Saturn's satellites

Saturn has a remarkably diverse set of satellites. They include gigantic Titan, which retains a thick atmosphere; Enceladus, possessing a vastly reworked surface that includes active geysers; Hyperion, a disk-shaped satellite whose rotation is erratic; Phoebe, moving in a retrograde orbit; and a co-orbiting pair called Janus and Epimetheus.

Overview

Prior to the space age, Saturn was known as the beautiful ringed world of the solar system. Many of its numerous larger satellites were discovered prior to the time of interplanetary spacecraft. The most notable of these was Titan, Saturn’s largest satellite and the only known moon from telescopic observation to maintain a thick atmosphere. Prior to the Voyager flybys, planetary scientists expected the other Saturn satellites to be relatively uninteresting, inactive, ice worlds. Only Iapetus was a curiosity as it displayed a very reflective and extremely dark side as well. The results of the Voyager flybys and the Cassini orbiter images revealed Saturn’s system to be a miniature solar system in its own right with a variety of extremely interesting and diverse satellites.

When Voyager1 passed by Saturn’s largest satellite Titan in November, 1980, scientists were somewhat disappointed with the imagery transmitted back to Earth. Titan appeared as a uniform orange sphere whose outline was blurred by a dense cloud cover. A closer examination found a higher layer of ultraviolet haze. The southern hemisphere has a slightly darker cast than the northern hemisphere. A clear equatorial boundary was noted, and a darker polar ring is evident in some photographs from Voyager 2. Beneath those clouds, Titan proved more interesting. Voyager 1’s close passage behind the disk of Titan allowed the use of its radio transmissions to probe the satellite’s atmosphere. The pressure at ground level is 1.5 times that of Earth. If Titan’s lower surface gravity is taken into account, the implication is that every square meter of Titan has ten times as much gas above its surface as Earth does.

Methane was spectroscopically detected from Earth, but the prime component of Titan’s atmosphere proved to be nitrogen. It was suspected that as much as 10 percent of the atmosphere is argon, and methane makes up between 1 and 6 percent of the rest of the atmosphere, increasing in concentration near Titan’s surface.

At higher altitudes, solar ultraviolet rays break methane down, and new molecules form as some hydrogen is lost. Spectroscopic observations show traces of hydrogen, ethane, propane, ethylene, diacetylene, hydrogen cyanide, carbon monoxide, and carbon dioxide. Together, these components form the petrochemical smog that so frustrated the Voyager imaging team.

The upper optical haze layer lies about 280 kilometers above the surface. The main cloud deck is about 200 kilometers from the surface. Titan’s solid surface is 400 kilometers smaller in diameter than previously thought, smaller than both Ganymede and Callisto in the Jupiter system. Why do these Jovian satellites not have atmospheres? Titan orbits at a greater distance from Saturn than either of these satellites do from Jupiter, so its tidal stress is less. Furthermore, Saturn is twice as far from the sun as is Jupiter, so solar radiation intensity at Titan is four times weaker than in the Jovian system.

Beneath those tantalizing orange clouds, the surface temperature is only ninety-four kelvins; combined with the fact that Titan’s atmospheric pressure is 1.5 bars, this temperature suggested the possibility of an ethane and/or methane sea on Titan’s surface. If tidal stresses heat the interior enough, there may even be icy geysers. The possibility of life arising at such low temperatures appears unlikely, but certainly the carbon chemistry on Titan must be very interesting.

