Can you imagine the Solar System without Saturn’s rings? Can you envision Earth at the time the dinosaurs roamed the planet? According to a new paper, the two may have coincided.
Data from the Cassini mission shows that Saturn’s rings may be only 10 to 100 million years old. They may not have been there during the reign of the dinosaurs, and may in fact be a fairly modern development in our Solar System.
It has been almost forty years since the Voyager 1 and 2 missions visited the Saturn system. As the probes flew by the gas giant, they were able to capture some stunning, high-resolution images of the planet’s atmosphere, its many moons, and its iconic ring system. In addition, the probes also revealed that Saturn was slowly losing its rings, at a rate that would see them gone in about 100 million years.
More recently, the Cassini orbiter visited the Saturn system and spent over 12 years studying the planet, its moons and its ring system. And according to new research based on Cassini’s data, it appears that Saturn is losing its rings at the maximum rate predicted by the Voyager missions. According to the study, Saturn’s rings are being gobbled up by the gas giant at a rate that means they could be gone in less 100 million years.
During the summer of 2018, the planets of Mars and Saturn (one after the other) have been in opposition. In astronomical terms, opposition refers to when a planet is on the opposite side of the Earth relative to the Sun. This not only means that the planet is closer to Earth in its respective orbit, but that is also fully lit by the Sun (as seen from Earth) and much more visible.
As a result, astronomers are able to observe these planets in greater detail. The Hubble Space Telescope took advantage of this situation to do what it has done best for the past twenty-eight years – capture some breathtaking images of both planets! Hubble made its observations of Saturn in June and Mars in July, and showed both planets close to their opposition.
Hubble’s high-resolution images of the planets and moons in our Solar System can only be surpassed by spacecraft that are either orbiting or conducting close flybys to them. However, Hubble has a major advantage over these types of missions, in that it can look at the Solar planets periodically and observe them over much longer periods of time than a passing spacecraft.
Hubble observed Saturn on June 6th, almost a month before it reached opposition on June 27th. At the time, the ringed gas giant was approximately 1.4 billion km (870 million mi) from Earth. Hubble was able to capture the planet’s magnificent ring system at a time when it was at its maximum tilt to Earth, which allowed for a spectacular view of the rings and the gaps between them.
Hubble’s new image of Saturn also managed to capture the hexagonal storm around the gas giant’s north pole. This stable and persistent jet stream was first observed by the Voyager 1 probe during its flyby of Saturn in 1981, and has been a mystery to astronomers ever since. On top of that, the new image also features six of Saturn’s 62 known moons – Dione, Enceladus, Tethys, Janus, Epimetheus, and Mimas.
Hubble’s new image of Mars was captured on July 18th, 13 days before it reached its closest approach to Earth. This year will see the Red Planet get as close as 57.6 million km from Earth, which is the closest approach made since 2003. On that occasion, Mars was just 55,757,930 km (34,647,420 mi) from Earth, which was the closest the planet had been to Earth in almost 60,000 years!
Mars is at opposition to Earth every two years, so Hubble has had many opportunities to capture detailed images of the planet’s surface. However, this new image is different in that it is dominated by a gigantic sandstorm that is currently encompassing the entire planet. This storm has been raging since May of 2018 and developed into a planet-wide dust storm within several weeks.
Dust storms are a common occurrence on Mars. They take place every year, usually stay contained to a local area, and normally last only about a few weeks. Larger dust storms that can grow to cover the entire planet are a rarer event, and can typically last for weeks or even months. These tend to happen during the spring and summer months in the southern hemisphere, which coincides with Mars being closer to the Sun in its elliptical orbit.
Due to increased temperatures, dust particles are lifted higher into the atmosphere, creating more wind. The resulting wind kicks up yet more dust, creating a feedback loop that NASA scientists are still trying to understand. While spacecraft orbiting Mars and rovers and lander can study storm behavior at lower altitudes or from the surface, Hubble observations allow astronomers to study changes in the higher atmosphere.
The combined observations will help planetary scientists to better understanding how these global storms arise. Despite the obscuring dust storm, Hubble still managed to capture several important Martian surface features like the polar ice caps, Terra Meridiani, the Schiaparelli Crater, and Hellas Basin – even though all of them appear slightly blurred in the image.
