Messier 10 (M10) – The NGC 6254 Globular Cluster

Welcome to another installment of Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by taking a look at Messier Object 10.

In the 18th century, French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky while searching for comets. Hoping to ensure that other astronomers did not make the same mistake, he began compiling a list of 1oo  of them. This list came to be known as the Messier Catalog, and would have far-reaching consequences.

In addition to being as a major milestone in the history of astronomy and the study of Deep Sky Objects. One of these objects is known as Messier 10 (aka. NGC 6254), a globular cluster that is located in the equatorial constellation of Ophiuchus. Of the many globular clusters that appear in this constellation (seven of which were cataloged by Messier himself) M10 is the brightest, and can be spotted with little more than a pair of binoculars. Continue reading “Messier 10 (M10) – The NGC 6254 Globular Cluster”

Uranus’ “Sprightly” Moon Ariel

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:

 

Saturn’s Moon Dione

Ringside With Dione

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.

Saturn’s “Yin-Yang” Moon Iapetus

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.

How Many Moons Does Uranus Have?

Uranus and Moons

In the outer Solar System, there are many worlds that are so large and impressive to behold that they will probably take your breath away. Not only are these gas/ice giants magnificent to look at, they are also staggering in size, have their own system a rings, and many, many moons. Typically, when one speaks of gas (and/or ice) giants and their moons, one tends to think about Jupiter (which has the most, at 67 and counting!).

But have you ever wondered how many moons Uranus has? Like all of the giant planets, it’s got rather a lot! In fact, astronomers can now account for 27 moons that are described as “Uranian”. Just like the other gas and ice giants, these moons are motley bunch that tell us much about the history of the Solar System. And, just like Jupiter and Saturn, the process of discovering these moons has been long and involved on multiple astronomers.

Continue reading “How Many Moons Does Uranus Have?”