The European Space Agency (ESA) provided the Huygens probe, a combination atmospheric entry probe and soft lander, for the National Aeronautics and Space Administration’s (NASA’s) Cassini program. The orbiter carried Huygens from its launch in October 1997 to its release on Christmas 2004. For the next two weeks the probe flew independently from the Cassini orbiter, and then it entered Titan’s atmosphere on January 14, 2005, and dropped down under a large parachute to a safe touchdown near Titan’s Xanadu region. Some researchers expected Huygens to splash down in a cryogenic sea of liquid hydrocarbons. It became clear rather quickly that Huygens had “plopped” down in what some referred to as Titanian mud. Evidence of liquid action on the surface was found, but the original idea of liquid hydrocarbon seas was dashed. Analysis of data sent from the probe on the way down to impact revealed several layers in the atmosphere, most notably a thick haze between eighteen and twenty kilometers above the surface. An Aerosol Collector and Pyrolyzer collected samples at different altitudes to determine the pressure of volatiles and organic materials. The probe was also outfitted with a gas chromatograph mass spectrometer to determine atmospheric composition. Titan’s atmosphere proved to be hazier than expected, as dust particle concentration was greater than previously believed. Wind data suggested that Titan’s atmosphere circulated gas from the south to the north pole and back again in periodic fashion.

Winds would play a large role in planetary dynamics for this complex world. Indeed, two years after the probe’s several hours of data were collected, Cassini scientists concluded that Titan’s crust moves on a subsurface ocean with crust movements in part driven by wind actions. That movement was noted by comparing radar data from the orbiter taken at different times during the mission in concert with available Huygens data. The proposed liquid subsurface layer would be located fifty to one hundred kilometers beneath the crust and include liquid ammonia in a water ice slush. Floating on this layer, the crust was seen to move as much as thirty kilometers over the course of several years of Cassini's observations.

Further examples of Huygens and Cassini images eventually found evidence proving the existence of ancient shorelines and the presence of liquid on the surface of Titan. Computer models of this dynamic world had to be greatly altered due to Huygens and Cassini data, and many new questions were raised to give Titan an even more mysterious nature. However, it still was believed to be a world rather similar in some ways to an early Earth, just a world frozen at an early point of physical and chemical evolution prior to the development of life. Down in the subsurface liquid layer higher temperatures could permit complex biochemistry, but there was no information produced by Cassini to investigate that supposition.

By measuring gravitational perturbations on the Voyagerspacecraft as they flew through the Saturnian system, scientists at the Jet Propulsion Laboratory (JPL) could determine the masses and densities of Saturn’s middle-sized satellites. Rhea’s bulk density of 1.3 grams per cubic centimeter suggests that it contains more ice and fewer silicates than Titan. It is worth noting that the density of bodies depends not only on their composition but also on how tightly packed they are. The greater the mass, the higher the gravity, and thus the greater the density. Thus Titan, Ganymede, and Callisto, which all approximate the size of the planet Mercury, have very similar densities, about 1.9 grams per cubic centimeter, and the smaller, icy satellites are less dense, even though the composition may be quite similar to that of larger satellites.

Rhea and all the smaller Saturnian satellites lack atmospheres and show some signs of older, cratered surfaces. The trailing side of Rhea is covered with pale, wispy streaks, a type of feature it shares with Dione. These streaks may be evidence of venting of water vapor from these satellites’ interiors, perhaps from tidally induced volcanism in the past. On Earth’s moon, such activity was on the side facing Earth rather than on the trailing side. Perhaps these wisps were once found on Rhea’s leading side but were eventually eroded, much as meteoric dust erases all but the youngest ray patterns around lunar craters.

During a Cassini flyby of Rhea in November 2005, some surprising results were obtained. While the spacecraft’s magnetometer did not pick up any interactions with Saturn’s magnetosphere that would have indicated even a meager atmosphere about Rhea, there was evidence of a broad debris disk and one structured ring about this satellite. The debris disk extended several thousand kilometers out from Rhea, hence it was several Rhea radii in expanse. Computer simulations of the gravitational interactions of Saturn and Rhea indicated that this ring could exist for a considerable time. The source for the ring particles about the small satellite could have been a large impact event with Rhea.

Slightly smaller than Rhea, Dione is a bit more dense (1.4 grams per cubic centimeter). Its wispy patterns are more marked than those of Rhea, and it also has some long cracks and large areas of fairly fresh ice, without large craters in evidence. Ice may have flowed through cracks during the cooling phase of the satellite. Unlike most substances, water expands when it freezes. Therefore, satellites made mainly of ice, or differentiated with rocky cores and mantles composed largely of water, might show such expansion cracks. Such cracks are evident on Tethys, and were noted by Voyager 2 on the Uranian moons Ariel and Titania in January 1986.

