The ‘Thrusters’ on Saturn’s Moon, Enceladus

The ‘Thrusters’ on Saturn’s Moon, Enceladus

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Enceladus is one of the moons of Saturn, bearing the Greek Mythology name of one of the Giants, child of Earth and Uranus, with serpent like lower part of the body. Enceladus is connected to earthquakes and according to the mythology he was buried under Mount Etna in Italy.

The moon was first observed in 1789 by a British astronomer and because of its volcanic behaviour it was given the name of Enceladus. It a sphere of about 500 km in size and the surface is icy. It was in 1980 that Voygaer passed at a distance of 200,000 km from Enceladus and we got the first low quality pictures of it. There is a possibility that Enceladus could have liquid water , as many scientists suggest.

The Cassini spacecraft has done an incredible job of getting high quality pictures since 2005, with multiple fly-bys close to the moon and more to come until 2017. In recent images taken by Cassin, Enceladus looks as if it has rockets to thrust it. This is the effect of icy salty water coming out from the south pole of the Moon, an effect that has been studied and was first observed in 2005.

The liquid water is expelled from the surface of the planet due to high pressure created by the volcanic activity of the planet. The jet streams that have been observed so far are 98 in number.

The most intriguing part of Enceladus is not the jet streams, but the possibility to have fluid water on the surface and a heat source (volcanic activity). Because then there is a large possibility that it also has life and maybe is a habitable environment (not for humans of course …).

    The best places to find extraterrestrial life in our solar system, ranked

    An illustration of Cassini diving through plumes erupting off Europa's surface. NASA/JPL-Caltech

    If you want to believe, now is the time: the hope that we might one day stumble upon alien life is greater than it ever was. No, it’s not going to be little green men speeding through space in flying disks—more likely microbes or primitive bacteria. But a discovery like that would nevertheless be a sign that we are not alone in the universe—that life elsewhere is a possibility.

    Where are we going to find that life? It was once thought the solar system was probably a barren wasteland apart from Earth. Rocky neighbors were too dry and cold like Mars, or too hot and hellish like Venus. The other planets were gas giants, and life on those worlds or their satellite moons was basically inconceivable. Earth seemed to be a miracle of a miracle.

    But life isn’t that simple. We now know that life on Earth is able to thrive in even the harshest, most brutal environments, in super cold and super dry conditions, depths of unimaginable pressures, and without the need to use sunlight as a source of energy. At the same time, our cursory understanding of these obscure worlds has expanded tremendously. Our rocky neighbors of Venus and Mars may have once been temperate and Earth-like, and some of the life might have lingered on after these planets’ climates took a turn for the worse. Several of the icy moons that hang around Jupiter and Saturn might have underground oceans that could sustain life. A couple may even have atmospheres. And still other places that seem to be too exotic for life continue to surprise us.

    Unlike the myriad of new exoplanets we’re identifying every year, when it comes to worlds in the solar system, we have the ability to send probes to these places and study them directly. “We can measure things that would be impossible to measure with telescopes,” says David Catling, an astrobiologist at the University of Washington. They could study things up close, maybe fly into the atmosphere or land on the surface, and perhaps one day even bring back samples that could reveal whether these planets and moons are home to materials or fossils that are evidence of life—or perhaps life itself.

    Here are the 10 best places in the solar system to look for extraterrestrial life, subjectively ranked by yours truly for how likely we are to find life—and how easy it would be to find it if it’s there.

    10. Triton

    Triton is the largest moon of Neptune, and one of the most exotic worlds in the solar system. It’s one of only five moons in the solar system known to be geologically active, as evidenced by its active geysers that spew sublimated nitrogen gas. Its surface is mostly frozen nitrogen, and its crust is made of water ice, and it has an icy mantle. Yes, this is a cold, cold world. But in spite of that, it seems to get some heat generated by tidal forces (gravitational friction between Triton and Neptune), and that could help warm up the waters and give rise to life through any organics that might exist on the moon.

    But actually finding life on Triton seems like a very distant possibility. The only mission to ever visit the world was Voyager 2 in 1989. The window for such a mission opens up only every 13 years. The best opportunity to visit Triton would be the proposed Trident mission (which seems unlikely to get launched after NASA just greenlighted two new missions to Venus later this decade). And lastly, the horrendous cold tempers hopes that life could stay unfrozen for long enough to make a home for itself.

    9. Ceres

    The largest asteroid and smallest dwarf planet in the solar system could be home to liquid water, sitting deep underground. Ceres, a dwarf planet that sits between Mars and Jupiter, was studied by NASA’s Dawn probe from orbit from 2015 to 2018. Scientists are still unpacking and analyzing that data, but tantalizing studies in the past few years suggest there’s an ocean sitting 25 miles below the surface, and could stretch for hundreds of miles. It would almost certainly be extremely salty—which would keep the water from freezing even well below 0°C. Dawn even found evidence of organic compounds on Ceres that could act as raw materials for life.

    But Ceres ranks second-to-last on our list because its habitability has too many questions attached. The evidence of subsurface water and the organic materials is still very new. Even if those things are there, it would need some source of heat and energy that could actually help encourage that water and organic material to react in such a way that it leads to life. And even if that occurred, finding that life means we have to drill at least two-dozen miles into the ground to access that water and study it. Lastly, Ceres is tiny—more than 13 times smaller than Earth. It’s not yet clear how that fraction of gravity could affect life on the dwarf planet, but if Earth is our compass for what’s habitable, Ceres’s small size is probably not an asset. There’s no shortage of new proposals for future missions to study the dwarf planet, including ones that would even attempt a sample return mission. But nothing is going up soon.

    8. Io

    Boasting over 400 active volcanoes, Io is the most geologically active world in the solar system. All of that activity is thought to be caused by tidal heating created as Io’s interior is gravitationally pulled between Jupiter and the other Jovian moons. The volcanism results in a huge coating of sulfur and sulfur dioxide frost (yes, that’s a thing!) across the globe, along with a super thin sulfur dioxide atmosphere. There might even be a subsurface ocean on Io, but it would be made of magma, not water.

    Life on Io is very unlikely. But all that heat is a bit of an encouraging sign. There may be locations on the surface or underground that aren’t overwhelmed by the volcanic activity—more temperate places where hardy forms of life have found a way to survive. We wouldn’t be able to study those spots directly, but a probe might be able to find evidence of life if it gets lucky.

    That’s easier said than done. The best chance of studying Io is through a proposed NASA mission called Io Volcano Observer (IVO), which if approved would launch in 2029 and do ten flybys of Io. But like Trident, IVO was vying for the same mission spots that were snatched by two upcoming Venus missions.

    7. Calisto

    Calisto’s claim to fame is that it has the oldest surface in the solar system. That doesn’t really mean much in terms of habitability though. Where Calisto shines for our purposes is that it’s another moon that’s thought to have a vast subsurface ocean, 155 miles underground. It also retains a thin atmosphere of hydrogen, carbon dioxide, and oxygen, which is more diverse and Earth-like than most of the other solar system moons that could be habitable.

    Still, Callisto’s chances of hosting life are not as favorable as other worlds, namely because it's still pretty damn cold. Our next best chance of really exploring it will be the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), launching next year and set to explore three of Jupiter’s moons. JUICE will make several close flybys of Callisto during its mission.

    6. Ganymede

    The largest moon to orbit Jupiter, and simply the largest moon in the solar system, is covered up in an icy shell. But underneath that surface is home to a global underground saltwater ocean that might contain more water than all of Earth’s own oceans combined. Naturally, all that water has scientists hopeful that some kind of life could exist on the moon. The moon even has a very thin oxygen atmosphere—nothing to write home about, but it’s something neat. And Ganymede has something else no other moon in the solar system has: a magnetic field. A magnetic field is critical for protecting worlds from harmful radiation spewed by the sun.

    But Ganymede isn’t perfect. A subsurface ocean is difficult to study, so if there’s life on the planet, we’re going to have a difficult time finding it. And so far, there has not yet been a dedicated mission to study Ganymede, although the JUICE will be the most in-depth investigation of Ganymede when it enters the moon’s orbit in 2032. It may have an opportunity to peer down at the surface and study the interior using radar, and clue scientists into Ganymede’s potential habitability.