The Hellas Basin – an impact basin that measures 2200 km (1367 mi) across and is nearly 8 km (4.97 mi) deep – is visible at the lower right and appears as a large and bright oval area. The orange area in the upper center of the image is Arabia Terra, a large upland area in northern Mars. This region is characterized by many impact craters and heavy erosion, which indicates that it could be among the oldest terrains on the planet.
South of Arabia Terra are the dark streaks known as Sinus Sabaeus and Sinus Meridiani, features that stretch from east to west along the equator and are made of up dark bedrock and sand deposits from ancient lava flows. Because it was autumn in the northern hemisphere when the image was taken, it is covered in a bright blanket of clouds – and clouds can also be seen above the northern and southern polar ice caps. Last, but not least, Mars’ two small moons – Phobos and Deimos – appear in the lower half of the image.
Comparing these new images of Mars and Saturn with older data gathered by Hubble, other telescopes and the many probes that have taken images of them over the years will allow astronomers to study how cloud patterns and large-scale structures on the Solar planets change over time. These latest images also show that even after almost three decades of being in operation, Hubble is still able to pull its weight!
And be sure to enjoy this video about the images acquired by Hubble, courtesy of Hubblecast:
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Welcome back to our planetary weather series! Next up, we take a look at the ringed-beauty, Saturn!
Saturn is famous for many things. Aside from its ring system, which are the most visible and beautiful of any gas giant, it is also known for its extensive system of moons (the second largest in the Solar System behind Jupiter). And then there its banded appearance and gold color, which are the result of its peculiar composition and persistent weather patterns.
Much like Jupiter, Saturn’s weather systems are known for being particularly extreme, giving rise to features that can be seen from great distances. It’s high winds periodically create massive oval-shaped storms, jet streams, hurricanes, and hexagonal wave patterns that are visible in both the northern and southern polar regions.
The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The gas giant is also known to contain heavier elements, though the proportions of these relative to hydrogen and helium is not known. It is assumed that they would match the primordial abundance from the formation of the Solar System.
Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide (NH4SH) or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion.
Saturn’s atmosphere exhibits a banded pattern similar to Jupiter’s, but Saturn’s bands are much fainter and wider near the equator. As with Jupiter’s cloud layers, they are divided into the upper and lower layers, which vary in composition based on depth and pressure. In the upper cloud layers, with temperatures in range of 100–160 K and pressures between 0.5–2 bar, the clouds consist of ammonia ice.
The presence of hydrogen gas results in clouds of deep red. However, these are obscured by clouds of ammonia, which are closer to the outer edge of the atmosphere and cover the entire planet. The exposure of this ammonia to the Sun’s ultraviolet radiation causes it to appear white. Combined with its deeper red clouds, this results in the planet having a pale gold color.
Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185–270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 290–235 K. Finally, the lower layers, where pressures are between 10–20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in an aqueous solution.
Great White Spot:
On occasion, Saturn’s atmosphere exhibits long-lived ovals, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval). This unique but short-lived phenomenon occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice.
These spots can be several thousands of kilometers wide, and have been observed in 1876, 1903, 1933, 1960, and 1990. Since 2010, a large band of white clouds called the Northern Electrostatic Disturbance have been observed enveloping Saturn, which was spotted by the Cassini space probe. If the periodic nature of these storms is maintained, another one will occur in about 2020.
The winds on Saturn are the second fastest among the Solar System’s planets, after Neptune’s. This is due in part to Saturn’s high rotational velocity – which is 9.87 km/s (6.13 mi/s), which works out to 35,500 km/h (22,058.7 mi/h). At this rate, it only takes the planet 10 hours 33 minutes to rotate once on its axis. However, due to it being a gas giant, there is a difference between the rotation of its atmosphere and its core.
Data obtained by the Voyager 1 and 2 missions indicated peak easterly winds of 500 m/s (1800 km/h). Saturn’s northern and southern poles have also shown evidence of stormy weather. At the north pole, this takes the form of a hexagonal wave pattern, whereas the south shows evidence of a massive jet stream.
The persisting hexagonal wave pattern around the north pole was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km (8,600 mi) long (which is longer than the diameter of the Earth) and the structure rotates with a period of 10h 39m 24s, which is assumed to be equal to the period of rotation of Saturn’s interior.
