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
Continuing with our “Definitive Guide to Terraforming“, Universe Today is happy to present our guide to terraforming Saturn’s Moons. Beyond the inner Solar System and the Jovian Moons, Saturn has numerous satellites that could be transformed. But should they be?
Around the distant gas giant Saturn lies a system of rings and moons that is unrivaled in terms of beauty. Within this system, there is also enough resources that if humanity were to harness them – i.e. if the issues of transport and infrastructure could be addressed – we would be living in an age a post-scarcity. But on top of that, many of these moons might even be suited to terraforming, where they would be transformed to accommodate human settlers.
As with the case for terraforming Jupiter’s moons, or the terrestrial planets of Mars and Venus, doing so presents many advantages and challenges. At the same time, it presents many moral and ethical dilemmas. And between all of that, terraforming Saturn’s moons would require a massive commitment in time, energy and resources, not to mention reliance on some advanced technologies (some of which haven’t been invented yet).
What would Earth look like if it had a ring system like Saturn's. Credit: Kevin Gill/Flickr
Saturn’s Rings are amazing to behold. Since they were first observed by Galileo in 1610, they have been the subject of endless scientific interest and popular fascination. Composed of billions of particles of dust and ice, these rings span a distance of about 282,000 km (175,000 miles) – which is three quarters of the distance between the Earth and its Moon – and hold roughly 30 quintillion kilograms (that’s 3.0. x 1018 kg) worth of matter.
All of the Solar System’s gas giants, from Jupiter to Neptune, have their own ring system – albeit less visible and picturesque ones. Sadly, none of the terrestrial planets (i.e. Mercury, Venus, Earth and Mars) have such a system. But just what would it look like if Earth did? Putting aside the physical requirements that it would take for a ring system to exist, what would it be like to look up from Earth and see beautiful rings reaching overhead?
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.
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
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
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, 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.
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.
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.
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 (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, 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 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, 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.
This portrait looking down on Saturn and its rings was created from images obtained by NASA's Cassini spacecraft on Oct. 10, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
The farthest planet from the Sun that can be observed with the naked eye, the existence of Saturn has been known for thousands of years. And much like all celestial bodies that can be observed with the aid of instruments – i.e. Mercury, Venus, Mars, Jupiter and the Moon – it has played an important role in the mythology and astrological systems of many cultures.
Saturn is one of the four gas giants in our Solar System, also known as the Jovian planets, and the sixth planet from the Sun. It’s ring system, which is it famous for, is also the most observable – consisting of nine continuous main rings and three discontinuous arcs.
Saturn’s Size, Mass and Orbit:
With a polar radius of 54364±10 km and an equatorial radius of 60268±4 km, Saturn has a mean radius of 58232±6 km, which is approximately 9.13 Earth radii. At 5.6846×1026 kg, and a surface area, at 4.27×1010 km2, it is roughly 95.15 as massive as Earth and 83.703 times it’s size. However, since it is a gas giant, it has significantly greater volume – 8.2713×1014 km3, which is equivalent to 763.59 Earths.
The sixth most distant planet, Saturn orbits the Sun at an average distance of 9 AU (1.4 billion km; 869.9 million miles). Due to its slight eccentricity, the perihelion and aphelion distances are 9.022 (1,353.6 million km; 841.3 million mi) and 10.053 AU (1,513,325,783 km; 940.13 million mi), on average respectively.
Saturn Compared to Earth. Image credit: NASA/JPL
With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days to complete a single revolution of the Sun. In other words, a single Cronian year is the equivalent of about 29.5 Earth years. However, as with Jupiter, Saturn’s visible features rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions.
The latest estimate of Saturn’s rotation as a whole are based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes. Saturn’s rotation causes it to have the shape of an oblate spheroid; flattened at the poles but bulging at the equator.
Saturn’s Composition:
As a gas giant, Saturn is predominantly composed of hydrogen and helium gas. With a mean density of 0.687 g/cm3, Saturn is the only planet in the Solar System that is less dense than water; which means that it lacks a definite surface, but is believed to have a solid core. This is due to the fact that Saturn’s temperature, pressure, and density all rise steadily toward the core.
Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles. This core is similar in composition to the Earth, but more dense due to the presence of metallic hydrogen, which as a result of the extreme pressure.
Diagram of Saturn’s interior. Credit: Kelvinsong/Wikipedia Commons
Saturn has a hot interior, reaching 11,700 °C at its core, and it radiates 2.5 times more energy into space than it receives from the Sun. This is due in part to the Kelvin-Helmholtz mechanism of slow gravitational compression, but may also be attributable to droplets of helium rising from deep in Saturn’s interior out to the lower-density hydrogen. As these droplets rise, the process releases heat by friction and leaves Saturn’s outer layers depleted of helium. These descending droplets may have accumulated into a helium shell surrounding the core.
In 2004, French astronomers Didier Saumon and Tristan Guillot estimated that the core must 9-22 times the mass of Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude. The outermost layer spans 1,000 km and consists of gas.
Saturn’s Atmosphere:
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.
NASA’s Cassini spacecraft captures a composite near-true-color view of the huge storm churning through the atmosphere in Saturn’s northern hemisphere. Image credit: NASA/JPL-Caltech/SSI
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.
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.
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 huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI
The winds on Saturn are the second fastest among the Solar System’s planets, after Neptune’s. Voyager data indicate 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.
Saturn’s Moons:
Saturn has at least 150 moons and moonlets, but only 53 of these moons have been given official names. Of these moons, 34 are less than 10 km in diameter and another 14 are between 10 and 50 km in diameter. However, some of its inner and outer moons are rather large, ranging from 250 to over 5000 km.
Moons of Saturn (from left to right): Mimas, Enceladus, Tethys, Dione, Rhea, Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
Traditionally, most of Saturn’s moons have been named after the Titans of Greek mythology, and are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.
The Inner Large Moons, which orbit within the E Ring (see below), includes the larger satellites Mimas, Enceladus, Tethys, and Dione. These moons are all composed primarily of water ice, and are believed to be differentiated into a rocky core and an icy mantle and crust. With a diameter of 396 km and a mass of 0.4×1020 kg, Mimas is the smallest and least massive of these moons. It is ovoid in shape and orbits Saturn at a distance of 185,539 km with an orbital period of 0.9 days.
Enceladus, meanwhile, has a diameter of 504 km, a mass of 1.1×1020 km and is spherical in shape. It orbits Saturn at a distance of 237,948 km and takes 1.4 days to complete a single orbit. Though it is one of the smaller spherical moons, it is the only Cronian moon that is endogenously active – and one of the smallest known bodies in the Solar System that is geologically active. This results in features like the famous “tiger stripes” – a series of continuous, ridged, slightly curved and roughly parallel faults within the moon’s southern polar latitudes.
Large geysers have also been observed in the southern polar region that periodically release plumes of water ice, gas and dust which replenish Saturn’s E ring. These jets are one of several indications that Enceladus has liquid water beneath it’s icy crust, where geothermal processes release enough heat to maintain a warm water ocean closer to its core. With a geometrical albedo of more than 140%, Enceladus is one of the brightest known objects in the Solar System.
Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Image Credit: NASA/JPL
At 1066 km in diameter, Tethys is the second-largest of Saturn’s inner moons and the 16th-largest moon in the Solar System. The majority of its surface is made up of heavily cratered and hilly terrain and a smaller and smoother plains region. Its most prominent features are the large impact crater of Odysseus, which measures 400 km in diameter, and a vast canyon system named Ithaca Chasma – which is concentric with Odysseus and measures 100 km wide, 3 to 5 km deep and 2,000 km long.
With a diameter and mass of 1,123 km and 11×1020 kg, Dione is the largest inner moon of Saturn. The majority of Dione’s surface is heavily cratered old terrain, with craters that measure up to 250 km in diameter. However, the moon is also covered with an extensive network of troughs and lineaments which indicate that in the past it had global tectonic activity.