Tethys is similar in size to Dione, but it features one huge impact crater. The crater’s floor is quite flat, suggesting internal flooding resulting from impact heating. Running from the crater three-quarters of the way around Tethys is a single gigantic valley system, Ithaca Chasma. Tethys’s craters are lower in relief than lunar craters. The icy crust of these Saturnian satellites is more plastic than is lunar crust. Older terrain cratering appears to have been just as heavy as on Earth’s moon, resulting in craters appearing less rugged than the lunar highlands.

The small satellite Mimas features a notable exception to the above rule. One huge crater, Herschel, is one-third as large as Mimas itself. This crater is very deep, about nine kilometers, with a central peak about six kilometers high. It constitutes one of the most striking geological features in the solar system. This crater on such a small roughly spherical body gives it the appearance of the Death Star station in the movie Star Wars. Many of the planetary scientists on the Voyager and Cassini teams, and astronomy professors worldwide, fondly refer to Mimas as the “Death Star Moon.”

It is likely that an impact of any greater force would have broken Mimas apart. Such huge impacts might also explain the very jumbled appearance of Uranus’s Miranda, which was apparently broken into several large pieces, and then haphazardly reassembled by gravity later.

One highly speculative hypothesis may account for such massive impacts and the intense cratering that is evident throughout the solar system. Perhaps a terrestrial planet in an unstable orbit beyond Mars and an outer icy Jovian satellite was totally fragmented in a high-energy head-on collision. The lighter, icy debris might account for some of the comets. Heavier chunks may have found a relatively stable orbit and formed the asteroid belt. Many chunks and particles, however, would have been scattered in all directions to impact other worlds. In some cases, the impact would have been forceful enough to send up debris which, in turn, would bombard neighboring worlds. This scenario might explain ice found on certain asteroids, the existence of captured satellites such as Phoebe and Phobos, and the fact that meteoritic material seems to have originated on a differentiated planet. It might also explain how fragments that scientists agree came from lunar basalts and from Mars are found on Earth.

While almost a twin of Mimas in size, and orbiting just beyond it in the satellite system, Enceladus is a very different world up close. Even the Pioneer 11 data indicated that it has an albedo near 100 percent. It seems to be made of fresher ice, reflecting far more sunlight than most Saturnian satellites. Had its material been older, dark meteoritic and cometary dust would have darkened it. Voyager’s cameras revealed that one of its hemispheres is heavily cratered and fairly old, but the opposite side features smooth plains cut by grooved terrain, similar to Ganymede. This evidence of much rifting and recent internal activity appears on a satellite about one-tenth the size of Ganymede. A count of ring particles in Saturn’s extended E ring also found that they peaked near Enceladus. Just as the dust ring of Jupiter is supplied by Io’s volcanoes, icy geysers on Enceladus periodically shoot debris above this active world. Why is this small world active at all? Mimas lies closer to Saturn and thus is more tidally stressed, yet it shows no such activity. Nor does the proximity of any other large satellite seem to account for the heating required to generate such activity. Such activity on smaller satellites is not unique; Uranus’s moon, Ariel, which is similar in size to Mimas, shows obvious broad rift valleys. The source of heating for this extensive and possibly continuing crustal activity is a mystery.

Cassini found geysers on Enceladus near its south pole along long cracks that essentially act like vents. Fresh crystalline ice forms at the site of these cracks and colors the features distinctively. Cassini scientists dubbed these nearly parallel cracks found at the south polar region “Tiger Stripes.” The Tiger Stripes were found to be 124 kilometers long and 40 kilometers apart. This activity was not new, so why did the Voyagers fail to see these geysers? Voyager 2 flew over Enceladus’s north pole and missed them. Cassini’s near-infrared mapping spectrometer and solid-state imager both examined the ice around the Tiger Stripes. Freshly formed ice was crystalline. As time progresses that pristine ice becomes radiation-damaged amorphous ice.