    5. Venus

    Here at the halfway point is where we start to get into the good stuff. Venus has surface temperatures that are hot enough to melt lead, and surface pressures that are more than 80 times as harsh as what we experience on Earth. And yet, maybe Venus is home to life! Those prospects ignited last year when researchers detected phosphine gas in very thick Venusian atmosphere. On Earth, phosphine is primarily produced naturally by life in oxygen-poor ecosystems, which raises the possibility that there might be life on Venus responsible for producing it as well. And the most likely scenario would be microbial life that’s hanging within the clouds—airborne life, basically.

    Now, the phosphine detections have come under scrutiny, and the idea of airborne life is certainly not something all scientists can get behind. But this and other work that’s explored Venus’s history of water have renewed a lot of great interest into the idea that Venus may have once been habitable, and might still be. The new DAVINCI+ and VERITAS missions that NASA will launch late this decade won’t find life, but they will get us closer to answering that question more concretely.

    4. Enceladus

    Saturn’s sixth largest moon is completely covered in clean ice, making it one of the most reflective bodies in the solar system. Its surface is ice cold, but there’s quite a bit of activity going on underneath. The moon ejects plumes that contain a myriad of different compounds, including salt water, ammonia, and organic molecules like methane and propane. Enceladus is thought to have a global salty ocean. And NASA’s found evidence of hydrothermal activity deep underground, which could very well provide a source of heat that’s necessary to give life a chance to evolve and thrive.

    In some ways, Enceladus ought to be higher up my list than Titan, were it not for the fact that there just simply isn’t any mission on the books right now to study it. Plenty of proposals have been debated for the last several years, including several under NASA. All are geared toward an astrobiological investigation that would look more closely for signs that Enceladus is habitable to life. While digging underground into the ocean would be the most surefire way to determine whether the moon is home to life, we might also catch a lucky break and be able to detect biosignatures that have been spewed up by the moon’s cryovolcanoes (volcanos that erupt vaporized materials like water or ammonia rather than molten rock). But not for a long time.

    3. Titan

    Titan, Saturn’s largest moon, is another world that sets itself apart from the rest of the solar system. It has one of the most robust atmospheres for a rocky world in the solar system outside of Earth and Venus. It's teeming with different bodies of liquid: lakes, rivers, and seas. But they’re not made of water—they’re made of methane and other hydrocarbons. Titan is extremely rich in organic materials, so it's already rich in the raw materials needed for life. And it may also have a subsurface ocean of water as well, though this will need to be verified.

    Scientists have just the mission lined up: the NASA Dragonfly mission, which will send a drone helicopter to explore Titan’s atmosphere directly and give us a much needed sense of exactly how developed its prebiotic chemistry runs. That mission launches in 2027 and will arrive at Titan in 2034.

    2. Europa

    Jupiter’s moon has an icy shell that’s 10 to 15 miles thick covering up a huge subsurface ocean that’s heated up by tidal forces. That heating is thought to help create an internal circulation system that keeps waters moving and replenishes the icy surface on a regular basis. This means the ocean floor is interacting with the surface—which means if we want to determine whether life exists in those subsurface oceans, we may not necessarily need to go all the way down there. Scientists have found deposits of clay-like minerals associated with organic materials on Europa. And it's suspected that radiation hitting the icy surface could result in oxygen that might find its way into the subsurface oceans and be used by emerging life. All the ingredients for life are potentially here.

    Luckily, we’re set to study Europa in great detail. JUICE will make two flybys of Europa during its time in the Jovian system. But the marquee mission on the books is Europa Clipper, a spacecraft that would conduct low-altitude flights that would attempt to study and characterize the surface, and investigate the subsurface environment as best it can. Clipper launches in 2024, and will reach Europa in 2030.

    1. Mars

    Mars takes the top spot for several reasons. We know it was once habitable billions of years ago, when it had lakes and rivers of liquid water on its surface. We know it had a robust atmosphere back then to keep things warm and comfy. And we currently have a rover on the surface, Perseverance, whose express goal is to look for signs of ancient life. It will even secure samples that we’ll one day bring back to Earth to study in the lab.

    So what does that have to do with finding current life? Well, if there are signs of ancient life, it’s possible life on Mars still exists. Probably not on the surface, but maybe underground. There have already been a few big studies that have used radar observations to show that reservoirs of liquid water probably exist a couple kilometers below the surface. We’ve found bacteria on Earth surviving in similar conditions, so it’s entirely possible something is living in those parts of Mars as well. Getting down there will be insanely difficult, but if we have reason to believe something is lurking in these reservoirs, it’ll be all hands on deck to figure out how we can get there and see for ourselves.


    The plumes discovered by the Cassini mission emanating from the south pole of Saturn׳s moon Enceladus and the unique chemistry found in them have fueled speculations that Enceladus may harbor life. The presumed aquiferous fractures from which the plumes emanate would make a prime target in the search for extraterrestrial life and would be more easily accessible than the moon׳s subglacial ocean.

    A lander mission that is equipped with a subsurface maneuverable ice melting probe will be most suitable to assess the existence of life on Enceladus. A lander would have to land at a safe distance away from a plume source and melt its way to the inner wall of the fracture to analyze the plume subsurface liquids before potential biosignatures are degraded or destroyed by exposure to the vacuum of space. A possible approach for the in situ detection of biosignatures in such samples can be based on the hypothesis of universal evolutionary convergence, meaning that the independent and repeated emergence of life and certain adaptive traits is wide-spread throughout the cosmos. We thus present a hypothetical evolutionary trajectory leading towards the emergence of methanogenic chemoautotrophic microorganisms as the baseline for putative biological complexity on Enceladus. To detect their presence, several instruments are proposed that may be taken aboard a future subglacial melting probe.

    The “Enceladus Explorer” (EnEx) project funded by the German Space Administration (DLR), aims to develop a terrestrial navigation system for a subglacial research probe and eventually test it under realistic conditions in Antarctica using the EnEx-IceMole, a novel maneuverable subsurface ice melting probe for clean sampling and in situ analysis of ice and subglacial liquids. As part of the EnEx project, an initial concept study is foreseen for a lander mission to Enceladus to deploy the IceMole near one of the active water plumes on the moon׳s South-Polar Terrain, where it will search for signatures of life.

    The general mission concept is to place the Lander at a safe distance from an active plume. The IceMole would then be deployed to melt its way through the ice crust to an aquiferous fracture at a depth of 100 m or more for an in situ examination for the presence of microorganisms.

    The driving requirement for the mission is the high energy demand by the IceMole to melt through the cold Enceladan ices. This requirement is met by a nuclear reactor providing 5 kW of electrical power. The nuclear reactor and the IceMole are placed on a pallet lander platform. An Orbiter element is also foreseen, with the main function of acting as a communications relay between Lander and Earth.

    After launch, the Lander and Orbiter will perform the interplanetary transfer to Saturn together, using the on-board nuclear reactor to power electric thrusters. After Saturn orbit insertion, the Combined Spacecraft will continue using Nuclear Electric Propulsion to reach the orbit of Enceladus. After orbit insertion at Enceladus, the Orbiter will perform a detailed reconnaissance of the South-Polar Terrain. At the end of the reconnaissance phase, the Lander will separate from the Orbiter and an autonomously guided landing sequence will place it near one of the active vapor plumes. Once landed, the IceMole will be deployed and start melting through the ice, while navigating around hazards and towards a target subglacial aquiferous fracture.

    An initial estimation of the mission׳s cost is given, as well as recommendations on the further development of enabling technologies. The planetary protection challenges posed by such a mission are also addressed.

    How could Enceladus possibly host life?

    Enceladus has water, organics and energy sources—the three ingredients for living organisms. NASA’s now-dead probe Cassini found traces of salt and sand, suggesting the ocean was in contact with the moon’s rocky core, as well as formaldehyde and acetylene.

    Then in 2017, Cassini detected molecular hydrogen as it flew through plumes from the Enceladean ocean that leaks into space through cracks in its ice-shell near the moon’s south pole. Abiotic microbes that exist around hydrothermal vents on the ocean floor on Earth feed-off hydrogen and carbon dioxide. Enceladus has carbon, hydrogen, nitrogen and oxygen.