The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice and rock. Of these, Rhea is the second largest – measuring 1,527 km in diameter and 23 × 1020 kg in mass – and the ninth largest moon of the Solar System. With an orbital radius of 527,108 km, it is the fifth-most distant of the larger moons, and takes 4.5 days to complete an orbit.
Like other Cronian satellites, Rhea has a rather heavily cratered surface, and a few large fractures on its trailing hemisphere. Rhea also has two very large impact basins on its anti-Saturnian hemisphere – the Tirawa crater (similar to Odysseus on Tethys) and an as-yet unnamed crater – that measure 400 and 500 km across, respectively.
A composite image of Titan’s atmosphere, created using blue, green and red spectral filters to create an enhanced-color view. Image Credit: NASA/JPL/Space Science Institute
At 5150 km in diameter, and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryo-volcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System beside Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.
With an orbital distance of 1,221,870 km, it is the second-farthest large moon from Saturn, and completes a single orbit every 16 days. Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.
Hyperion is Titan’s immediate neighbor. At an average diameter of about 270 km, it is smaller and lighter than Mimas. It is also irregularly shaped and quite odd in composition. Essentially, the moon is an ovoid, tan-colored body with an extremely porous surface (which resembles a sponge). The surface of Hyperion is covered with numerous impact craters, most of which are 2 to 10 km in diameter. It also has a highly unpredictable rotation, with no well-defined poles or equator.
The two sides of Iapetus, which is known as “Saturn’s yin yang moon” because of the contrast in its color composition. Credit: NASA/JPL
At 1,470 km in diameter and 18×1020 kg in mass, Iapetus is the third-largest of Saturn’s large moons. And at a distance of 3,560,820 km from Saturn, it is the most distant of the large moons, and takes 79 days to complete a single orbit. Due to its unusual color and composition – its leading hemisphere is dark and black whereas its trailing hemisphere is much brighter – it is often called the “yin and yang” of Saturn’s moons.
Beyond these larger moons are Saturn’s Irregular Moons. These satellites are small, have large-radii, are inclined, have mostly retrograde orbits, and are believed to have been acquired by Saturn’s gravity. These moons are made up of three basic groups – the Inuit Group, the Gallic Group, and the Norse Group.
The Inuit Group consists of five irregular moons that are all named from Inuit mythology – Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. All have prograde orbits that range from 11.1 to 17.9 million km, and from 7 to 40 km in diameter. They are all similar in appearance (reddish in hue) and have orbital inclinations of between 45 and 50°.
The Gallic group are a group of four prograde outer moons named for characters in Gallic mythology -Albiorix, Bebhionn, Erriapus, and Tarvos. Here too, the moons are similar in appearance and have orbits that range from 16 to 19 million km. Their inclinations are in the 35°-40° range, their eccentricities around 0.53, and they range in size from 6 to 32 km.
Saturns rings and moons, shown to scale. Credit: NASA
Last, there is the Norse group, which consists of 29 retrograde outer moons that take their names from Norse mythology. These satellites range in size from 6 to 18 km, their distances from 12 and 24 million km, their inclinations between 136° and 175°, and their eccentricities between 0.13 and 0.77. This group is also sometimes referred to as the Phoebe group, due to the presence of a single larger moon in the group – which measures 240 km in diameter. The second largest, Ymir, measures 18 km across.
Within the Inner and Outer Large Moons, there are also those belonging to Alkyonide group. These moons – Methone, Anthe, and Pallene – are named after the Alkyonides of Greek mythology, are located between the orbits of Mimas and Enceladus, and are among the smallest moons around Saturn.
Some of the larger moons even have moons of their own, which are known as Trojan moons. For instance, Tethys has two trojans – Telesto and Calypso, while Dione has Helene and Polydeuces.
Saturn’s Ring System:
Saturn’s rings are believed to be very old, perhaps even dating back to the formation of Saturn itself. There are two main theories as to how these rings formed, each of which have variations. One theory is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces.
