Saturn & Moons

            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.	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 denser due to the presence of metallic hydrogen, which as a result of the extreme pressure.	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.	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 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.  (c) Universe Today.com

Lakes on the surface of Titan

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.	    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.	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.	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.	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.	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.
(c) Universe Today.com

Saturn's Rings

            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 one 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.	    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. (c) Universe Today.com

Observation and Exploration of 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.	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 18th 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.	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.	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.	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 conducted multiple flybys of Titan and other icy satellites until its mission ended in 2017.	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 was extended twice – first to September 2010 and again to 2017. The mission officially ended in 2017 when Cassini was purposefully sent to burn up in Saturn’s atmosphere.  (c) Universe Today.com

Huygens descends to the surface of Titan

Saturn's Moons: Facts About the Ringed Planet's Satellites

© Nola Taylor Redd, Space.com. June 30, 2016
            [Over 100 moons] travel around Saturn. They come in a variety of sizes and compositions, from almost pure ice to rocky material, as well as a combination of both. Their journeys around the ringed planet range from half an Earth day to just over four Earth years.
            One of Saturn's moons, Titan, makes up 96 percent of the mass orbiting the planet. Scientists think that Saturn's system may have originally housed two such moons, but the second broke up, creating the debris that formed the rings and smaller, inner moons. Another theory suggests that the system originally housed several large moons, similar to Jupiter's Galilean moons, but two fused into Titan. The violent collision could have scattered the debris that would have later drawn together into the smaller moons.
            The colorful globe of Saturn's largest moon, Titan, passes in front of the planet and its rings in this true color snapshot from NASA's Cassini spacecraft. The north polar hood can be seen on Titan (3,200 miles or 5,150 kilometers across) and appears as a detached layer at the top of the moon here. Image released Dec. 22, 2011.
            Some of the moons travel inside the gaps of the rings, clearing paths through the debris. Others orbit farther out. Several of the moons interact with one another, with their orbits moving in resonance with each other. Larger moons may trap smaller moons, keeping them nearby. Sixteen of the moons are tidally locked, with one face permanently turned toward Saturn.
            The first moon was discovered in 1655. Over the next 200 years, the other seven major satellites were spotted. By 1997, astronomers on Earth had found 18 moons in orbit around the planet. The close orbit of NASA's Cassini mission, along with advances in technology for Earth-based telescopes, enabled the discovery of the rest.
            In 1847, British astronomer Sir John Herschel suggested that the moons of Saturn take their names from the Titans. The mythical siblings of the Greek god Cronus — Saturn to the Romans — the Titans battled the Olympian gods and lost. Once the names of the Titans were used, the moons began to be called after other characters from Roman and Greek mythology. Only 53 of Saturn's moons have names; the rest are identified by a numerical designation relating to their year of discovery.
    Let's look at the eight major moons of Saturn:
Titan
            Titan is the largest of Saturn's moons and the first to be discovered. Titan is the only moon in the solar system known to have a significant atmosphere. Nitrogen and methane extend around the moon 10 times as far into space as Earth's atmosphere, sometimes falling to the surface in the form of methane rain. This atmosphere makes it one of the best potential candidates for hosting life. Titan is larger than the planet Mercury, though not nearly as massive. It hosts many hydrocarbon-filled lakes as well as extremely tall mountains, with the largest one rising to nearly 11,000 feet. This moon is the only one to have a landing craft arrive on its surface, when the Cassini mission sent the Huygens lander there in 2005.
Dione
            Dione is thought to be a dense rocky core surrounded by water-ice. The tidally locked moon is heavily cratered not on its leading side but on its back side. Astronomers think a collision could have spun the moon on its axis. The moon hosts a thin oxygen atmosphere and may have a liquid ocean beneath its surface.
Enceladus
            Enceladus contains more than 100 geysers at its south pole. Tidal heating causes portions of the icy planet to melt, spewing icy material into space from its "tiger stripes." The tiny bits of ice travel together to create Saturn's E ring. The satellite's icy surface makes it one of the brightest objects in the solar system. The moon has a subsurface ocean that may be friendly to life.
Hyperion
            Hyperion was the last of the major satellites to be discovered. Hyperion is a small moon with an irregular appearance. The flattened object resembles an elongated potato rather than a sphere, a form that may have been created when an impact demolished a larger moon long ago. Hyperion has a spongy shape, possibly due to its low density and porous surface. Impacts seem to be absorbed by the moon, and most of the ejecta is thrown into space.
Iapetus
            Iapetus features light and dark contrasts on its surface, giving the moon a yin-yang shape. Dark hydrocarbons falling to the moon long ago, perhaps from the nearby moon Phoebe, may have had more time to absorb more heat, gradually growing and spreading over time. Iapetus has a walnut-like shape, with its center bulging outward, and a ridge running around its equator. The moon also contains some of the highest mountains in the solar system, which may have been material from another moon. Scientists are studying ice movements (such as landslides) to do comparative work with these types of features on Earth.
Mimas
            Mimas has a gaping crater that gives the rocky moon a strong resemblance to fictional Death Star in the "Star Wars" movies. The impact stands out despite the fact that Mimas is one of the most heavily cratered bodies in the solar system, with overlapping impacts covering the surface. The smallest and closest orbiting of Saturn's major moons, Mimas cleared the gap known as the Cassini division between two of the planet's rings. Mimas is made up primarily of water-ice, but despite its proximity to the planet (and the resulting tidal heating that should occur), the surface of the moon remains unchanged; none of the ice seems to be melting, though such melting occurs on other, more distant moons. It is possible that there is a liquid ocean beneath its surface, although scientists say that an oval-shaped core could also explain some of Mimas' libration movements.
Rhea
            Rhea is a heavily cratered moon and lacks a core at its center. Instead, the entire body is composed of ice, with traces of rock mixed in, causing it to resemble a dirty snowball. The second largest of the planet's major moons, Rhea is still rather small, about half the size of Earth's moon. The satellite contains a faint oxygen atmosphere, about 5 trillion times less dense than the one found on Earth, but the only known oxygen atmosphere in the solar system. Radiation from Saturn's magnetosphere could release oxygen and carbon dioxide from the icy surface.
Tethys
            Tethys travels close to Saturn and feels the gravitational pull of the planet. The heat from Saturn may allow the moon's icy surface to melt slightly, filling in craters and other signs of impact. Made up almost entirely of water ice, the surface is highly reflective. A large trench crosses the moon, running diagonally from its north to south pole and spanning three-quarters of the satellite's circumference. A large crater on the other side of the moon covers nearly two-fifth of the moon's diameter and is nearly the size of Mimas. Scientists have found strange red arcs on the moon and are still struggling to explain how the arcs got there.