Data and the geyser actions strongly suggested the presence of a subsurface ocean on Enceladus. However, calculations about heat transport inside Enceladus led researchers to believe that the satellite’s subsurface ocean would not be able to exist for more than 30 million years if it were warmed only by heat escaping from the core toward the crust. Since that ocean most likely has been in existence for more than 30 million years, the heating mechanism for both the subsurface and the cryovolcanic activity at the satellite’s south pole must be from tidal flexing. Only that could provide the 5.8 gigawatts of heat Cassini saw emerging from the Tiger Stripes over which it flew on a close fly. Since internal heat sources apart from tidal flexing produce only 0.32 gigawatts, without tidal heating Enceladus’s subsurface ocean would have frozen. But the story here is more complex than that. Without the subsurface ocean, tidal flexing of the magnitude necessary to produce the observed heat would not be possible, and without the heat for the tidal flexing the ocean would freeze. In 2023, astonishing images taken by NASA’s James Webb telescope showed a water vapor plume, nearly 9,700 kilometers (6,000 miles) long that extended from Enceladus and into Saturn’s E ring. Powerful water geysers, located from cracks in Enceladus’ icy surface, were the source of the emitted water vapor. The Webb telescope allowed, for the first time, an accurate estimation of this flow of liquid which was calculated as 360 liters (79 gallons) per second. These geysers add further credence to the possibility of life on Encelades.

Iapetus confronts scientists with another mystery. A portion of this satellite is extremely dark, whereas the rest of Iapetus has an albedo typical of an icy surface. Iapetus’s dark side is six times lower in albedo than that icy portion. Within the darker portion is an irregular dark spot. This pattern of darker leading hemispheres is also seen on Rhea and Dione, but to a far lesser degree. The brighter, icy side does have an albedo of 50 percent. This is typical of older water-ice crusts, and it shows heavy cratering typical of other similar satellites. Some larger craters near the boundary between the hemispheres have light-colored walls, with darker flat floors, like some larger craters on Earth’s moon. What is the reddish-black material that gives the darker side an albedo of only 5 percent? It is probably an organic tar, and it appears to be a good match with carbonaceous chondrite meteorites, the dark rings of Uranus, and the black crust of Halley’s comet. Carbon almost always appears in an oxidized form (carbon dioxide, carbonic acid, carbonate rocks, carbohydrates) in the inner solar system. Dark neutral carbon is a major solid material found in the outer solar system. It was impossible to judge the age of Iapetus’ dark spot, as Voyager’s cameras could not pick up any details on the dark side. In fact, some photographs even make the dark side disappear into the blackness of space. The concentration of the dark material on the leading side suggested an external source for this coating. It had been hypothesized that dark Phoebe was responsible. Phoebe’s color, however, does not match the black side of Iapetus. The dark floor of some of Iapetus’s craters constitutes evidence for an internal origin. New data and insights would have to wait for Cassini to pass by Iapetus at a much closer distance than had the Voyagers.

Cassini flew within 1640 kilometers of Iapetus on September 10, 2007. Unfortunately during the encounter, Cassini entered a safe mode after on-board delicate solid-state electronics suffered a cosmic ray hit. Fortunately, most of the science harvest was recovered after a short delay in playback. Among the findings was a raised area around the satellite’s mid-section that gave Iapetus the appearance of a walnut. Why the equatorial bulge on this unusual satellite? Julie Castillo of the Jet Propulsion Laboratory advanced an innovative explanation for the unique feature. Castillo invoked a high rotational speed early in the satellite’s history coupled with heat from internal radioactivity, perhaps from aluminum 26 and iron 60 isotopes, that softened the satellite to form the equatorial bulge. Consideration of the time frame in which tidal forces forced the spin rate to diminish led to the conclusion that this particular pair of isotopes would be required, since they would be abundant and would generate heat quickly due to rapid radioactive decay. Then, from a softened and malleable state, the satellite’s bulge was frozen in place before Iapetus’s spin rate slowed down. More investigation would be needed to confirm or refute this theory. Unless extended mission priorities are changed and trajectories reevaluated, this could be the closest Cassini would ever get to this highly unusual satellite.