In version of this theory, the moon was struck by a large comet or asteroid – possible during the Late Heavy Bombardment – that pushed it beneath the Roche Limit. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material from which Saturn formed billions of years ago.
The structure is subdivided into seven smaller ring sets, each of which has a division (or gap) between it and its neighbor. The A and B Rings are the densest part of the Cronian ring system and are 14,600 and 25,500 km in diameter, respectively. They extend to a distance of 92,000 – 117,580 km (B Ring) and 122,170 – 136,775 km (A Ring) from Saturn’s center, and are separated by the 4,700 km wide Cassini Division.
Saturn’s rings. Credit: NASA/JPL/Space Science Institute.
The C Ring, which is separated from the B Ring by the 64 km Maxwell Gap, is approximately 17,500 km in width and extends 74,658 – 92,000 from Saturn’s center. Together with the A and B Rings, they comprise the main rings, which are denser and contain larger particles than the “dusty rings”.
These tenuous rings are called “dusty” due to the small particles that make them up. They include the D Ring, a 7,500 km ring that extends inward to Saturn’s cloud tops (66,900 – 74,510 km from Saturn’s center) and is separated from the C Ring by the 150 km Colombo Gap. On the other end of the system, the G and E Rings are located, which are also “dusty” in composition.
The G Ring is 9000 km in width and extends 166,000 – 175,000 km from Saturn’s center. The E Ring, meanwhile, is the largest single ring section, measuring 300,000 km in width and extending 166,000 to 480,000 km from Saturn’s center. It is here where the majority of Saturn’s moons are located (see above).
The narrow F Ring, which sits on the outer edge of the A Ring, is more difficult to categorize. While some parts of it are very dense, it also contains a great deal of dust-size particles. For this reason, estimates on its width range from 30 to 500 km, and it extends roughly 140,180 km from Saturn’s center.
History of Observing Saturn:
Because it is visible to the naked eye in the night sky, human beings have been observing Saturn for thousands of years. In ancient times, it was considered the most distant of five known the planets, and thus was accorded special meaning in various mythologies. The earliest recorded observations come from the Babylonians, where astronomers systematically observed and recorded its movements through the zodiac.
Roman astrological calendar, from the stone plate of the 3rd—4th centuries CE, Rome. Credit: Museo della civiltà romana
To the ancient Greeks, this outermost planet was named Cronus (Kronos), after the Greek god of agriculture and youngest of the Titans. The Greek scientist Ptolemy made calculations of Saturn’s orbit based on observations of the planet while it was in opposition.The Romans followed in this tradition, identifying it with their equivalent of Cronos (named Saturnus).
In ancient Hebrew, Saturn is called ‘Shabbathai’, whereas in Ottoman Turkish, Urdu and Malay, its name is ‘Zuhal’, which derived is from the original Arabic. In Hindu astrology, there are nine astrological objects known as Navagrahas. Saturn, which is one of them, is known as “Shani”, who judges everyone based on the good and bad deeds performed in life. In ancient China and Japan, the planet was designated as the “earth star” – based on the Five Elements of earth, air, wind, water and fire.
However, the planet was not directly observed until 1610, when Galileo Galilee first discerned the presence of rings. At the time, he mistook them for two moons that were located on either side. It was not until Christiaan Huygens used a telescope with greater magnification that this was corrected. Huygens also discovered Saturn’s moon Titan, and Giovanni Domenico Cassini later discovered the moons of Iapetus, Rhea, Tethys and Dione.
No further discoveries of significance were made again until the 181th and 19th centuries. The first occurred in 1789 when William Herschel discovered the two distant moons of Mimas and Enceladus, and then in 1848 when a British team discovered the irregularly-shaped moon of Hyperion.
Drawing of Saturn by Robert Hook, taken from Philosophical Transactions (1666). Credit: Wikipedia Commons
In 1899 William Henry Pickering discovered Phoebe, noting that it had a highly irregular orbit that did not rotate synchronously with Saturn as the larger moons do. This was the first time any satellite had been found to move about a planet in retrograde orbit. And by 1944, research conducted throughout the early 20th century confirmed that Titan has a thick atmosphere – a feature unique among the Solar System’s moons.