    Could there be hydrothermal vents on Enceladus’s seafloor? Either way, Enceladus has chemistry that could support simple microbial life.


    After 20 years in space, the Cassini spacecraft is running out of fuel. In 2010, Cassini began a seven-year mission extension in which the plan was to expend all of the spacecraft’s propellant exploring Saturn, which led to the Grand Finale and ends with a plunge into the planet’s atmosphere.

    Cassini’s final 22 orbits carried the spacecraft on an elliptical path, diving at tens of thousands of miles per hour through the 1,500-mile-wide (2,400-kilometer-wide) space between the rings and the planet, where no spacecraft has explored before.

    "In the Grand Finale orbits [we will] for the first time address the question of the origin and the age of the rings. We'll do this by measuring the mass of the rings very accurately. If the rings are a lot more massive than we expect, perhaps they're old, as old as Saturn itself, and they've been massive enough to survive the micrometeoroid bombardment and erosion and leave us with the rings we see today."

    Linda Spilker

    Cassini Project Scientist

    Each of these last 22 orbits took about six and a half days to complete. They began April 22 and end Sept. 15. When Cassini was nearest to Saturn during each orbit, the spacecraft’s speed ranged between 75,000 and 78,000 miles per hour (121,000 and 126,000 kilometers per hour), depending on the orbit.

    The Goodbye Kiss

    Cassini got within 75,000 miles (120,000 kilometers) of Saturn’s giant moon Titan, whose gravity changed the spacecraft’s trajectory ever so slightly, ensuring that Cassini’s next transit through Saturn’s atmosphere will be too deep for the spacecraft to survive.


    Cassini is at the point in its elliptical orbit that’s farthest from Saturn. The spacecraft will never be this far from the planet again. From here on Cassini only gets closer to Saturn and accelerates for about three days until it enters Saturn’s atmosphere.

    Final Downlink

    Cassini turns to Earth and transmits everything on its data recorders. Because of Earth’s rotation, this 11-hour downlink begins with the NASA Deep Space Network (or DSN) antenna station in California, which then hands off receiving to a station in Australia.

    From this point the spacecraft holds this orientation — its antenna pointed toward Earth — for the remaining 14.5 hours of the mission.

    Powering up for the final plunge

    Throughout the mission, Cassini has primarily relied upon its reaction wheels for fine adjustments to its orientation, especially during science observations.

    But from now through end of mission, the spacecraft will only use thrusters because their power is necessary to fight against the push of Saturn’s atmosphere. After 20 years, the reaction wheels retire.

    The Final Handoff

    As soon as Earth rotates enough for the DSN’s station in Australia to detect Cassini’s signal, that station begins downlinking the spacecraft’s data. The station in California continues receiving so that data overlaps with that received in Australia. About 20 minutes later, Earth’s rotation pulls Saturn out of view of the California antennas, and the Australia station alone receives Cassini’s signal.

    Real-Time downlink is initiated

    From here on, Cassini's purpose is to transmit every bit of data possible before the spacecraft is destroyed. Typically, Cassini holds onto science data for hours or days after it’s recorded, but the spacecraft is running out of time. So Cassini now transmits data just a few seconds after recording it.

    The deeper the spacecraft descends into Saturn’s atmosphere, the more precious the science data gets. Cassini won’t get a second chance to send this unique data to Earth.

    September 15, 3:30:50 am PDT

    Atmospheric entry begins

    Cassini is traveling about 77,000 miles (123,000 kilometers) per hour as it enters Saturn's upper atmosphere. The attitude control thrusters fire at 10 percent of their capacity, and the spacecraft is approximately 1,200 miles (1,900 kilometers) above Saturn’s cloud tops.

    September 15, 3:31:48 am PDT

    Thrusters at maximum

    The attitude control thrusters keeping the spacecraft's antenna pointed at Earth are firing at 100 percent of capacity. The spacecraft is directly sampling Saturn’s atmosphere from about 190 miles (300 kilometers) deeper into Saturn than on any of its previous orbits. The molecules in Saturn's atmosphere can't get out of Cassini's way fast enough, so their heat starts building up on the spacecraft's forward-facing surfaces. Cassini begins getting warmer.

    September 15, 3:32:00 am PDT

    Loss of Signal

    At about 930 miles (1,500 kilometers) above the cloud tops, the attitude control thrusters fighting to keep Cassini stable can't win against the increasingly dense atmosphere. Cassini begins to slowly tumble, and permanently loses contact with Earth.

    The last bits of Cassini's final signal won't reach Earth for nearly an hour and a half, due to the travel time for its radio signal at the speed of light. Technically, its mission is now at an end.

    “The reaction control system thrusters are at 100 percent. A minute before that, it was 10 percent — the atmospheric density goes up about an order of magnitude per minute.”

    Mission engineers have used computer models to predict what will happen after loss of signal. Though they know what will ultimately become of the spacecraft, it’s difficult to be absolutely certain about the timing and chronology of some of the events. That said, here’s what they predict:

    The spacecraft rams through Saturn’s atmosphere at four times the speed of a re-entry vehicle entering Earth’s atmosphere, and Cassini has no heat shield. So temperatures around the spacecraft will increase by 30-to-100 times per minute, and every component of the spacecraft will disintegrate over the next couple of minutes…

    The spacecraft is now traveling about 77,200 miles (144,200 kilometers) per hour through Saturn’s upper atmosphere, about 700 miles (1,100 kilometers) above Saturn’s cloud tops. Under other circumstances, Cassini's gyroscopes, star trackers, and excessive thruster-firing would prompt the computers to begin a series of actions which would eventually lead to a precautionary standby mode known as “safe mode.”

    Per its programming, the spacecraft's computers would typically command all science instruments and other non-essential systems to shut down so that all available power can focus on re-establishing communication with Earth. Cassini would then attempt to stop tumbling using its thrusters, find the Sun with its solar detectors, center its antenna on the Sun, use its star-trackers to tweak its orientation to point at Earth, and radio home. But by this time, the spacecraft’s computer will likely have overheated, causing it to fail.

    Cassini’s gold-colored multi-layer insulation blankets will char and break apart, and then the spacecraft's carbon fiber epoxy structures, such as the 11-foot (3-meter) wide high-gain antenna and the 30-foot (11-meter) long magnetometer boom, will weaken and break apart. Components mounted on the outside of the central body of the spacecraft will then break apart, followed by the leading face of the spacecraft itself.

    Temperatures around what remains of the spacecraft eventually exceed those on the surface of the Sun. Heating and expansion of gases inside the propellant tanks may cause them to explode. The tanks make up the spacecraft's central body, so their rupture would blast apart what's left of the spacecraft. The debris is then completely consumed in the planet's atmosphere. Cassini's materials will sink deep into Saturn and mix with the hot, high-pressure atmosphere of the giant planet to be completely diluted.