Phoebe may be a captured asteroid from the far reaches of the main belt, and therefore similar to Chiron. Phoebe’s orbit is retrograde, like those of Jupiter’s four outermost moonlets. All these small worlds are quite distant from the gas giants whose gravity trapped them. Cassini encountered Phoebe on the way into Saturn orbit insertion. Photographs revealed Phoebe’s surface to look almost spongelike, not at all like the other Saturnian satellites.

Like the dark side of Iapetus, Phoebe has an albedo of about 5 percent, which is similar to those of two other captured asteroids, Deimos and Phobos, which orbit Mars. At two hundred kilometers in diameter, Phoebe is much rounder than the odd “hamburger moon,” Hyperion, which orbits between Tethys and Iapetus. Puck, a satellite of Uranus, is similarly round and dark, and about the same size as Phoebe. This fact suggests that a round shape is the norm for dark, primitive bodies such as these, and that something unusual happened with Hyperion.

Hyperion’s shape is quite striking. It is a huge disk, about 250 kilometers across but only 150 kilometers thick. Like Phoebe, its surface is dark, old, and heavily cratered. Stranger still is Hyperion’s rotation period. It has not yet been well defined. Like Earth’s moon, Saturn’s other satellites are tidally locked, with one side permanently facing the planet. The Voyager 2 team tried to orient photographs of Hyperion to map it, however, they found that it was rotating chaotically. It appears to have no regular rotation period; it tumbles irregularly. Close coupling between Hyperion’s eccentric orbit and that of Titan may cause this unique effect.

The nine satellites discussed to this point were known well before the Voyager missions, but Voyager photographs found or confirmed eight more satellites, making Saturn the most numerous satellite system. Several photographs suggested the existence of even more Saturnian satellites, but their periods of revolution and orbits had to be determined before they were formally recognized. All these new satellites are much closer to Saturn than Phoebe and Iapetus, and they show a much more reflective, icy surface like Enceladus. None of these satellites is large enough to be nearly spherical or to have become differentiated. All of them have quite interesting orbits.

Just as the Trojan asteroids share Jupiter’s orbit, so two of Saturn’s middle-sized satellites have smaller companions in their orbits. Dione has two; Helene, the leading one, appears quite elongated, while the following one appears rounder. Lagrangian, Tethys’s companion, is a smaller version of Mimas, with a huge crater from an impact that almost destroyed it.

Saturn’s coorbital satellites were first spotted in 1966, but for more than a decade thereafter they were mistaken for a single satellite with an orbit under that of Mimas. Even prior to the Voyager flights, however, observers repeatedly noticed inconsistencies that led some to argue that there must be two satellites sharing the same orbit. Janus, the larger, is about two hundred kilometers across, and Epimetheus is about 150 kilometers across. Actually, their orbits are not quite identical. The inner satellite has a period of 16.664 hours; the outer one has a period of 16.672 hours, or a difference of twenty-nine seconds per orbit. Every four years, the inner satellite overtakes the outer at the speed of nine meters per second, and they exchange orbits. This close relationship and the irregular, elongated appearances of these satellites suggest they were once part of a single larger one split apart by a collision into the two pieces now sharing the same orbit.

The inner three satellites discovered by Voyagers 1 and 2 are all closely associated with Saturn’s rings. Atlas, a tiny, football-shaped body, orbits just outside the bright A ring of Saturn. Prometheus is a shepherding moon, keeping the particles in Saturn’s F ring in place from the inside of that ring. Pandora plays a similar role on the outside of the F ring. Their close relationship to this thin set of ringlets may explain why the F ring sometimes appears braided. Additional satellites were identified in subsequent reviews of Voyager and other available data. Then, with the arrival of the Cassini probe in the Saturn system, the number of recognized satellites again increased significantly, reaching sixty-two by 2013.