The surface of Titan  (c) Julian Baum

Titan

Diameter: 5,149.4 km
Mass: 1.35 × 10^23 kg (1.8 Moons)
Orbits: Saturn
Orbit Distance: 1,221,865 km
Orbit Length: 15.9 days
Surface Temperature: -179 °C
Discovery Date: March 25, 1655
Discovered By:    Christiaan Huygens
            Titan’s diameter is 50 percent larger than Earth’s Moon, making it among the largest natural satellites in the solar system.        Titan’s most obvious feature is its heavy, hazy atmosphere. The most abundant gas is nitrogen, with methane and ethane clouds and a thick organic smog.        It was discovered in 1655 by Dutch astronomer Christiaan Huygens. It is named for mythological Titans, the brothers and sisters of the Greek god Cronus.        The composition of Titan is known to be water ice over a rocky interior. Its surface has liquid hydrocarbon lakes and the vents of cryovolcanoes, distributed among areas of bright and dark terrain that show evidence of some impact cratering.        Titan is thought to have several layers: a rocky core, surrounded by layers of crystalline ice. It is likely that the core is still hot, with a layer of liquid water and ammonia.        Like other moons around their primary planets, Titan has a rotation period that is the same as its orbital period. That means it turns on its axis in the same length of time as it takes to orbit Saturn.        Titan may have formed as material in orbit around early Saturn began to accrete. Giant impacts and collisions may have disturbed the orbits of Titan and other moons into their current positions.        Several probes have imaged Titan, but only one has visited the surface — the Huygens lander. It arrived on January 14, 2005, and sent data for about an hour and a half, making it the most distant landing of any mission in the solar system.  © Space Facts.com