    Early observations Edit

    Before the advent of telescopic photography, eight moons of Saturn were discovered by direct observation using optical telescopes. Saturn's largest moon, Titan, was discovered in 1655 by Christiaan Huygens using a 57-millimeter (2.2 in) objective lens [12] on a refracting telescope of his own design. [13] Tethys, Dione, Rhea and Iapetus (the "Sidera Lodoicea") were discovered between 1671 and 1684 by Giovanni Domenico Cassini. [14] Mimas and Enceladus were discovered in 1789 by William Herschel. [14] Hyperion was discovered in 1848 by W.C. Bond, G.P. Bond [15] and William Lassell. [16]

    The use of long-exposure photographic plates made possible the discovery of additional moons. The first to be discovered in this manner, Phoebe, was found in 1899 by W.H. Pickering. [17] In 1966 the tenth satellite of Saturn was discovered by Audouin Dollfus, when the rings were observed edge-on near an equinox. [18] It was later named Janus. A few years later it was realized that all observations of 1966 could only be explained if another satellite had been present and that it had an orbit similar to that of Janus. [18] This object is now known as Epimetheus, the eleventh moon of Saturn. It shares the same orbit with Janus—the only known example of co-orbitals in the Solar System. [19] In 1980, three additional Saturnian moons were discovered from the ground and later confirmed by the Voyager probes. They are trojan moons of Dione (Helene) and Tethys (Telesto and Calypso). [19]

    Observations by spacecraft Edit

    The study of the outer planets has since been revolutionized by the use of unmanned space probes. The arrival of the Voyager spacecraft at Saturn in 1980–1981 resulted in the discovery of three additional moons – Atlas, Prometheus and Pandora, bringing the total to 17. [19] In addition, Epimetheus was confirmed as distinct from Janus. In 1990, Pan was discovered in archival Voyager images. [19]

    The Cassini mission, [20] which arrived at Saturn in the summer of 2004, initially discovered three small inner moons including Methone and Pallene between Mimas and Enceladus as well as the second trojan moon of Dione – Polydeuces. It also observed three suspected but unconfirmed moons in the F Ring. [21] In November 2004 Cassini scientists announced that the structure of Saturn's rings indicates the presence of several more moons orbiting within the rings, although only one, Daphnis, had been visually confirmed at the time. [22] In 2007 Anthe was announced. [23] In 2008 it was reported that Cassini observations of a depletion of energetic electrons in Saturn's magnetosphere near Rhea might be the signature of a tenuous ring system around Saturn's second largest moon. [24] In March 2009 , Aegaeon, a moonlet within the G Ring, was announced. [25] In July of the same year, S/2009 S 1, the first moonlet within the B Ring, was observed. [4] In April 2014, the possible beginning of a new moon, within the A Ring, was reported. [26] (related image)

    Outer moons Edit

    Study of Saturn's moons has also been aided by advances in telescope instrumentation, primarily the introduction of digital charge-coupled devices which replaced photographic plates. For the entire 20th century, Phoebe stood alone among Saturn's known moons with its highly irregular orbit. Beginning in 2000, however, three dozen additional irregular moons have been discovered using ground-based telescopes. [27] A survey starting in late 2000 and conducted using three medium-size telescopes found thirteen new moons orbiting Saturn at a great distance, in eccentric orbits, which are highly inclined to both the equator of Saturn and the ecliptic. [28] They are probably fragments of larger bodies captured by Saturn's gravitational pull. [27] [28] In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons, [29] [30] in 2006, astronomers using the Subaru 8.2 m telescope reported the discovery of nine more irregular moons, [31] in April 2007 , Tarqeq (S/2007 S 1) was announced and in May of the same year S/2007 S 2 and S/2007 S 3 were reported. [32] In 2019, twenty new irregular satellites of Saturn were reported, resulting in Saturn overtaking Jupiter as the planet with the most known moons for the first time since 2000. [11] [33]

    Some of the 82 known satellites of Saturn are considered lost because they have not been observed since their discovery and hence their orbits are not known well enough to pinpoint their current locations. [34] [35] Work has been done to recover many of them in surveys from 2009 onwards, but five – S/2004 S 13, S/2004 S 17, S/2004 S 12, S/2004 S 7, and S/2007 S 3 – still remain lost today. [33]

    Naming Edit

    The modern names for Saturnian moons were suggested by John Herschel in 1847. [14] He proposed to name them after mythological figures associated with the Roman titan of time, Saturn (equated to the Greek Cronus). [14] In particular, the then known seven satellites were named after Titans, Titanesses and Giants—brothers and sisters of Cronus. [17] In 1848, Lassell proposed that the eighth satellite of Saturn be named Hyperion after another Titan. [16] When in the 20th century the names of Titans were exhausted, the moons were named after different characters of the Greco-Roman mythology or giants from other mythologies. [36] All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and after Norse ice giants. [37]

    Some asteroids share the same names as moons of Saturn: 55 Pandora, 106 Dione, 577 Rhea, 1809 Prometheus, 1810 Epimetheus, and 4450 Pan. In addition, two more asteroids previously shared the names of Saturnian moons until spelling differences were made permanent by the International Astronomical Union (IAU): Calypso and asteroid 53 Kalypso and Helene and asteroid 101 Helena.

    Saturn's satellite system is very lopsided: one moon, Titan, comprises more than 96% of the mass in orbit around the planet. The six other planemo (ellipsoidal) moons constitute roughly 4% of the mass, and the remaining 75 small moons, together with the rings, comprise only 0.04%. [a]

    Saturn's major satellites, compared to the Moon
    (km) [38]
    (kg) [39]
    Orbital radius
    (km) [40]
    Orbital period
    (days) [40]
    Mimas 396
    (12% Moon)
    4×10 19
    (0.05% Moon)
    (48% Moon)
    (3% Moon)
    Enceladus 504
    (14% Moon)
    1.1×10 20
    (0.2% Moon)
    (62% Moon)
    (5% Moon)
    Tethys 1,062
    (30% Moon)
    6.2×10 20
    (0.8% Moon)
    (77% Moon)
    (7% Moon)
    Dione 1,123
    (32% Moon)
    1.1×10 21
    (1.5% Moon)
    (98% Moon)
    (10% Moon)
    Rhea 1,527
    (44% Moon)
    2.3×10 21
    (3% Moon)
    (137% Moon)
    (20% Moon)
    Titan 5,149
    (148% Moon)
    (75% Mars)
    1.35×10 23
    (180% Moon)
    (318% Moon)
    (60% Moon)
    Iapetus 1,470
    (42% Moon)
    1.8×10 21
    (2.5% Moon)
    (926% Moon)
    (290% Moon)

    Although the boundaries may be somewhat vague, Saturn's moons can be divided into ten groups according to their orbital characteristics. Many of them, such as Pan and Daphnis, orbit within Saturn's ring system and have orbital periods only slightly longer than the planet's rotation period. [41] The innermost moons and most regular satellites all have mean orbital inclinations ranging from less than a degree to about 1.5 degrees (except Iapetus, which has an inclination of 7.57 degrees) and small orbital eccentricities. [33] On the other hand, irregular satellites in the outermost regions of Saturn's moon system, in particular the Norse group, have orbital radii of millions of kilometers and orbital periods lasting several years. The moons of the Norse group also orbit in the opposite direction to Saturn's rotation. [37]

    Ring moonlets Edit

    During late July 2009, a moonlet, S/2009 S 1, was discovered in the B Ring, 480 km from the outer edge of the ring, by the shadow it cast. [4] It is estimated to be 300 m in diameter. Unlike the A Ring moonlets (see below), it does not induce a 'propeller' feature, probably due to the density of the B Ring. [42]

    In 2006, four tiny moonlets were found in Cassini images of the A Ring. [43] Before this discovery only two larger moons had been known within gaps in the A Ring: Pan and Daphnis. These are large enough to clear continuous gaps in the ring. [43] In contrast, a moonlet is only massive enough to clear two small—about 10 km across—partial gaps in the immediate vicinity of the moonlet itself creating a structure shaped like an airplane propeller. [44] The moonlets themselves are tiny, ranging from about 40 to 500 meters in diameter, and are too small to be seen directly. [9]

    In 2007, the discovery of 150 more moonlets revealed that they (with the exception of two that have been seen outside the Encke gap) are confined to three narrow bands in the A Ring between 126,750 and 132,000 km from Saturn's center. Each band is about a thousand kilometers wide, which is less than 1% the width of Saturn's rings. [9] This region is relatively free from the disturbances caused by resonances with larger satellites, [9] although other areas of the A Ring without disturbances are apparently free of moonlets. The moonlets were probably formed from the breakup of a larger satellite. [44] It is estimated that the A Ring contains 7,000–8,000 propellers larger than 0.8 km in size and millions larger than 0.25 km. [9] In April 2014, NASA scientists reported the possible consolidation of a new moon within the A Ring, implying that Saturn's present moons may have formed in a similar process in the past when Saturn's ring system was much more massive. [26]

    Similar moonlets may reside in the F Ring. [9] There, "jets" of material may be due to collisions, initiated by perturbations from the nearby small moon Prometheus, of these moonlets with the core of the F Ring. One of the largest F Ring moonlets may be the as-yet unconfirmed object S/2004 S 6. The F Ring also contains transient "fans" which are thought to result from even smaller moonlets, about 1 km in diameter, orbiting near the F Ring core. [45]