Knowledge Gained

While Jupiter possesses four satellites comparable in size to Earth’s moon or even to Mercury, Saturn has only one, Titan. Like Jupiter’s Ganymede and Callisto, Titan is comparable to Mercury in size but only about one-third as massive and dense. Its exact dimensions were still in debate prior to the Voyager missions. Its visible orange disk made it appear to be the largest known natural satellite; out-of-date astronomy textbooks will list Titan as the largest satellite in the solar system. Spectroscopic observations plainly revealed an atmosphere with gaseous methane and other hydrocarbons. Just how deep was the atmosphere, and what was it made of? These questions led the Voyager 1 team to target Titan as a main mission objective and to guide one probe closer to this satellite than to any other body on its mission.

The chief discoveries that resulted concerned Titan’s atmosphere. It is thick, twice as dense as Earth’s, but like Earth’s, Titan’s atmosphere is made primarily of nitrogen. Orange clouds appear to be a hydrocarbon smog, with complex organic chemistry taking place there. Surface temperatures and pressures lie close to the triple point of methane, so the surface might experience methane rains that would build up into lakes of liquid methane and freeze into methane ice at the poles. Confirmation of that would have to wait for a probe outfitted with imaging radar and/or a lander. Thus, the origin of the Cassini mission. A combination of radar images taken from orbit with data from the Huygens lander eventually confirmed the presence of cryogenic lakes and found ancient shorelines of lakes no longer existent. Huygens appeared to have landed in a wet slushlike material at cryogenic temperatures rather than floating on a lake or sea or even having hit a hard icy surface.

By the time its primary mission was completed, Cassini had flown past Titan several dozen times at varying distances. Perhaps one of Cassini’s biggest surprises was the detection that the surface of Titan moved as much as thirty kilometers between the earliest flyby of the Cassini primary mission (2004) and some near the time that the extended mission was approved (2008). This suggested that the crust floated on a layer of fluid, meaning the large satellite likely had an underground ocean, presumably a mixture of water and ammonia.

All of Saturn’s remaining satellites are smaller than Earth’s moon. Rhea is next in size, about half as large as the moon at fifteen hundred kilometers in diameter; Voyager 1 showed its icy surface to be cratered, but with fresher ice creating wispy terrain. Dione is next in size, at 1,100 kilometers in diameter, and has even more wispy terrain than Rhea. Tethys featured a huge, flattened crater on one side, with a great crack or rift running to the other side.

The innermost of the satellites well-known prior to the Voyager flybys are Mimas and Enceladus, both about five hundred kilometers in diameter. Mimas was found by Voyager 1 to have a dramatic impact crater one-third as large as the satellite. Enceladus is one of the most puzzling satellites, with tidal stresses producing plate activity, according to Voyager 2 data. These satellites are, in order from Titan inward: Dione, Tethys, Rhea, Enceladus, and Mimas. Prior to the Voyagers, only their orbital periods and approximate diameters were known, based on their brightness. No one had actually seen their disks. Cassini provided data that made Enceladus much more interesting to planetary scientists.

The brightness of Iapetus presented a major problem. When Gian Domenico Cassini found it in 1671, he realized that this odd satellite must be far brighter on one side (the leading hemisphere as it orbits Saturn) than on the other. Diameter measurements were impossible until Voyager photographed the disk. It proved to be about half as big as the moon, with one side bright and icy. The other side is mostly covered with a layer of tarlike black material that hid any surface features from the cameras on Voyager 2. Similarly, little was known about Hyperion, another dark satellite orbiting between Titan and Iapetus; it was found by Voyager 2 to be irregularly shaped and tumbling without any rotational period.

Phoebe, the outermost known satellite, is distinguished by its retrograde orbit, like four of the outermost satellites of Jupiter. Like the dark side of Iapetus, Phoebe may be covered with carbon-rich material. More puzzling, the existence of Janus, a tenth moon, had been suspected, but before Voyager, photographs showed it in the wrong place. Voyager 1 detected two satellites sharing the same orbit. The other seven of Saturn’s major satellites were not known prior to the Voyager missions. Cassini added considerably to the total list of Saturn’s family of satellites.