Exploration of Saturn:
By the late 20th century, unmanned spacecraft began to conduct flybys of Saturn, gathering information on its composition, atmosphere, ring structure, and moons. The first flyby was conducted by NASA using the Pioneer 11 robotic space probe, which passed Saturn at a distance of 20,000 km in September of 1979.
Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. The spacecraft also studied Saturn’s rings, revealing the thin F Ring and the fact that dark gaps in the rings are bright when facing towards the Sun, meaning that they contain fine light-scattering material. In addition, Pioneer 11 measured the temperature of Titan.
The next flyby took place in November of 1980 when the Voyager 1 space probe passed through the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites – which included features of various moons that had never before been seen.
These six narrow-angle color images were made from the first ever ‘portrait’ of the solar system taken by Voyager 1 in November 1980. Credit: NASA/JPL
In August 1981, Voyager 2 conducted its flyby and gathered more close-up images of Saturn’s moons, as well as evidence of changes in the atmosphere and the rings. The probes discovered and confirmed several new satellites orbiting near or within the planet’s rings, as well as the small Maxwell Gap and Keeler gap (a 42 km wide gap in the A Ring).
In June of 2004, the Cassini–Huygens space probe entered the Saturn system and conducted a close flyby of Phoebe, sending back high-resolution images and data. By July 1st, 2004, the probe entered orbit around Saturn, and by December, it had completed two flybys of Titan before releasing the Huygens probe. This lander reached the surface and began transmitting data on the atmospheric and surface by by Jan. 14th, 2005. Cassini has since conducted multiple flybys of Titan and other icy satellites.
In 2006, NASA reported that Cassini had found evidence of liquid water reservoirs that erupt in geysers on Saturn’s moon Enceladus. Over 100 geysers have since been identified, which are concentrated around the southern polar region. In May 2011, NASA scientists at an Enceladus Focus Group Conference reported that Enceladus’ interior ocean may be the most likely candidate in the search for extra-terrestrial life.
In addition, Cassini photographs have revealed a previously undiscovered planetary ring, eight new satellites, and evidence of hydrocarbon lakes and seas near Titan’s north pole. The probe was also responsible for sending back high-resolution images of the intense storm activity at Saturn’s northern and southern poles.
Cassini’s primary mission ended in 2008, but the probe’s mission has been extended twice since then – first to September 2010 and again to 2017. In the coming years, NASA hopes to use the probe to study a full period of Saturn’s seasons.
Artist Illustration of the Cassini space probe to Saturn and Titan, a joint NASA, ESA mission. Credit: NASA/JPL
From being a very important part of the astrological systems of many cultures to becoming the subject of ongoing scientific fascination, Saturn continues to occupy a special place in our hearts and minds. Whether it’s Saturn’s fantastically large and beautiful ring system, its many many moons, its tempestuous weather, or its curious composition, this gas giant continues to fascinate and inspire.
In the coming years and decades, additional robotic explorer missions will likely to be sent to investigate Saturn, its rings and its system of moons in greater detail. What we find may constitute some of the most groundbreaking discoveries of all time, and will likely teach us more about the history of our Solar System.
The strangest feature on Iapetus is the equatorial ridge. What could possibly create a feature like this?
To paraphrase the British geneticist J.B.S Haldane, “in my suspicion, the Universe is not only stranger than we suppose, it’s stranger than we can suppose.” The context was life and evolution, but he might as well been talking about Saturn’s moons. Those teeny worlds are some of the strangest places we’ve ever seen.
Titan is a massive moon with an atmosphere thicker than Earth’s. If it wasn’t for the bone crippling cold and unbreathable atmosphere, you could wear a pair of wings and fly around in the Titanic skies.
There’s Enceladus, an icy moon that blasts water out into space through geysers at its southern pole. But the Saturnian moon that fascinates me the most has got to be Iapetus, also known as Saturn’s yin-yang moon.