    One of the recently discovered moons, Aegaeon, resides within the bright arc of G Ring and is trapped in the 7:6 mean-motion resonance with Mimas. [25] This means that it makes exactly seven revolutions around Saturn while Mimas makes exactly six. The moon is the largest among the population of bodies that are sources of dust in this ring. [46]

    Ring shepherds Edit

    Shepherd satellites are small moons that orbit within, or just beyond, a planet's ring system. They have the effect of sculpting the rings: giving them sharp edges, and creating gaps between them. Saturn's shepherd moons are Pan (Encke gap), Daphnis (Keeler gap), Atlas (A Ring), Prometheus (F Ring) and Pandora (F Ring). [21] [25] These moons together with co-orbitals (see below) probably formed as a result of accretion of the friable ring material on preexisting denser cores. The cores with sizes from one-third to one-half the present-day moons may be themselves collisional shards formed when a parental satellite of the rings disintegrated. [41]

    Co-orbitals Edit

    Janus and Epimetheus are called co-orbital moons. [19] They are of roughly equal size, with Janus being slightly larger than Epimetheus. [41] Janus and Epimetheus have orbits with only a few kilometers difference in semi-major axis, close enough that they would collide if they attempted to pass each other. Instead of colliding, however, their gravitational interaction causes them to swap orbits every four years. [47]

    Inner large moons Edit

    The innermost large moons of Saturn orbit within its tenuous E Ring, along with three smaller moons of the Alkyonides group.

      is the smallest and least massive of the inner round moons, [39] although its mass is sufficient to alter the orbit of Methone. [47] It is noticeably ovoid-shaped, having been made shorter at the poles and longer at the equator (by about 20 km) by the effects of Saturn's gravity. [48] Mimas has a large impact crater one-third its diameter, Herschel, situated on its leading hemisphere. [49] Mimas has no known past or present geologic activity, and its surface is dominated by impact craters. The only tectonic features known are a few arcuate and linear troughs, which probably formed when Mimas was shattered by the Herschel impact. [49] is one of the smallest of Saturn's moons that is spherical in shape—only Mimas is smaller [48] —yet is the only small Saturnian moon that is currently endogenously active, and the smallest known body in the Solar System that is geologically active today. [50] Its surface is morphologically diverse it includes ancient heavily cratered terrain as well as younger smooth areas with few impact craters. Many plains on Enceladus are fractured and intersected by systems of lineaments. [50] The area around its south pole was found by Cassini to be unusually warm and cut by a system of fractures about 130 km long called "tiger stripes", some of which emit jets of water vapor and dust. [50] These jets form a large plume off its south pole, which replenishes Saturn's E ring [50] and serves as the main source of ions in the magnetosphere of Saturn. [51] The gas and dust are released with a rate of more than 100 kg/s. Enceladus may have liquid water underneath the south-polar surface. [50] The source of the energy for this cryovolcanism is thought to be a 2:1 mean-motion resonance with Dione. [50] The pure ice on the surface makes Enceladus one of the brightest known objects in the Solar System—its geometrical albedo is more than 140%. [50] is the third largest of Saturn's inner moons. [39] Its most prominent features are a large (400 km diameter) impact crater named Odysseus on its leading hemisphere and a vast canyon system named Ithaca Chasma extending at least 270° around Tethys. [49] The Ithaca Chasma is concentric with Odysseus, and these two features may be related. Tethys appears to have no current geological activity. A heavily cratered hilly terrain occupies the majority of its surface, while a smaller and smoother plains region lies on the hemisphere opposite to that of Odysseus. [49] The plains contain fewer craters and are apparently younger. A sharp boundary separates them from the cratered terrain. There is also a system of extensional troughs radiating away from Odysseus. [49] The density of Tethys (0.985 g/cm 3 ) is less than that of water, indicating that it is made mainly of water ice with only a small fraction of rock. [38] is the second-largest inner moon of Saturn. It has a higher density than the geologically dead Rhea, the largest inner moon, but lower than that of active Enceladus. [48] While the majority of Dione's surface is heavily cratered old terrain, this moon is also covered with an extensive network of troughs and lineaments, indicating that in the past it had global tectonic activity. [52] The troughs and lineaments are especially prominent on the trailing hemisphere, where several intersecting sets of fractures form what is called "wispy terrain". [52] The cratered plains have a few large impact craters reaching 250 km in diameter. [49] Smooth plains with low impact-crater counts are also present on a small fraction of its surface. [53] They were probably tectonically resurfaced relatively later in the geological history of Dione. At two locations within smooth plains strange landforms (depressions) resembling oblong impact craters have been identified, both of which lie at the centers of radiating networks of cracks and troughs [53] these features may be cryovolcanic in origin. Dione may be geologically active even now, although on a scale much smaller than the cryovolcanism of Enceladus. This follows from Cassini magnetic measurements that show Dione is a net source of plasma in the magnetosphere of Saturn, much like Enceladus. [53]

    Alkyonides Edit

    Three small moons orbit between Mimas and Enceladus: Methone, Anthe, and Pallene. Named after the Alkyonides of Greek mythology, they are some of the smallest moons in the Saturn system. Anthe and Methone have very faint ring arcs along their orbits, whereas Pallene has a faint complete ring. [54] Of these three moons, only Methone has been photographed at close range, showing it to be egg-shaped with very few or no craters. [55]

    Trojan moons Edit

    Trojan moons are a unique feature only known from the Saturnian system. A trojan body orbits at either the leading L4 or trailing L5 Lagrange point of a much larger object, such as a large moon or planet. Tethys has two trojan moons, Telesto (leading) and Calypso (trailing), and Dione also has two, Helene (leading) and Polydeuces (trailing). [21] Helene is by far the largest trojan moon, [48] while Polydeuces is the smallest and has the most chaotic orbit. [47] These moons are coated with dusty material that has smoothed out their surfaces. [56]

    Outer large moons Edit

    These moons all orbit beyond the E Ring. They are:

      is the second-largest of Saturn's moons. [48] In 2005 Cassini detected a depletion of electrons in the plasma wake of Rhea, which forms when the co-rotating plasma of Saturn's magnetosphere is absorbed by the moon. [24] The depletion was hypothesized to be caused by the presence of dust-sized particles concentrated in a few faint equatorial rings. [24] Such a ring system would make Rhea the only moon in the Solar System known to have rings. [24] However, subsequent targeted observations of the putative ring plane from several angles by Cassini's narrow-angle camera turned up no evidence of the expected ring material, leaving the origin of the plasma observations unresolved. [57] Otherwise Rhea has rather a typical heavily cratered surface, [49] with the exceptions of a few large Dione-type fractures (wispy terrain) on the trailing hemisphere [58] and a very faint "line" of material at the equator that may have been deposited by material deorbiting from present or former rings. [59] Rhea also has two very large impact basins on its anti-Saturnian hemisphere, which are about 400 and 500 km across. [58] The first, Tirawa, is roughly comparable to the Odysseus basin on Tethys. [49] There is also a 48 km-diameter impact crater called Inktomi[60][b] at 112°W that is prominent because of an extended system of bright rays, [61] which may be one of the youngest craters on the inner moons of Saturn. [58] No evidence of any endogenic activity has been discovered on the surface of Rhea. [58] , at 5,149 km diameter, is the second largest moon in the Solar System and Saturn's largest. [62][39] Out of all the large moons, Titan is the only one with a dense (surface pressure of 1.5 atm), cold atmosphere, primarily made of nitrogen with a small fraction of methane. [63] The dense atmosphere frequently produces bright white convective clouds, especially over the south pole region. [63] On June 6, 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan. [64] On June 23, 2014, NASA claimed to have strong evidence that nitrogen in the atmosphere of Titan came from materials in the Oort cloud, associated with comets, and not from the materials that formed Saturn in earlier times. [65] The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters and is probably very young. [63] It contains a pattern of light and dark regions, flow channels and possibly cryovolcanos. [63][66] Some dark regions are covered by longitudinal dune fields shaped by tidal winds, where sand is made of frozen water or hydrocarbons. [67] Titan is 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. [68] The largest lake, Kraken Mare, is larger than the Caspian Sea. [69] 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. [66] On July 2, 2014, NASA reported the ocean inside Titan may be "as salty as the Earth's Dead Sea". [70][71] is Titan's nearest neighbor in the Saturn system. The two moons are locked in a 4:3 mean-motion resonance with each other, meaning that while Titan makes four revolutions around Saturn, Hyperion makes exactly three. [39] With an average diameter of about 270 km, Hyperion is smaller and lighter than Mimas. [72] It has an extremely irregular shape, and a very odd, tan-colored icy surface resembling a sponge, though its interior may be partially porous as well. [72] The average density of about 0.55 g/cm 3 [72] indicates that the porosity exceeds 40% even assuming it has a purely icy composition. The surface of Hyperion is covered with numerous impact craters—those with diameters 2–10 km are especially abundant. [72] It is the only moon besides the small moons of Pluto known to have a chaotic rotation, which means Hyperion has no well-defined poles or equator. While on short timescales the satellite approximately rotates around its long axis at a rate of 72–75° per day, on longer timescales its axis of rotation (spin vector) wanders chaotically across the sky. [72] This makes the rotational behavior of Hyperion essentially unpredictable. [73] is the third-largest of Saturn's moons. [48] Orbiting the planet at 3.5 million km, it is by far the most distant of Saturn's large moons, and also has the largest orbital inclination, at 15.47°. [40] Iapetus has long been known for its unusual two-toned surface its leading hemisphere is pitch-black and its trailing hemisphere is almost as bright as fresh snow. [74]Cassini images showed that the dark material is confined to a large near-equatorial area on the leading hemisphere called Cassini Regio, which extends approximately from 40°N to 40°S. [74] The pole regions of Iapetus are as bright as its trailing hemisphere. Cassini also discovered a 20 km tall equatorial ridge, which spans nearly the moon's entire equator. [74] Otherwise both dark and bright surfaces of Iapetus are old and heavily cratered. The images revealed at least four large impact basins with diameters from 380 to 550 km and numerous smaller impact craters. [74] No evidence of any endogenic activity has been discovered. [74] A clue to the origin of the dark material covering part of Iapetus's starkly dichromatic surface may have been found in 2009, when NASA's Spitzer Space Telescope discovered a vast, nearly invisible disk around Saturn, just inside the orbit of the moon Phoebe – the Phoebe ring. [75] Scientists believe that the disk originates from dust and ice particles kicked up by impacts on Phoebe. Because the disk particles, like Phoebe itself, orbit in the opposite direction to Iapetus, Iapetus collides with them as they drift in the direction of Saturn, darkening its leading hemisphere slightly. [75] Once a difference in albedo, and hence in average temperature, was established between different regions of Iapetus, a thermal runaway process of water ice sublimation from warmer regions and deposition of water vapor onto colder regions ensued. Iapetus's present two-toned appearance results from the contrast between the bright, primarily ice-coated areas and regions of dark lag, the residue left behind after the loss of surface ice. [76][77]

    Irregular moons Edit

    Irregular moons are small satellites with large-radii, inclined, and frequently retrograde orbits, believed to have been acquired by the parent planet through a capture process. They often occur as collisional families or groups. [27] The precise size as well as albedo of the irregular moons are not known for sure because the moons are very small to be resolved by a telescope, although the latter is usually assumed to be quite low—around 6% (albedo of Phoebe) or less. [28] The irregulars generally have featureless visible and near infrared spectra dominated by water absorption bands. [27] They are neutral or moderately red in color—similar to C-type, P-type, or D-type asteroids, [37] though they are much less red than Kuiper belt objects. [27] [c]

    Inuit group Edit

    The Inuit group includes seven prograde outer moons that are similar enough in their distances from the planet (186–297 radii of Saturn), their orbital inclinations (45–50°) and their colors that they can be considered a group. [28] [37] The moons are Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq, [37] along with two unnamed moons S/2004 S 29 and S/2004 S 31. The largest among them is Siarnaq with an estimated size of about 40 km.

    Gallic group Edit

    The Gallic group are four prograde outer moons that are similar enough in their distance from the planet (207–302 radii of Saturn), their orbital inclination (35–40°) and their color that they can be considered a group. [28] [37] They are Albiorix, Bebhionn, Erriapus, and Tarvos. [37] The largest among these moons is Albiorix with an estimated size of about 32 km. There is an additional satellite S/2004 S 24 that could belong to this group, but more observations are needed to confirm or disprove its categorization. S/2004 S 24 has the most distant prograde orbit of Saturn's known satellites.

    Norse group Edit

    The Norse (or Phoebe) group consists of 46 retrograde outer moons. [28] [37] They are Aegir, Bergelmir, Bestla, Farbauti, Fenrir, Fornjot, Greip, Hati, Hyrrokkin, Jarnsaxa, Kari, Loge, Mundilfari, Narvi, Phoebe, Skathi, Skoll, Surtur, Suttungr, Thrymr, Ymir, [37] and twenty-five unnamed satellites. After Phoebe, Ymir is the largest of the known retrograde irregular moons, with an estimated diameter of only 18 km. The Norse group may itself consist of several smaller subgroups. [37]

      , at 213 ± 1.4 km in diameter, is by far the largest of Saturn's irregular satellites. [27] It has a retrograde orbit and rotates on its axis every 9.3 hours. [78] Phoebe was the first moon of Saturn to be studied in detail by Cassini, in June 2004 during this encounter Cassini was able to map nearly 90% of the moon's surface. Phoebe has a nearly spherical shape and a relatively high density of about 1.6 g/cm 3 . [27]Cassini images revealed a dark surface scarred by numerous impacts—there are about 130 craters with diameters exceeding 10 km. Spectroscopic measurement showed that the surface is made of water ice, carbon dioxide, phyllosilicates, organics and possibly iron bearing minerals. [27] Phoebe is believed to be a captured centaur that originated in the Kuiper belt. [27] It also serves as a source of material for the largest known ring of Saturn, which darkens the leading hemisphere of Iapetus (see above). [75]

    Confirmed moons Edit

    The Saturnian moons are listed here by orbital period (or semi-major axis), from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in bold, while the irregular moons are listed in red, orange and gray background. The orbits and mean distances of the irregular moons are strongly variable over short timescales due to frequent planetary and solar perturbations, [79] therefore the orbit epochs of all irregular moons are based on the same Julian date of 2459200.5, or 17 December 2020. [80]

    Unconfirmed moons Edit

    The following objects (observed by Cassini) have not been confirmed as solid bodies. It is not yet clear if these are real satellites or merely persistent clumps within the F Ring. [21]

    Name Image Diameter (km) Semi-major
    axis (km) [47]
    period (d) [47]
    Position Discovery year Status
    S/2004 S 3 and S 4 [m] ≈ 3–5 ≈ 140 300 ≈ + 0.619 uncertain objects around the F Ring 2004 Were undetected in thorough imaging of the region in November 2004, making their existence improbable
    S/2004 S 6 ≈ 3–5 ≈ 140 130 + 0.618 01 2004 Consistently detected into 2005, may be surrounded by fine dust and have a very small physical core

    Hypothetical moons Edit

    Two moons were claimed to be discovered by different astronomers but never seen again. Both moons were said to orbit between Titan and Hyperion. [87]

      which was supposedly sighted by Hermann Goldschmidt in 1861, but never observed by anyone else. [87] was allegedly discovered in 1905 by astronomer William Pickering, but never seen again. Nevertheless, it was included in numerous almanacs and astronomy books until the 1960s. [87]

    Past temporary moons Edit

    Much like Jupiter, asteroids and comets will infrequently make close approaches to Saturn, even more infrequently becoming captured into orbit of the planet. The comet P/2020 F1 (Leonard) is calculated to have made a close approach of 978 000 ± 65 000 km ( 608 000 ± 40 000 mi to Saturn on 8 May 1936, closer than the orbit of Titan to the planet, with an orbital eccentricity of only 1.098 ± 0.007 . The comet may have been orbiting Saturn prior to this as a temporary satellite, but difficulty modelling the non-gravitational forces makes whether or not it was indeed a temporary satellite uncertain. [88]

    Other comets and asteroids may have temporarily orbited Saturn at some point, but none are presently known to have.