The south pole vortex, meanwhile, was first observed using the Hubble Space Telescope. These images indicated the presence of a jet stream, but not a hexagonal standing wave. These storms are estimated to be generating winds of 550 km/h, are comparable in size to Earth, and believed to have been going on for billions of years.
In 2006, the Cassini space probe observed a hurricane-like storm that had a clearly defined eye. Such storms had not been observed on any planet other than Earth – even on Jupiter. This storm appeared to be caused by heat that was generated in the depths of the warm interior of Saturn, which then escaped to the upper atmosphere and escaped the planet.
Saturn has also been noted for its “string of pearls” feature, which was captured by Cassini’s visual and infrared mapping spectrometer in 2006. This feature, which appeared in it’s northern latitudes (and has not been seen on any other gas giant) is a series of cloud clearings spaced at regular intervals that show how Saturn’s atmosphere is lit by its own internal, thermal glow.
So how is the weather on Saturn? Pretty violent and stormy! And not surprising given the planet’s mass, composition, powerful gravity, and rapid rotation. Makes you feel happy we live on Earth, where the Earth is (comparatively speaking) pretty calm and boring!
Welcome back to our series on Colonizing the Solar System! Today, we take a look at the largest of Saturn’s Moons – Titan, Rhea, Iapetus, Dione, Tethys, Enceladus, and Mimas.
From the 17th century onward, astronomers made some profound discoveries around the planet Saturn, which they believed was the most distant planet of the Solar System at the time. Christiaan Huygens and Giovanni Domenico Cassini were the first, spotting the largest moons of Saturn – Titan, Tethys, Dione, Rhea and Iapetus. More discoveries followed; and today, what we recognized as the Saturn system includes 62 confirmed satellites.
What we know of this system has grown considerably in recent decades, thanks to missions like Voyager and Cassini. And with this knowledge has come multiple proposals that claim how Saturn’s moons should someday be colonized. In addition to boasting the only body other than Earth to have a dense, nitrogen-rich atmosphere, there are also abundant resources in this system that could be harnessed.
Much like the idea of colonizing the Moon, Mars, the moons of Jupiter, and other bodies in the Solar System, the idea of establishing colonies on Saturn’s moons has been explored extensively in science fiction. At the same time, scientific proposals have been made that emphasize how colonies would benefit humanity, allowing us to mount missions deeper into space and ushering in an age of abundance!
Examples in Fiction:
The colonization of Saturn has been a recurring theme in science fiction over the decades. For example, in Arthur C. Clarke’s 1976 novel Imperial Earth, Titan is home to a human colony of 250,000 people. The colony plays a vital role in commerce, where hydrogen is taken from the atmosphere of Saturn and used as fuel for interplanetary travel.
In Piers Anthony’s Bio of a Space Tyrant series (1983-2001), Saturn’s moons have been colonized by various nations in a post-diaspora era. In this story, Titan has been colonized by the Japanese, whereas Saturn has been colonized by the Russians, Chinese, and other former Asian nations.
In the novel Titan (1997) by Stephen Baxter, the plot centers on a NASA mission to Titan which must struggle to survive after crash landing on the surface. In the first few chapters of Stanislaw Lem’s Fiasco(1986), a character ends up frozen on the surface of Titan, where they are stuck for several hundred years.
In Kim Stanley Robinson’s Mars Trilogy (1996), nitrogen from Titan is used in the terraforming of Mars. In his novel 2312 (2012), humanity has colonized several of Saturn’s moons, which includes Titan and Iapetus. Several references are made to the “Enceladian biota” in the story as well, which are microscopic alien organisms that some humans ingest because of their assumed medicinal value.
As part of his Grand Tour Series, Ben Bova’s novels Saturn (2003) and Titan (2006) address the colonization of the Cronian system. In these stories, Titan is being explored by an artificially intelligent rover which mysteriously begins malfunctioning, while a mobile human Space Colony explores the Rings and other moons.
In his book Entering Space: Creating a Spacefaring Civilization (1999), Robert Zubrin advocated colonizing the outer Solar System, a plan which included mining the atmospheres of the outer planets and establishing colonies on their moons. In addition to Uranus and Neptune, Saturn was designated as one of the largest sources of deuterium and helium-3, which could drive the pending fusion economy.