Here’s a photo captured by Cassini. Check out the bizarre surface features, where half of the moon is icy white and the other is brownish red. Astronomers believe this strange coloration comes from the ice on the warmer side sublimating away, leaving this darker material beneath.
Sure that’s a bit odd, but the strangest feature on Iapetus is the equatorial ridge. This feature measures 1,300 km long and it makes the moon look like a space walnut. Because of the heavy cratering on the ridge, astronomers know that it’s ancient, nearly as old as the moon itself. At 13 kms high, it’s tall enough to keep out the most persnickety white walker or wildling mammoth & giant battalion.
What could possibly create a feature like this?
Astronomers are of a few camps. The first think it formed through convective activity early on in the moon’s history. Saturn pulls Iapetus with its tremendous gravity, and the moon undergoes massive tidal forces. This generates heat in the moon’s interior, and it might have caused it to blob out at the equator.
A ridge that follows the equator of Saturn’s moon Iapetus gives it the appearance of a giant walnut. This image was taken by the Cassini spacecraft. Credit: NASA/JPL/SSI
A second idea is that Iapetus consumed one of Saturn’s rings, billions of years ago. The moon might have slowly wandered through the ring plane, and accreted all the ring material, like snow piling up in front of a plow.
A third is that Iapetus was smashed into by a massive asteroid billions of years ago. This impact caused the moon to fly apart, but then mutual gravity pulled it back together. The force of this recombination squeezed out material at the equator, which then solidified in place.
Alternately, it might be a walnut from a Galactus family Christmas stocking. So which is it?
It turns out that Saturn has two more moons in its system with similar equatorial ridges. Its moon Atlas is just 15 km across, but it’s dominated by an equatorial ridge. It looks like a UFO, and Pan has a similar feature.
Saturn’s relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)
Astronomers know that both of these created their ridges by pulling material out of the rings and piling it up on their surface. This is the mechanism that seems to match what’s going on with Iapetus.
One mystery, is how distantly Iapetus orbits Saturn. There’s no ring that far out, so where did it get the material to consume? Is it possible that Iapetus drifted outward, or had a ring system of its own?
You want puzzles? Iapetus is one of the strangest places in the Solar System, and it would be my candidate for a future orbiter or lander. Let’s explore it closer.
What’s your favorite bizarre object in the Solar System? Tell us in the comments below.
A graphic depicting Earth and Saturn's rings to scale. Credit: John Brady/Astronomy Central.
We Earthlings love to dream, conjure and extrapolate. If you pose a question such as, “What if Earth had Saturn’s rings?” with all the resources available these days someone will not only answer the question but create some beautiful graphics to depict it! Yesterday, we saw this amazing graphic posted on reddit of a to-scale depiction of how Earth would look like with Saturn’s rings, and thanks to those who helped find the original source, the original image was created by John Brady at Astronomy Central. (We apologize… we originally credited the wrong person).
See more great size comparisons of things in our Solar System and Universe at John Brady’s post on Astronomy Central, including a look at how many Earth’s would span across Saturn’s rings.
Saturn's F ring appears distorted in this October 2013 picture from the Cassini spacecraft. The twisting may be because the F ring is crashing repeatedly into a "single small object", NASA stated. Credit: NASA/JPL-Caltech/Space Science Institute
What’s up with this distortion? This picture from the Cassini spacecraft shows some kind of twist happening in the F ring of Saturn. Scientists in fact have seen other strange shapes in this delicate ring, indicating that something is disturbing it from time to time.
“Saturn’s F ring often appears to do things other rings don’t. In this Cassini spacecraft image, a strand of ring appears to separate from the core of the ring as if pulled apart by mysterious forces. Some ring scientists believe that this feature may be due to repeated collisions between the F ring and a single small object,” NASA stated this month.