FACTS ABOUT TITAN
            Saturn's largest moon, Titan, is an icy world whose surface is completely obscured by a golden hazy atmosphere. Titan is the second largest moon in our solar system. Only Jupiter's moon Ganymede is larger, by just 2 percent. Titan is bigger than Earth's moon, and larger than even the planet Mercury.            This mammoth moon is the only moon in the solar system with a dense atmosphere, and it’s the only world besides Earth that has standing bodies of liquid, including rivers, lakes and seas, on its surface. Like Earth, Titan’s atmosphere is primarily nitrogen, plus a small amount of methane. It is the sole other place in the solar system known to have an earthlike cycle of liquids raining from clouds, flowing across its surface, filling lakes and seas, and evaporating back into the sky (akin to Earth’s water cycle). Titan is also thought to have a subsurface ocean of water.Size and DistanceTitan has a radius of about 1,600 miles (2,575 kilometers), and is nearly 50 percent wider than Earth’s moon. Titan is about 759,000 miles (1.2 million kilometers) from Saturn, which itself is about 886 million miles (1.4 billion kilometers) from the Sun, or about 9.5 astronomical units (AU). One AU is the distance from Earth to the Sun. Light from the Sun takes about 80 minutes to reach Titan; because of the distance, sunlight is about 100 times fainter at Saturn and Titan than at Earth.Orbit and RotationTitan takes 15 days and 22 hours to complete a full orbit of Saturn. Titan is also tidally locked in synchronous rotation with Saturn, meaning that, like Earth’s Moon, Titan always shows the same face to the planet as it orbits. Saturn takes about 29 Earth years to orbit the Sun (a Saturnian year), and Saturn’s axis of rotation is tilted like Earth’s, resulting in seasons. But Saturn’s longer year produces seasons that each last more than seven Earth years. Since Titan orbits roughly along Saturn’s equatorial plane, and Titan’s tilt relative to the sun is about the same as Saturn’s, Titan’s seasons are on the same schedule as Saturn’s—seasons that last more than seven Earth years, and a year that lasts 29 Earth years.FormationScientists aren’t certain about Titan’s origin. However, its atmosphere provides a clue. Several instruments on the NASA and ESA Cassini-Huygens mission measured the isotopes nitrogen-14 and nitrogen-15 in Titan’s atmosphere. The instruments found Titan’s nitrogen isotope ratio most resembles that found in comets from the Oort Cloud—a sphere of hundreds of billions of icy bodies thought to orbit the Sun at a distance between 5,000 and 100,000 astronomical units from the Sun (Earth is about one astronomical unit from the Sun—roughly 93 million miles or 150 million kilometers). Titan’s atmospheric nitrogen ratio suggests the moon’s building blocks formed early in the solar system's history, in the same cold disk of gas and dust that formed the Sun (called the protosolar nebula), rather than forming in the warmer disk of material that Saturn later formed from (called the Saturn sub-nebula).StructureTitan’s internal structure isn’t entirely known, but one model based on data from the Cassini-Huygens mission suggests Titan has five primary layers. The innermost layer is a core of rock (specifically, water-bearing silicate rock) about 2,500 miles (4,000 kilometers) in diameter. Surrounding the core is a shell of water ice—a special type called ice-VI that is only found at extremely high-pressures. The high-pressure ice is surrounded by a layer of salty liquid water, on top of which sits an outer crust of water ice. This surface is coated with organic molecules that have rained or otherwise settled out of the atmosphere in the form of sands and liquids. The surface is hugged by a dense atmosphere.SurfaceThe surface of Titan is one of the most Earthlike places in the solar system, albeit at vastly colder temperatures and with different chemistry. Here it is so cold (-290 degrees Fahrenheit or -179 degrees Celsius) that water ice plays the role of rock. Titan may have volcanic activity as well, but with liquid water “lava” instead of molten rock. Titan’s surface is sculpted by flowing methane and ethane, which carves river channels and fills great lakes with liquid natural gas. No other world in the solar system, aside