    It is thought that the Saturnian system of Titan, mid-sized moons, and rings developed from a set-up closer to the Galilean moons of Jupiter, though the details are unclear. It has been proposed either that a second Titan-sized moon broke up, producing the rings and inner mid-sized moons, [89] or that two large moons fused to form Titan, with the collision scattering icy debris that formed the mid-sized moons. [90] On June 23, 2014, NASA claimed to have strong evidence that nitrogen in the atmosphere of Titan came from materials in the Oort cloud, associated with comets, and not from the materials that formed Saturn in earlier times. [65] Studies based on Enceladus's tidal-based geologic activity and the lack of evidence of extensive past resonances in Tethys, Dione, and Rhea's orbits suggest that the moons inward of Titan may be only 100 million years old. [91]

    1. ^ The mass of the rings is about the mass of Mimas, [8] whereas the combined mass of Janus, Hyperion and Phoebe—the most massive of the remaining moons—is about one-third of that. The total mass of the rings and small moons is around 5.5 × 10 19 kg .
    2. ^ Inktomi was once known as "The Splat". [61]
    3. ^ The photometric color may be used as a proxy for the chemical composition of satellites' surfaces.
    4. ^ Order refers to the position among other moons with respect to their average distance from Saturn.
    5. ^ A confirmed moon is given a permanent designation by the IAU consisting of a name and a Roman numeral. [36] The nine moons that were known before 1900 (of which Phoebe is the only irregular) are numbered in order of their distance from Saturn the rest are numbered in the order by which they received their permanent designations. Many small moons have not yet received a permanent designation.
    6. ^ The diameters and dimensions of the inner moons from Pan through Janus, Methone, Pallene, Telepso, Calypso, Helene, Hyperion and Phoebe were taken from Thomas 2010, Table 3. [38] Diameters and dimensions of Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus are from Thomas 2010, Table 1. [38] The approximate sizes of other satellites are from the website of Scott Sheppard. [33]
    7. ^ Masses of the large moons were taken from Jacobson, 2006. [39] Masses of Pan, Daphnis, Atlas, Prometheus, Pandora, Epimetheus, Janus, Hyperion and Phoebe were taken from Thomas, 2010, Table 3. [38] Masses of other small moons were calculated assuming a density of 1.3 g/cm 3 .
    8. ^ abc The orbital parameters were taken from Spitale, et al. 2006, [47] IAU-MPC Natural Satellites Ephemeris Service, [81] and NASA/NSSDC. [40]
    9. ^ Negative orbital periods indicate a retrograde orbit around Saturn (opposite to the planet's rotation).
    10. ^ To Saturn's equator for the regular satellites, and to the ecliptic for the irregular satellites
    11. ^ Only known prograde outer satellite, inclination similar to other satellites of the Gallic group
    12. ^ Probably a captured asteroid due to its unusually high eccentricity, though orbit is similar to the Norse group
    13. ^ S/2004 S 4 was most likely a transient clump—it has not been recovered since the first sighting. [21]
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    Ocean Currents Predicted on Enceladus

    Buried beneath 20 kilometers of ice, the subsurface ocean of Enceladus—one of Saturn's moons—appears to be churning with currents akin to those on Earth.

    The theory, derived from the shape of Enceladus's ice shell, challenges the current thinking that the moon's global ocean is homogenous, apart from some vertical mixing driven by the warmth of the moon's core.

    Enceladus, a tiny frozen ball about 500 kilometers in diameter (about 1/7th the diameter of Earth's moon), is the sixth largest moon of Saturn. Despite its small size, Enceladus attracted the attention of scientists in 2014 when a flyby of the Cassini spacecraft discovered evidence of its large subsurface ocean and sampled water from geyser-like eruptions that occur through fissures in the ice at the south pole. It is one of the few locations in the solar system with liquid water (another is Jupiter's moon Europa), making it a target of interest for astrobiologists searching for signs of life.

    The ocean on Enceladus is almost entirely unlike Earth's. Earth's ocean is relatively shallow (an average of 3.6 km deep), covers three-quarters of the planet's surface, is warmer at the top from the sun's rays and colder in the depths near the seafloor, and has currents that are affected by wind Enceladus, meanwhile, appears to have a globe-spanning and completely subsurface ocean that is at least 30 km deep and is cooled at the top near the ice shell and warmed at the bottom by heat from the moon's core.

    Despite their differences, Caltech graduate student Ana Lobo (MS ✗) suggests that oceans on Enceladus have currents akin to those on Earth. The work builds on measurements by Cassini as well as the research of Andrew Thompson, professor of environmental science and engineering, who has been studying the way that ice and water interact to drive ocean mixing around Antarctica.

    The oceans of Enceladus and Earth share one important characteristic: they are salty. And as shown by findings published in Nature Geoscience on March 25, variations in salinity could serve as drivers of the ocean circulation on Enceladus, much as they do in Earth's Southern Ocean, which surrounds Antarctica.

    Lobo and Thompson collaborated on the work with Steven Vance and Saikiran Tharimena of JPL, which Caltech manages for NASA.

    Gravitational measurements and heat calculations from Cassini had already revealed that the ice shell is thinner at the poles than at the equator. Regions of thin ice at the poles are likely associated with melting and regions of thick ice at the equator with freezing, Thompson says. This affects the ocean currents because when salty water freezes, it releases the salts and makes the surrounding water heavier, causing it to sink. The opposite happens in regions of melt.

    "Knowing the distribution of ice allows us to place constraints on circulation patterns," Lobo explains. An idealized computer model, based on Thompson's studies of Antarctica, suggests that the regions of freezing and melting, identified by the ice structure, would be connected by the ocean currents. This would create a pole-to-equator circulation that influences the distribution of heat and nutrients.

    "Understanding which regions of the subsurface ocean might be the most hospitable to life as we know it could one day inform efforts to search for signs of life," Thompson says.

    The paper is titled "A pole-to-equator ocean overturning circulation on Enceladus." This work was supported by JPL's Strategic Research and Technology Development program the Icy Worlds node of NASA's Astrobiology Institute and the David and Lucile Packard Foundation.

    Cassini Spacecraft

    The Cassini–Huygens mission, commonly called Cassini, was a collaboration between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a probe to study the planet Saturn and its system, including its rings and natural satellites.

    The Flagship-class robotic spacecraft comprised both NASA’s Cassini probe (the fourth space probe to visit Saturn and the first to enter its orbit) and ESA’s Huygens lander which landed on Saturn’s largest moon, Titan. The spacecraft was named after the Italian astronomer Giovanni Cassini (8 June 1625 – 14 September 1712) and the Dutch astronomer Christiaan Huygens (14 April 1629 – 8 July 1695).

    At the end of its mission, the Cassini spacecraft, one of the most important scientific instruments humanity has ever built, executed a “Grand Finale”, a series of 22 orbits that each passed between the planet and its rings. The purpose of this phase was to maximize Cassini’s scientific outcome before the spacecraft was disposed of.

    On September 15, 2017, Cassini made its final approach to the giant planet Saturn. But this encounter was like no other. This time, it dived into the planet’s atmosphere, sending science data for as long as its small thrusters could keep the spacecraft’s antenna pointed at Earth. Soon after, Cassini burned up and disintegrated like a meteor. The atmospheric entry of Cassini ended the mission, but analyses of the returned data will continue for many years.

    Composition and Surface Features:

    Enceladus has a density of 1.61 g/cm³, which is higher than Saturn’s other mid-sized, icy satellites, suggesting a composition that includes a greater percentage of silicates and iron. It is also believed to be largely differentiated between a geologically active core and an icy mantle, with a liquid water ocean nestled between.

    Gravity measurements by NASA’s Cassini spacecraft and Deep Space Network suggest that Saturn’s moon Enceladus harbors a large interior ocean beneath it’s south pole. Credit: NASA/JPL-Caltech

    The existence of this liquid water ocean has been the subject of scientific debate since 2005, when scientists first observed plumes containing water vapor spewing from Enceladus’s south polar surface. These jets are capable of dispensing 250 kg of water vapor every second at speeds of up to 2,189 km/h (1,360 mph), and reaching 500 km into space.