There’s a debate in the scientific community about where the rings arose in the first place. “It’s been going back and forth for ages and it still goes back and forth. Are they old, or have they been there a long period of time? Are they new? I don’t know what to think, to be quite honest. I’m not being wishy-washy, I just don’t know what to think anymore,” Kevin Grazier, a planetary scientist with the Cassini mission for over 15 years, told Universe Today in December.
While this picture dates from October, you can check out Cassini images as they come in to NASA’s raw image database. Even in unprocessed form, the planet and its rings look beautiful — as you can clearly see in samples below.
The bulk of Saturn looms to the side of this shot of Saturn’s rings taken in February 2014 by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute
The variety of Saturn’s rings is visible in this raw shot from the Cassini spacecraft taken in February 2014. Credit: NASA/JPL/Space Science Institute
Although Saturn’s rings look solid and substantial in images such as this, they are made up of many tiny, icy objects collecting as thin as 6.2 miles (10 kilometers) deep. Image taken by the Cassini spacecraft in February 2014. Credit: NASA/JPL/Space Science Institute
ive images of Saturn's rings, taken by NASA's Cassini spacecraft between 2009 and 2012, show clouds of material ejected from impacts of small objects into the rings. Image Credit: NASA/JPL-Caltech/Space Science Institute/Cornell.
From tell-tale evidence, we know that Earth, our Moon and other bodies in our Solar System are constantly barraged with both small meteoroids and larger asteroids or comets. And sometimes – like in the case of seeing meteors fling across our sky, or flashes on the Moon or Jupiter getting hit by Comet Shoemaker-Levy 9 — we even get to watch as it happens. Now, for the first time the Cassini spacecraft has provided direct evidence of small meteoroids crashing into Saturn’s rings.
Researchers say that studying the impact rate of meteoroids from outside the Saturnian system helps scientists understand how different planet systems in our solar system formed.
Saturn’s rings act as very effective detectors of many kinds of surrounding phenomena, including the interior structure of the planet and the orbits of its moons. For example, a subtle but extensive corrugation that ripples 12,000 miles (19,000 kilometers) across the innermost rings tells of a very large meteoroid impact in 1983.
“These new results imply the current-day impact rates for small particles at Saturn are about the same as those at Earth — two very different neighborhoods in our solar system — and this is exciting to see,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “It took Saturn’s rings acting like a giant meteoroid detector — 100 times the surface area of the Earth — and Cassini’s long-term tour of the Saturn system to address this question.”
The Saturnian equinox in summer 2009 was an especially good time to see the debris left by meteoroid impacts. The very shallow sun angle on the rings caused the clouds of debris to look bright against the darkened rings in pictures from Cassini’s imaging science subsystem.
This animation depicts the shearing of an initially circular cloud of debris as a result of the particles in the cloud having differing orbital speeds around Saturn. Image credit: NASA/Cornell
“We knew these little impacts were constantly occurring, but we didn’t know how big or how frequent they might be, and we didn’t necessarily expect them to take the form of spectacular shearing clouds,” said Matt Tiscareno, lead author of the paper and a Cassini participating scientist at Cornell University in Ithaca, N.Y. “The sunlight shining edge-on to the rings at the Saturnian equinox acted like an anti-cloaking device, so these usually invisible features became plain to see.”
Tiscareno and his colleagues now think meteoroids of this size probably break up on a first encounter with the rings, creating smaller, slower pieces that then enter into orbit around Saturn. The impact into the rings of these secondary meteoroid bits kicks up the clouds. The tiny particles forming these clouds have a range of orbital speeds around Saturn. The clouds they form soon are pulled into diagonal, extended bright streaks.
“Saturn’s rings are unusually bright and clean, leading some to suggest that the rings are actually much younger than Saturn,” said Jeff Cuzzi, a co-author of the paper and a Cassini interdisciplinary scientist specializing in planetary rings and dust at NASA’s Ames Research Center in Moffett Field, Calif. “To assess this dramatic claim, we must know more about the rate at which outside material is bombarding the rings. This latest analysis helps fill in that story with detection of impactors of a size that we weren’t previously able to detect directly.”