from Earth, has that kind of liquid activity on its surface. Vast regions of dark dunes stretch across Titan’s landscape, primarily around the equatorial regions. The "sand" in these dunes is composed of dark hydrocarbon grains thought to look something like coffee grounds. In appearance, the tall, linear dunes are not unlike those seen in the desert of Namibia in Africa. Titan has few visible impact craters, meaning its surface must be relatively young and some combination of processes erases evidence of impacts over time. Earth is similar in that respect as well; craters on our planet are erased by the relentless forces of flowing liquid (water, in Earth's case), wind, and the recycling of the crust via plate tectonics. These forces are present on Titan as well, in modified forms. In particular, tectonic forces—the movement of the ground due to pressures from beneath—appear to be at work on the icy moon, although scientists do not see evidence of plates like on Earth.AtmosphereOur solar system is home to more than 150 moons, but Titan is unique in being the only moon with a thick atmosphere. At the surface of Titan, the atmospheric pressure is about 60 percent greater than on Earth—roughly the same pressure a person would feel swimming about 50 feet (15 meters) below the surface in theocean on Earth. Because Titan is less massive than Earth, its gravity doesn't hold onto its gaseous envelope as tightly, so the atmosphere extends to an altitude 10 times higher than Earth's—nearly 370 miles (600 kilometers) into space. Titan's atmosphere is mostly nitrogen (about 95 percent) and methane (about 5 percent), with small amounts of other carbon-rich compounds. High in Titan’s atmosphere, methane and nitrogen molecules are split apart by the Sun's ultraviolet light and by high-energy particles accelerated in Saturn's magnetic field. The pieces of these molecules recombine to form a variety of organic chemicals (substances that contain carbon and hydrogen), and often include nitrogen, oxygen and other elements important to life on Earth. Some of the compounds produced by that splitting and recycling of methane and nitrogen create a kind of smog—a thick, orange-colored haze that makes the moon's surface difficult to view from space. (Spacecraft and telescopes can, however, see through the haze at certain wavelengths of light outside of those visible to human eyes.) Some of the heavy, carbon-rich compounds settle to the moon’s surface—these hydrocarbons play the role of “sand” in Titan’s vast dune fields. And methane condenses into clouds that occasionally drench the surface in methane storms. The methane in Titan’s atmosphere is what makes its complex atmospheric chemistry possible, but where all that methane comes from is a mystery. Because sunlight continuously breaks down methane in Titan’s atmosphere, some source must be replenishing it or it would be depleted over time. Researchers suspect methane could be belched into Titan's atmosphere by cryovolcanism—volcanoes releasing chilled water instead of molten rock lava—but they’re not certain if this or some other process is responsible.Potential for LifeThe Cassini spacecraft’s numerous gravity measurements of Titan revealed that the moon is hiding an underground ocean of liquid water (likely mixed with salts and ammonia). The European Space Agency’s Huygens probe also measured radio signals during its descent to the surface, in 2005, that strongly suggested the presence of an ocean 35 to 50 miles (55 to 80 kilometers) below the icy ground. The discovery of a global ocean of liquid water adds Titan to the handful of worlds in our solar system that could potentially contain habitable environments. Additionally, Titan’s rivers, lakes and seas of liquid methane and ethane might serve as a habitable environment on the moon’s surface, though any life there would likely be very different from Earth’s life. Thus, Titan could potentially harbor environments with conditions suitable for life—meaning both life as we know it (in the subsurface ocean) and life as we don’t know it (in the hydrocarbon liquid on the surface). Although there is so far no evidence of life on Titan, its complex chemistry and unique environments are certain to make it a destination for continued exploration.  © SOLAR SYSTEM. NASA.gov