He further identified Saturn as being the most important and most valuable of the three, because of its relative proximity, low radiation, and excellent system of moons. Zubrin claimed that Titan is a prime candidate for colonization because it is the only moon in the Solar System to have a dense atmosphere and is rich in carbon-bearing compounds.
On March 9th, 2006, NASA’s Cassini space probe found possible evidence of liquid water on Enceladus, which was confirmed by NASA in 2014. According to data derived from the probe, this water emerges from jets around Enceladus’ southern pole, and is no more than tens of meters below the surface in certain locations. This would would make collecting water considerably easier than on a moon like Europa, where the ice sheet is several km thick.
Data obtained by Cassini also pointed towards the presence of volatile and organic molecules. And Enceladus also has a higher density than many of Saturn’s moons, which indicates that it has a larger average silicate core. All of these resources would prove very useful for the sake of constructing a colony and providing basic operations.
In October of 2012, Elon Musk unveiled his concept for an Mars Colonial Transporter (MCT), which was central to his long-term goal of colonizing Mars. At the time, Musk stated that the first unmanned flight of the Mars transport spacecraft would take place in 2022, followed by the first manned MCT mission departing in 2024.
In September 2016, during the 2016 International Astronautical Congress, Musk revealed further details of his plan, which included the design for an Interplanetary Transport System (ITS) and estimated costs. This system, which was originally intended to transport settlers to Mars, had evolved in its role to transport human beings to more distant locations in the Solar System – which could include the Jovian and Cronian moons.
Compared to other locations in the Solar System – like the Jovian system – Saturn’s largest moons are exposed to considerably less radiation. For instance, Jupiter’s moons of Io, Ganymede and Europa are all subject to intense radiation from Jupiter’s magnetic field – ranging from 3600 to 8 rems day. This amount of exposure would be fatal (or at least very hazardous) to human beings, requiring that significant countermeasures be in place.
In contrast, Saturn’s radiation belts are significantly weaker than Jupiter’s – with an equatorial field strength of 0.2 gauss (20 microtesla) compared to Jupiter’s 4.28 gauss (428 microtesla). This field extends from about 139,000 km from Saturn’s center out to a distance of about 362,000 km – compared to Jupiter’s, which extends to a distance of about 3 million km.
Of Saturn’s largest moons, Mimas and Enceladus fall within this belt, while Dione, Rhea, Titan, and Iapetus all have orbits that place them from just outside of Saturn’s radiation belts to well beyond it. Titan, for example, orbits Saturn at an average distance (semi-major axis) of 1,221,870 km, putting it safely beyond the reach of the gas giant’s energetic particles. And its thick atmosphere may be enough to shield residents from cosmic rays.
In addition, frozen volatiles and methane harvested from Saturn’s moons could be used for the sake of terraforming other locations in the Solar System. In the case of Mars, nitrogen, ammonia and methane have been suggested as a means of thickening the atmosphere and triggering a greenhouse effect to warm the planet. This would cause water ice and frozen CO² at the poles to sublimate – creating a self-sustaining process of ecological change.
Colonies on Saturn’s moons could also serve as bases for harvesting deuterium and helium-3 from Saturn’s atmosphere. The abundant sources of water ice on these moons could also be used to make rocket fuel, thus serving as stopover and refueling points. In this way, a colonizing the Saturn system could fuel Earth’s economy, and the facilitate exploration deeper into the outer Solar System.
Naturally, there are numerous challenges to colonizing Saturn’s moons. These include the distance involved, the necessary resources and infrastructure, and the natural hazards colonies on these moons would have to deal with. For starters, while Saturn may be abundant in resources and closer to Earth than either Uranus or Neptune, it is still very far.
On average, Saturn is approximately 1,429 billion km away from Earth; or ~8.5 AU, the equivalent of eight and a half times the average distance between the Earth and the Sun. To put that in perspective, it took the Voyager 1 probe roughly thirty-eight months to reach the Saturn system from Earth. For crewed spacecraft, carrying colonists and all the equipment needed to colonize the surface, it would take considerably longer to get there.
These ships, in order to avoid being overly large and expensive, would need to rely on cryogenics or hibernation-related technology in order to save room on storage and accommodations. While this sort of technology is being investigated for crewed missions to Mars, it is still very much in the research and development phase.