Context

Practically nothing was known about Saturn’s satellites prior to the Voyager flybys. Titan, Dione, Mimas, and Rhea were examined most fully by Voyager 1, in November, 1980. Until the arrival of Cassini in Saturn orbit, most information about Enceladus, Iapetus, Hyperion, and Tethys had come from Voyager 2 in August, 1981. Much about these satellites was discovered or confirmed by Voyager 1, but thanks to improved orbital data, they were best photographed by Voyager 2. Clearly, a strong argument can be made for using two spacecraft in flyby missions.

In brief, the Voyager missions found Saturn’s satellite family to be a very diverse lot. Even satellites similar in size and mass, such as Mimas and Enceladus, appeared very different up close, and obviously were shaped by different processes. Each satellite has its own history of impacts. Tidal stress has played an important role in the evolution of many of these bodies, as it has in the Jovian satellite system. Each satellite has its own fascinating evolutionary story to be interpreted by geologists.

With Cassini repeatedly orbiting Saturn and conducting numerous flybys of many of the satellites, planetary scientists were able to make comparisons over time. Just as the Voyagers had piqued interest in satellites that had once been thought to be merely crater-pocked ice balls, Cassini images revealed many of the satellites not well studied by the Voyagers to also be rather intriguing in totally unexpected ways. Interest in Endeladus, for example, increased greatly due to Cassini observations.

In 2023 another major discovery was made concerning Saturn. This was the confirmation of an additional 62 satellites. This increased the total number of known moons orbiting the planet to 145. Jupiter was previously believed to host the largest number of moons in the solar system with 95. Not only is Saturn now confirmed as having more satellites than the rest of the planets combined, but it is also the most of any planet observed in any galaxy. These newly discovered satellites had to first be established as not being asteroids. They are also considered to be “irregular moons” as they are theorized as having been free-floating objects captured by the pull of Saturn’s gravity.

Bibliography:

"Cassini." NASA Solar System Exploration, 25 Sept. 2023, solarsystem.nasa.gov/missions/cassini/science/rings. Accessed 26 Sept. 2023.

Consolmagno, Guy. Worlds Apart: A Textbook in Planetary Sciences. Englewood Cliffs, NJ: Prentice Hall, 1994. Print.

Encrenaz, Thérèse, et al. The Solar System. New York: Springer, 2004. Print.

Harland, David M. Cassini at Saturn: Huygens Results. New York: Springer, 2007. Print.

Harland, David M. Mission to Saturn: Cassini and the Huygens Probe. New York: Springer Praxis, 2002. Print.

Hartmann, William K. Moons and Planets. 5th ed. Belmont, CA: Thomson Brooks/Cole, 2005. Print.

Irwin, Patrick G. J. Giant Planets of Our Solar System: An Introduction. 2d ed. New York: Springer, 2006. Print.

Lea, Robert. "Saturn Reclaims 'Moon King' Title with 62 Newfound Satellites, Bringing Total to 145". Space.com, 12 May, 2023, www.space.com/saturn-moon-king-62-newfound-satellites. Accessed 27 Sept. 2023.

Leverington, David. Babylon to Voyager and Beyond: A History of Planetary Astronomy. New York: Cambridge UP, 2003. Print.

Lorenz, Ralph, and Jacqueline Mitton. Lifting Titan’s Veil: Exploring the Giant Moon of Saturn. New York: Cambridge UP, 2002. Print.

Morrison, David, and Tobias Owen. The Planetary System. 3d ed. San Francisco: Pearson/Addison-Wesley, 2003. Print.

Müllerller-Wodarg, Ingo, et al. Titan: Interior, Surface, Atmosphere, and Space Environment. New York: Cambridge UP, 2013. Print.

"Saturn." NASA Solar System Exploration, 25 Sept. 2023, solarsystem.nasa.gov/planets/saturn/in-depth/#." Accessed 26 Sept. 2023.

Van Pelt, Michel. Space Invaders: How Robotic Spacecraft Explore the Solar System. New York: Springer, 2006. Print.