Enceladus (c) Mark Garlick

Enceladus

Overview
            Few worlds in our solar system are as compelling as Saturn’s icy ocean moon Enceladus. A handful of worlds are thought to have liquid water oceans beneath their frozen shell, but Enceladus sprays its ocean out into space where a spacecraft can sample it. From these samples, scientists have determined that Enceladus has most of the chemical ingredients needed for life, and likely has hydrothermal vents spewing out hot, mineral-rich water into its ocean.            About as wide as Arizona, Enceladus also has the whitest, most reflective surface in the solar system. The moon creates a ring of its own as it orbits Saturn—its spray of icy particles spreads out into the space around its orbit, circling the planet to form Saturn’s E ring.            Enceladus is named after a giant in Greek mythology.            Pictures from the Voyager spacecraft in the 1980s indicated that although this moon is small—only about 310 miles (500 kilometers) across — its icy surface is remarkably smooth in some places, and bright white all over. In fact, Enceladus is the most reflective body in the solar system. For decades, scientists didn’t know why.            Because Enceladus reflects so much sunlight, the surface temperature is extremely cold, about minus 330 degrees Fahrenheit (minus 201 degrees Celsius). But it is not as cold and inactive a place as it appears.            Enceladus orbits Saturn at a distance of 148,000 miles (238,000 kilometers) between the orbits of two other moons, Mimas and Tethys. Enceladus is tidally locked with Saturn, keeping the same face toward the planet. It completes one orbit every 32.9 hours within the densest part of Saturn's E Ring. Also, like some other moons in the extensive systems of the giant planets, Enceladus is trapped in what’s called an orbital resonance, which is when two or more moons line up with their parent planet at regular intervals and interact gravitationally. Enceladus orbits Saturn twice every time Dione, a larger moon, orbits once. Dione’s gravity stretches Enceladus’ orbit into an elliptical shape, so Enceladus is sometimes closer and other times farther from Saturn, causing tidal heating within the moon.             Parts of Enceladus show craters up to 22 miles (35 kilometers) in diameter, while other regions have few craters, indicating major resurfacing events in the geologically recent past. In particular, the south polar region of Enceladus is almost entirely free of impact craters. The area is also littered with house-sized ice boulders and regions carved by tectonic patterns unique to this region of the moon.            In 2005, NASA’s Cassini spacecraft discovered that icy water particles and gas gush from the moon’s surface at approximately 800 miles per hour (400 meters per second). The eruptions appear to be continuous, generating an enormous halo of fine ice dust around Enceladus, which supplies material to Saturn's E-ring. Only a small fraction of the material ends up in the ring, however, with most of it falling like snow back to the moon’s surface, which helps keep Enceladus bright white.            The water jets come from relatively warm fractures in the crust, which scientists informally call the “tiger stripes.” Several gases, including water vapor, carbon dioxide, methane, perhaps a little ammonia and either carbon monoxide or nitrogen gas make up the gaseous envelope of the plume, along with salts and silica. And the density of organic materials in the plume was about 20 times denser than scientists expected.            From gravity measurements based on the Doppler effect and the magnitude of the moon’s very slight wobble as it orbits Saturn, scientists determined that the jets were being supplied by a global ocean inside the moon. Scientists think that the moon’s ice shell may be as thin as half a mile to 3 miles (1 to 5 kilometers) at the south pole. The average global thickness of the ice is thought to be about 12 to 16 miles (20 to 25 kilometers).            Since the ocean in Enceladus supplies the jets, and the jets produce Saturn’s E ring, to study material in the E ring is to study Enceladus’ ocean. The E ring is mostly made of ice droplets, but among them are peculiar nanograins of silica, which can only be generated where liquid water and rock interact at temperatures above about 200 degrees Fahrenheit (90 degrees Celsius). This, among other evidence, points to hydrothermal vents deep beneath Enceladus’ icy shell, not unlike the hydrothermal vents that dot Earth’s ocean floor.With its global ocean, unique chemistry and internal heat, Enceladus has become a promising lead in our search for worlds where life could exist.Discovery
            British astronomer William Herschel spotted Enceladus orbiting Saturn on 28 August 1789.How Enceladus Got Its Name
            Enceladus is named after the giant Enceladus of Greek mythology. William Herschel's son John Herschel suggested the name in his 1847 publication Results of Astronomical Observation made at the Cape of Good Hope, in which he suggested names for the first seven Saturnian moons discovered. He chose these names in particular because Saturn, known in Greek mythology as Cronus, was the leader of the Titans. © SOLAR SYSTEM. NASA.gov