Any vessels involved in the colonization efforts, or used to ship resources to and from the Cronian system, would also need to have advanced propulsion systems to ensure that they could make the trips in a realistic amount of time. Given the distances involved, this would likely require rockets that used nuclear-thermal propulsion, or something even more advanced (like anti-matter rockets).
And while the former is technically feasible, no such propulsion systems have been built just yet. Anything more advanced would require many more years of research and development, and a major commitment in resources. All of this, in turn, raises the crucial issue of infrastructure.
Basically, any fleet operating between Earth and Saturn would require a network of bases between here and there to keep them supplied and fueled. So really, any plans to colonize Saturn’s moons would have to wait upon the creation of permanent bases on the Moon, Mars, the Asteroid Belt, and most likely the Jovian moons. This process would be punitively expensive by current standards and (again) would require a fleet of ships with advanced drive systems.
And while radiation is not a major threat in the Cronian system (unlike around Jupiter), the moons have been subject to a great deal of impacts over the course of their history. As a result, any settlements built on the surface would likely need additional protection in orbit, like a string of defensive satellites that could redirect comets and asteroids before they reached orbit.
Given its abundant resources, and the opportunities it would present for exploring deeper into the Solar System (and maybe even beyond), Saturn and its system of moons is nothing short of a major prize. On top of that, the prospect of colonizing there is a lot more appealing than other locations that come with greater hazards (i.e. Jupiter’s moons).
However, such an effort would be daunting and would require a massive multi-generational commitment. And any such effort would most likely have to wait upon the construction of colonies and/or bases in locations closer to Earth first – such as on the Moon, Mars, the Asteroid Belt, and around Jupiter. But we can certainly hold out hope for the long run, can’t we?
Cassini was launched in 1997 and reached Saturn in 2004. It will end its mission by plunging into the gas giant. But before then, it will dive through Saturn’s rings a total of 20 times.
The first dive through the rings was just completed, and represents the beginning of Cassini’s final mission phase. On December 4th at 5:09 PST the 2,150 kg, plutonium-powered probe, crossed through a faint and dusty ring created by the moons Janus and Epimetheus. This brought it to within 11,000 km of Saturn’s F-ring.
Though the end of a mission might seem sad, people behind the mission are excited about this final phase, a series of close encounters with the most iconic structures in our Solar System: Saturn’s glorious rings.
“This is a remarkable time in what’s already been a thrilling journey.” – Linda Spilker, NASA/JPL
“It’s taken years of planning, but now that we’re finally here, the whole Cassini team is excited to begin studying the data that come from these ring-grazing orbits,” said Linda Spilker, Cassini project scientist at JPL. “This is a remarkable time in what’s already been a thrilling journey.”
Even casual followers of space news have enjoyed the steady stream of eye candy from Cassini. But this first orbit through Saturn’s rings is more about science than pictures. The probe’s cameras captured images 2 days before crossing through the plane of the rings, but not during the closest approach. In future ring-grazing orbits, Cassini will give us some of the best views yet of Saturn’s outer rings and some of the small moons that reside there.
Cassini is about more than just beautiful images though. It’s a vital link in a series of missions that have opened up our understanding of the Solar System we inhabit. Here are some of Cassini’s important discoveries:
The Cassini mission discovered 7 new moons orbiting Saturn. Methone, Pallene and Polydeuces were all discovered in 2004. Daphnis, Anthe, and Aegaeon were discovered between 2005 and 2009. The final moon is currently named S/2009 S 1.
In 2014, NASA reported that yet another new moon may be forming in Saturn’s A ring.
Huygens lands on Titan
The Huygens lander detached from the Cassini orbiter on Christmas Day 2004. It landed on the frigid surface of Saturn’s moon Titan after a 2 1/2 hour descent. The lander transmitted 350 pictures of Titan’s descent to the surface. An unfortunate software error caused the loss of another 350 pictures.
Cassini performed several flybys of the moon Enceladus. The first was in 2005, and the last one was in 2015. The discovery of ice-plumes and a salty liquid ocean were huge for the mission. The presence of liquid water on Enceladus makes it one of the most likely places for microbial life to exist in our Solar System.
Each of Cassini’s final ring-grazing orbits will last one week. Cassini’s final orbit will bring it close to Saturn’s moon Titan. That encounter will change Cassini’s path. Cassini will leap over the rings and make the first of 22 plunges through the gap between Saturn and its rings.
