Science Institute/ Lunar and Planetary Institute

The band is likely caused by the influence of energetic electrons from Saturn’s magnetosphere, which drift in the direction opposite to the rotation of the planet and impact areas on the leading hemisphere close to the equator. Temperature maps of Tethys obtained by Cassini have shown this bluish region to be cooler at midday than surrounding areas.

At present, Tethys’ water-rich composition remains unexplained. One of the most interesting explanations proposed is that the rings and inner moons accreted from the ice-rich crust of a much larger, Titan-sized moon before it was swallowed up by Saturn. This, and other mysteries, will likely be addressed by future space probe missions.

We have many great articles about Tethys here at Universe Today. Here’s one about the story about Tethys, with a photograph taken by NASA’s Cassini spacecraft, and another about a feature on the surface of Tethys called Ithaca Chasma.

Want more info on Tethys? Check out this article from Solar Views, and this one from Nine Planets.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

Posted on by Matt Williams

Saturn’s Moon Rhea

Saturn's moon Rhea, as imaged by the Cassini-Huygens spaceprobe. Credit: NASA/JPL-Caltech

The Cronian system (i.e. Saturn and its system of rings and moons) is breathtaking to behold and intriguing to study. Besides its vast and beautiful ring system, it also has the second-most satellites of any planet in the Solar System. In fact, Saturn has an estimated 150 moons and moonlets – and only 53 of them have been officially named – which makes it second only to Jupiter.

For the most part, these moons are small, icy bodies that are believed to house interior oceans. And in all cases, particularly Rhea, their interesting appearances and compositions make them a prime target for scientific research. In addition to being able to tell us much about the Cronian system and its formation, moons like Rhea can also tell us much about the history of our Solar System.

Discovery and Naming:

Rhea was discovered by Italian astronomer Giovanni Domenico Cassini on December 23rd, 1672. Together with the moons of Iapetus, Tethys and Dione, which he discovered between 1671 and 1672, he named them all Sidera Lodoicea (“the stars of Louis”) in honor of his patron, King Louis XIV of France. However, these names were not widely recognized outside of France.

In 1847, John Herschel (the son of famed astronomer William Herschel, who discovered Uranus, Enceladus and Mimas) suggested the name Rhea – which first appeared in his treatise Results of Astronomical Observations made at the Cape of Good Hope. Like all the other Cronian satellites, Rhea was named after a Titan from Greek mythology, the “mother of the gods” and one the sisters of Cronos (Saturn, in Roman mythology).

The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Some small moons are also shown. All to scale. Credit: NASA/JPL/Space Science Institute
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan (background), Iapetus (top), and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute

Size, Mass and Orbit:

With a mean radius of 763.8±1.0 km and a mass of 2.3065 ×1021 kg, Rhea is equivalent in size to 0.1199 Earths (and 0.44 Moons), and about 0.00039 times as massive (or 0.03139 Moons). It orbits Saturn at an average distance (semi-major axis) of 527,108 km, which places it outside the orbits of  Dione and Tethys, and has a nearly circular orbit with a very minor eccentricity (0.001).

With an orbital velocity of about 30,541 km/h, Rhea takes approximately 4.518 days to complete a single orbit of its parent planet. Like many of Saturn’s moons, its rotational period is synchronous with its orbit, meaning that the same face is always pointed towards it.

Composition and Surface Features:

With a mean density of about 1.236 g/cm³, Rhea is estimated to be composed of 75% water ice (with a density of roughly 0.93 g/cm³) and 25% of silicate rock (with a density of around 3.25 g/cm³). This low density means that although Rhea is the ninth-largest moon in the Solar System, it is also the tenth-most massive.

In terms of its interior, Rhea was originally suspected of being differentiated between a rocky core and an icy mantle. However, more recent measurements would seem to indicate that Rhea is either only partly differentiated, or has a homogeneous interior – likely consisting of both silicate rock and ice together (similar to Jupiter’s moon Callisto).

Views of Saturn's moon Rhea. Credit: NASA/JPL/Space Science Institute
Views of Saturn’s moon Rhea, with false-color image showing elevation data at the right. Credit: NASA/JPL/Space Science Institute

Models of Rhea’s interior also suggest that it may have an internal liquid-water ocean, similar to Enceladus and Titan. This liquid-water ocean, should it exist, would likely be located at the core-mantle boundary, and would be sustained by the heating caused by from decay of radioactive elements in its core.

Rhea’s surface features resemble those of Dione, with dissimilar appearances existing between their leading and trailing hemispheres – which suggests that the two moons have similar compositions and histories. Images taken of the surface have led astronomers to divide it into two regions – the heavily cratered and bright terrain, where craters are larger than 40 km (25 miles) in diameter; and the polar and equatorial regions where craters are noticeably smaller.

Another difference between Rhea’s leading and trailing hemisphere is their coloration. The leading hemisphere is heavily cratered and uniformly bright while the trailing hemisphere has networks of bright swaths on a dark background and few visible craters. It had been thought that these bright areas (aka. wispy terrain) might be material ejected from ice volcanoes early in Rhea’s history when its interior was still liquid.

However, observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, has cast this into doubt. It is now believed that the wispy terrain are tectonically-formed ice cliffs (chasmata) which resulted from extensive fracturing of the moon’s surface. Rhea also has a very faint “line” of material at its equator which was thought to be deposited by material deorbiting from its rings (see below).

Hemispheric color differences on Saturn's moon Rhea are apparent in this false-color view from NASA's Cassini spacecraft. This image shows the side of the moon that always faces the planet. Image Credit: NASA/JPL/SSI
Hemispheric color differences on Saturn’s moon Rhea are apparent in this false-color view of the anti-Cronian side, from NASA’s Cassini spacecraft. Image Credit: NASA/JPL/SSI

Rhea has two particularly large impact basins, both of which are situated on Rhea’s anti-Cronian side (aka. the side facing away from Saturn). These are known as Tirawa and Mamaldi basins, which measure roughly 360 and 500 km (223.69 and 310.68 mi) across. The more northerly and less degraded basin of Tirawa overlaps Mamaldi – which lies to its southwest – and is roughly comparable to the Odysseus crater on Tethys (which gives it its “Death-Star” appearance).

Atmosphere:

Rhea has a tenuous atmosphere (exosphere) which consists of oxygen and carbon dioxide, which exists in a 5:2 ratio. The surface density of the exosphere is from 105 to 106 molecules per cubic centimeter, depending on local temperature. Surface temperatures on Rhea average 99 K (-174 °C/-281.2 °F) in direct sunlight, and between 73 K (-200 °C/-328 °F) and 53 K (-220 °C/-364 °F) when sunlight is absent.

The oxygen in the atmosphere is created by the interaction of surface water ice and ions supplied from Saturn’s magnetosphere (aka. radiolysis). These ions cause the water ice to break down into oxygen gas (O²) and elemental hydrogen (H), the former of which is retained while the latter escapes into space. The source of the carbon dioxide is less clear, and could be either the result of organics in the surface ice being oxidized, or from outgassing from the moon’s interior.

Saturn's second-largest moon Rhea, in front of the rings and a blurred Epimetheus (or Janus) whizzing behind. Acquired March 29, 2012.
Saturn’s second-largest moon Rhea, pictured by the Cassini probe on March 29, 2012. Credit: NASA/JPL

Rhea may also have a tenuous ring system, which was inferred based on observed changes in the flow of electrons trapped by Saturn’s magnetic field. The existence of a ring system was temporarily bolstered by the discovered presence of a set of small ultraviolet-bright spots distributed along Rhea’s equator (which were interpreted as the impact points of deorbiting ring material).

However, more recent observations made by the Cassini probe have cast doubt on this. After taking images of the planet from multiple angles, no evidence of ring material was found, suggesting that there must be another cause for the observed electron flow and UV bright spots on Rhea’s equator. If such a ring system were to exist, it would be the first instance where a ring system was found orbiting a moon.

Exploration:

The first images of Rhea were obtained by the Voyager 1 and 2 spacecraft while they studied the Cronian system, in 1980 and 1981, respectively. No subsequent missions were made until the arrival of the Cassini orbiter in 2005. After it’s arrival in the Cronian system, the orbiter made five close targeted fly-bys and took many images of Saturn from long to moderate distances. 

The Cronian system is definitely a fascinating place, and we’ve really only begun to scratch its surface in recent years. In time, more orbiters and perhaps landers will be traveling to the system, seeking to learn more about Saturn’s moons and what exists beneath their icy surfaces. One can only hope that any such mission includes a closer look at Rhea, and the other “Death Star Moon”, Dione.

We have many great articles on Rhea and Saturn’s system of moons here at Universe Today. Here is one about its possible ring system, its tectonic activity, it’s impact basins, and images provided by Cassini’s flyby.

Astronomy Cast also has an interesting interview with Dr. Kevin Grazier, who worked on the Cassini mission.

For more information, check out NASA’s Solar System Exploration page on Rhea.

Posted on by Matt Williams

Io, Jupiter’s Volcanic Moon

This global view of Jupiter's moon, Io, was obtained during the tenth orbit of Jupiter by NASA's Galileo spacecraft. Credit: NASA
This global view of Jupiter's moon, Io, was obtained during the tenth orbit of Jupiter by NASA's Galileo spacecraft. Credit: NASA

Exploring the Solar System is like peeling an onion. With every layer removed, one finds fresh mysteries to ponder over, each one more confounding than the last. And this is certainly the case when it comes to Jupiter’s system of moons, particularly its four largest – Io, Europa, Ganymede and Callisto. Known as the Galilean Moons, in honor of their founder, these moons possess enough natural wonders to keep scientists busy for centuries.

As Jupiter’s innermost moon, it is also the fourth-largest moon in the Solar System, has the highest density of any known moon, and is the driest known object in the Solar System. It is also one of only four known bodies that experiences active volcanism and – with over 400 active volcanoes – it is the most geologically active body in the Solar System.

Continue reading “Io, Jupiter’s Volcanic Moon”