In September 2017, the Cassini probe will finally reach the end of its epic mission. In order to prevent any possible contamination of Saturn’s moons, the probe will make one last glorious plunge into Saturn’s atmosphere, transmitting data until it is destroyed.
During the Scientific Revolution, which took place between the 15th and 18th centuries, numerous inventions and discoveries were made that forever changed the way humanity viewed the Universe. And while this explosion in learning owed its existence to countless individuals, a few stand out as being especially worthy of praise and remembrance.
One such individual is Gionvanni Domenico Cassini, also known by his French name Jean-Dominique Cassini. An Italian astronomer, engineer, and astrologer, Cassini made many valuable contributions to modern science. However, it was his discovery of the gaps in Saturn’s rings and four of its largest moons for which he is most remembered, and the reason why the Cassini spacecraft bears his name.
Early Life and Education:
Giovanni Domenico Cassini was born on June 8th, 1625, in the small town of Perinaldo (near Nice, France) to Jacopo Cassini and Julia Crovesi. Educating by Jesuit scientists, he showed an aptitude for mathematics and astronomy from an early age. In 1648, he accepted a position at the observatory at Panzano, near Bologna, where he was employed by a rich amateur astronomer named Marquis Cornelio Malvasia.
During his time at the Panzano Observatory, Cassini was able to complete his education and went on to become the principal chair of astronomy at the University of Bologna by 1650. While there, he made several scientific contributions that would have a lasting mark.
This included the calculation of an important meridian line, which runs along the left aisle of the San Petronio Basilica in Bologna. At 66.8 meters (219 ft) in length, it is one of the largest astronomical instruments in the worl and allowed for measurements that were (at the time) uniquely precise. This meridian also helped to settle the debate about whether or not the Universe was geocentric or heliocentric.
During his time in Italy, Cassini determined the obliquity of the Earth’s ecliptic – aka. it’s axial tilt, which he calculated to be 23° and 29′ at the time. He also studied the effects of refraction and the Solar parallax, worked on planetary theory, and observed the comets of 1664 and 1668.
In recognition of his engineering skills, Pope Clement IX employed Cassini with regard to fortifications, river management and flooding along the Po River in northern Italy. In 1663, Cassini was named superintendent of fortifications and oversaw the fortifying of Urbino. And in 1665, he was named the inspector for the town of Perugia in central Italy.
In 1669, Cassini received an invitation by Louis XIV of France to move to Paris and help establish the Paris Observatory. Upon his arrival, he joined the newly-founded Academie Royale des Sciences (Royal Academy of Sciences), and became the first director of the Paris Observatory, which opened in 1671. He would remain the director of the observatory until his death in 1712.
In 1673, Cassini obtained his French citizenship and in the following year, he married Geneviève de Laistre, the daughter of the lieutenant general of the Comte de Clermont. During his time in France, Cassini spent the majority of his time dedicated to astronomical studies. Using a series of very long air telescopes, he made several discoveries and collaborated with Christiaan Huygens in many projects.
In the 1670s, Cassini began using the triangulation method to create a topographic map of France. It would not be completed until after his death (1789 or 1793), when it was published under the name Carte de Cassini. In addition to being the first topographical map of France, it was the first map to accurately measure longitude and latitude, and showed that the nation was smaller than previously thought.
In 1672, Cassini and his colleague Jean Richer made simultaneous observations of Mars (Cassini from Paris and Richer from French Guiana) and determined its distance to Earth through parallax. This enabled him to refine the dimensions of the Solar System and determine the value of the Astronomical Unit (AU) to within 7% accuracy. He and English astronomer Robert Hooke share credit for the discovery of the Great Red Spot on Jupiter (ca. 1665).
In 1683, Cassini presented an explanation for “zodiacal light” – the faint glow that extends away from the Sun in the ecliptic plane of the sky – which he correctly assumed to be caused by a cloud of small particles surrounding the Sun. He also viewed eight more comets before his death, which appeared in the night sky in 1672, 1677, 1698, 1699, 1702 (two), 1706 and 1707.
In ca. 1690, Cassini was the first to observe differential rotation within Jupiter’s atmosphere. He created improved tables for the positions of Jupiter’s Galilean moons, and discovered the periodic delays between the occultations of Jupiter’s moons and the times calculated. This would be used by Ole Roemer, his colleague at the Paris Observatory, to calculate the velocity of light in 1675.
In 1683, Cassini began the measurement of the arc of the meridian (longitude line) through Paris. From the results, he concluded that Earth is somewhat elongated. While in fact, the Earth is flattened at the poles, the revelation that Earth is not a perfect sphere was groundbreaking.
Cassini also observed and published his observations about the surface markings on Mars, which had been previously observed by Huygens but not published. He also determined the rotation periods of Mars and Jupiter, and his observations of the Moon led to the Cassini Laws, which provide a compact description of the motion of the Moon. These laws state that:
The Moon takes the same amount of time to rotate uniformly about its own axis asit takes to revolve around the Earth. As a consequence, the same face is always pointed towards Earth.
The Moon’s equator is tilted at a constant angle (about 1°32′ of arc) to the plane of the Earth’s orbit around the Sun (i.e. the ecliptic)
The point where the lunar orbit passes from south to north on the ecliptic (aka. the ascending node of the lunar orbit) always coincides with the point where the lunar equator passes from north to south on the ecliptic (the descending node of the lunar equator).
Thanks to his leadership, Giovanni Cassini was the first of four successive Paris Observatory directors that bore his name. This would include his son, Jaques Cassini (Cassini II, 1677-1756); his grandson César François Cassini (Cassini III, 1714-84); and his great grandson, Jean Dominique Cassini (Cassini IV, 1748-1845).
Observations of Saturn:
During his time in France, Cassini also made his famous discoveries of many of Saturn’s moons – Iapetus in 1671, Rhea in 167, and Tethys and Dione in 1684. Cassini named these moons Sidera Lodoicea (the stars of Louis), and correctly explained the anomalous variations in brightness to the presence of dark material on one hemisphere (now called Cassini Regio in his honor).
In 1675, Cassini discovered that Saturn’s rings are separated into two parts by a gap, which is now called the “Cassini Division” in his honor. He also theorized that the rings were composed of countless small particles, which was proven to be correct.
Death and Legacy:
After dedicating his life to astronomy and the Paris Observatory, Cassini went blind in 1711 and then died on September 14th, 1712, in Paris. And although he resisted many new theories and ideas that were proposed during his lifetime, his discoveries and contributions place him among the most important astronomers of the 17th and 18th centuries.
As a traditionalist, Cassini initially held the Earth to be the center of the Solar System. In time, he would come to accept the Solar Theory of Nicolaus Copernicus within limits, to the point that he accepted the model proposed by Tycho Brahe. However, he rejected the theory of Johannes Kepler that planets travel in ellipses and proposed hat their paths were certain curved ovals (i.e. Cassinians, or Ovals of Cassini)
Cassini also rejected Newton’s Theory of Gravity, after measurements he conducted which (wrongly) suggested that the Earth was elongated at its poles. After forty years of controversy, Newton’s theory was adopted after the measurements of the French Geodesic Mission (1736-1744) and the Lapponian Expedition in 1737, which showed that the Earth is actually flattened at the poles.
For his lifetime of work, Cassini has been honored in many ways by the astronomical community. Because of his observations of the Moon and Mars, features on their respective surfaces were named after him. Both the Moon and Mars have their own Cassini Crater, and Cassini Regio on Saturn’s moon Iapetus also bears his name.
Then there is Asteroid (24101) Cassini, which was discovered by C.W. Juels at in 1999 using the Fountain Hills Observatory telescope. Most recently, there was the joint NASA-ESA Cassini-Huygens missions which recently finished its mission to study Saturn and its moons. This robotic orbiter and lander mission was named in honor of the two astronomers who were chiefly responsible for discovering Saturn system of moons.
In the end, Cassini’s passion for astronomy and his contributions to the sciences have ensured him a lasting place in the annals of history. In any discussion of the Scientific Revolution and of the influential thinkers who made it happen, his name appears alongside such luminaries as Copernicus, Galileo, and Newton.
Back in 2007, astronomers observed a series of unusual eclipses coming from a star 420 light years from Earth. In 2012, a team from Japan and the Netherlands reasoned that this phenomena was due to the presence of a large exoplanet – designated J1407b – with a massive ring system orbiting the star. Since then, several surprising finds have been made.
For example, in 2015, the same team concluded that the ring system is one-hundred times larger and heavier than Saturn’s (and may be similarly sculpted by exomoons). And in their most recent study, they have shown that these giant rings may last for over 100,000 years, assuming they have a rare and unusual orbit around their planet.
In their previous work, Rieder and Kenworth determined that the ring system around J1407b consisted about 37 rings that extend to a distance of 0.6 AU (90 million km) from the planet. They also estimated that these rings are 100 times as massive as our Moon – 7342 trillion trillion metric tons. What’s more, while J1407b’s existence is yet to be confirmed, they were able to rule out the possibility of it having a circular orbit around the star.
As a result, there were doubts that such a ring system could exist. Given the fact that the planet periodically gets closer to its star, the ring system would experience gravitational disruption. Therefore, Steven Rieder (of the RIKEN institute in Japan) and Matthew Kenworth (of Leiden University in the Netherlands) set out to assess how long such a ring system could remain stable for.
In other words, the ring system that they hypothesized back in 2012 could endure for 110,000 years. As Rieder (the lead author on the paper) explained in a statement, the results were surprising, but happened to fit the facts:
“The system is only stable when the rings rotate opposite to how the planet orbits the star. It might be far-fetched: massive rings that rotate in opposite direction, but we now have calculated that a ‘normal’ ring system cannot survive.”
How such a ring system could have come about is a mystery, as retrograde ring systems are quite uncommon. But Rieder and Kenworth have stated that they think it might be the result of a catastrophic event – such as a massive collision – that caused the rings (or the planet) to change the direction of their rotation.
Their results also indicated that a retrograde ring system would allow for eclipses, like the one that was observed in 2007. While there was some chance of these being caused by another object, the results suggested otherwise. “The chance of that is minimal,” said Rieder. “Also, the velocity measured with previous observations may not be right, but that would be very strange, because those measurements are very accurate.”
In the future, Rieder and Kenswoth hope to investigate the mysteries of this ring formation more closely. This will include how it could have formed in the first place, and how it has evolved over time. Their study has been accepted for publication in the journal Astronomy & Astrophysics and be viewed online at arXiv.
Fans of astronomy are no doubt familiar with the work of Kevin Gill. In the past, he has brought us visualizations of what the Earth would look like if it had a system of rings, what a “Living Mars” would look like – i.e. if it was covered in oceans and lush vegetation – and an artistic rendition of the places we’ve been in our Solar System.
In his latest work, which once again merges the artistic and astronomical, Gill has created a series of images that show Saturn’s moon of Daphnis, and the effect it has on Saturn’s Keeler Gap. Through these images – titled “Daphnis in the Keeler Gap” and “Daphnis and Waves Along the Keeler Gap” – we get to see an artistic rendition of how one of Saturn’s moons interacts with its beautiful ring system.
As one of Saturn’s smallest moons – measuring just 8 km (~5 mi) in diameter – the existence of Daphnis had been previously inferred by astronomers based on the gravitational ripples that were observed on the outer edge of the Keeler Gap. This 42 km (26 mi) wide gap, which lies in Saturn’s A Ring and is approximately 250 km from the its outer edge, is kept clear by Daphnis’ orbit around the planet.
In 2005, the Cassini space probe finally confirmed the existence of this tiny moon. After analyzing images provided by the probe, the Cassini Imaging Science Team concluded that Daphnis’ path and orbit induce a wavy pattern in the edge of the gap. These waves reach a distance of 1.5 km (0.93 mi) above the ring, due to Daphnis being slightly inclined to the ring’s plane.
However, all the images taken by Cassini showed this effect from a great distance. In order to help people appreciate what it must look like close-up, Gill decided to create the visuals you see here. From his images, the passage of Daphnis is shown to give the A Ring a rippled, wavy appearance. In addition, one can see how Daphnis is inclined slightly above the plane of the A Ring, causing the waves to reach upward.
As Kevin Gill told Universe Today via email, these images were the largely inspired by the most recent images of Saturn’s rings that were provided by Cassini space probe, which returned to an equatorial orbit a few months ago after spending two years in high-inclination orbits:
“These are inspired by a general interest in the moon-ring interactions and some recent Cassini views of Daphnis on the 15th (shown below). This is one of the many aspects of the Saturn system that I imagine would be absolutely breathtaking if you could see it in person and ended up being rather simple to model in Maya.”