    In 2006, it was determined that Enceladus’s plumes are the source of Saturn’s E Ring and actively replenish it. According to measurements made by the Cassini-Huygens probe, these emissions are composed mostly of water vapor, as well as minor components like molecular nitrogen, methane, and carbon dioxide. Further observations noted the presence of simple hydrocarbons such as methane, propane, acetylene and formaldehyde.

    The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized subsurface chambers. The intensity of the eruptions varies significantly due to changes in Enceladus’s orbit. Basically, the plumes are about four times brighter when Enceladus is at apoapsis (farthest from Saturn), which is consistent with geophysical calculations that predict that the south polar fissures will be under less compression, thus opening them wider.

    The existence of subsurface water was confirmed thanks to evidence provided by the Cassini mission in 2014. This included gravity measurements obtained during the flybys of 2010-2012, which confirmed the existence of a south polar subsurface ocean of liquid water within Enceladus with a thickness of around 10 km.

    Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL

    In addition, during the July 14, 2005 flyby, the Cassini probe also detected the presence of escaping internal heat in the southern polar region. These temperatures were too high to be attributed to solar heating, and combined with the geyser activity, seemed to indicate that the interior of the planet is still geologically active.

    Further studies from measurements of Enceladus’s libration as it orbits Saturn strongly suggest that the entire icy crust is detached from the rocky core, which would mean that the ocean beneath its surface is planet-wide. The amount of libration implies that this global ocean is about 26 to 31 kilometers in depth (compared to Earth’s average ocean depth of 3.7 kilometers).

    Observations of Enceladus’ surface has revealed five types of terrain – cratered terrain, smooth (young) terrain, ridged terrain (often bordering on smooth areas), linear cracks, scarps, troughs, and grooves. Surveys of the cratered terrain, smooth plains, and other features indicate a level of resurfacing that suggests that tectonics are an important factor in the geological history of Enceladus.

    Recent observations by Cassini have provided a closer look at the crater distribution and size. These features have been named by the IAU after characters and places from Burton’s translation of The Book of One Thousand and One Nights – i.e. the Shahrazad crater, the Diyar plains, the Anbar depression.

    Artist impression of the view of Saturn from Enceladus, with geysers erupting at the right in the foreground. Credit: Michael Carroll

    The smooth plains are dominated by fresh clean ice, which gives Enceladus what is possibly the most reflective surface in the Solar System (with a visual geometric albedo of 1.38). These areas have few craters, which indicate that they are likely younger than a few hundred million years old. In addition, the relative youthfulness of these regions are an indication that cryovolcanism and other processes actively renew the surface.

    The older terrain is not only cratered, but numerous fractures have also been observed – suggesting that the surface has been subject to extensive deformation since the craters formed. Some areas show regions with no craters, indicating major resurfacing events in the geologically recent past. The fissures, plains, corrugated terrain and other crustal deformations also indicate that Enceladus is geologically active.

    One of the more dramatic types of tectonic features found on Enceladus are its rift canyons. These canyons can be up to 200 km long, 5–10 km wide, and 1 km deep. Such features are geologically young, because they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.

    Evidence of tectonics on Enceladus is also derived from grooved terrain, consisting of lanes of curved formations and ridges that often separate smooth plains from cratered regions. Deep fractures are another, which are often found in bands cutting across cratered terrain, and which were probably influenced by the formation of weakened regolith produced by impact craters.

    Enceladus, showing the famous “Tiger Stripes” feature – a series of fractures bound on either side by colorful ice. Credit: NASA/JPL/Space Science Institute

    Linear grooves can also be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they are among the youngest features on Enceladus. However, some linear grooves have been softened like the craters nearby, suggesting that they are older. Ridges have also been observed on Enceladus, though they are relatively limited in extent and are up to one kilometer tall.

    Other interesting features include the “Tiger stripes“: a series of fractures bounded on either side by ridges in the southern polar region that are are surrounded by mint-green-colored, coarse-grained water ice. These fractures appear to be the youngest features in this region, and combined with a lack of impact craters in this area, are further evidence of geological activity.

    NASA Finds Evidence of “Fresh Ice” on Saturn’s Moon Enceladus

    By digging through detailed infrared images of Saturn’s icy moon Enceladus — courtesy of NASA’s Cassini spacecraft, which met its demise back in 2017 after 13 years of Saturn exploration — NASA scientists say they’ve found “strong evidence” of fresh ice in the moon’s northern hemisphere.

    The ice, thought to have originated and resurfaced from Enceladus’ interior, could be good news for the prospect of life on Enceladus, which is considered by many scientists to be one of the most promising places to look for life in the solar system.

    The dataset, the most detailed global infrared views ever produced of the moon according to the agency, was created using data collected by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS). It includes scans of variable wavelengths, including visible light and infrared.

    In 2005, scientists first made the discovery that Enceladus shoots giant plumes of ice grains and vapor from a suspected subsurface ocean hiding underneath a thick crust of ice.



    The new infrared signals perfectly match the location of this activity, made highly visible in the form of neon red “tiger stripe” gashes on the moon’s south pole.

    Similar features have also been spotted in the northern hemisphere as well, leading scientists to believe that the same process is happening on both hemispheres.

    “The infrared shows us that the surface of the south pole is young, which is not a surprise because we knew about the jets that blast icy material there,” Gabriel Tobie, VIMS scientist at the University of Nantes, France and co-author of a new paper about the findings published in the journal Icarus, said in a NASA statement.

    “Now, thanks to these infrared eyes, you can go back in time and say that one large region in the northern hemisphere appears also young and was probably active not that long ago, in geologic timelines,” he added.



    In October 2019, a team of researchers from the Free University of Berlin found traces of organic compounds in the moon’s icy plumes that appear to be the building blocks of amino acids, the precursors of Earth-based lifeforms.

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    Cassini finale: Saturn space probe completes its fiery demise

    Thirteen years after reaching Saturn , NASA's nuclear-powered Cassini spacecraft raced through its 294th and final orbit Thursday, collecting priceless data while hurtling toward a kamikaze-like plunge into the ringed planet's atmosphere Friday, going out in a blaze of glory to wrap up an "insanely" successful mission.

    Friday morning, NASA confirmed "Cassini's final dive is happening" and its final signal to Earth had been received. "Cassini is now part of the planet it studied. Thanks for the science #GrandFinale," NASA tweeted.

    Earth received @CassiniSaturn&rsquos final signal at 7:55am ET. Cassini is now part of the planet it studied. Thanks for the science #GrandFinale

    &mdash NASA (@NASA) September 15, 2017

    During its last orbit, Cassini was programmed to snap a final few pictures of Saturn , its vast ring system, Titan and the small moon Enceladus Thursday in what mission managers were calling "the last picture show," before turning its large dish antenna toward Earth to transmit the images and other data back to waiting scientists.

    Titan and Enceladus, which harbors a saltwater ocean beneath an icy crust, host potentially habitable environments and rather than risk an eventual collision with an out-of-gas Cassini -- and earthly contamination -- NASA managers opted to crash the spacecraft into Saturn to eliminate any possible threat.

    Saturn's moon Enceladus sinks behind the giant planet as NASA's Cassini spacecraft makes its final approach before burning up in Saturn's atmosphere. NASA/JPL-Caltech/SSI

    Virtually out of propellant, Cassini used a final gravitational nudge -- a "goodbye kiss" -- from Saturn's smog-shrouded moon Titan earlier this week to precisely aim itself at a point on the planet's dayside 10 degrees above the equator.

    "That final flyby of Titan . put Cassini on an impacting trajectory and there is absolutely no coming out of it," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We are going so deep into the atmosphere the spacecraft doesn't have a chance of coming out."

    Space & Astronomy

    "These final images are sort of like taking a last look around your house or apartment just before you move out," said Linda Spilker, the Cassini project scientist at NASA's Jet Propulsion Laboratory. "You look at your old rooms, and memories across the years come flooding back. In the same way, Cassini is taking a last look around the Saturn system . and with those pictures come heart-warming memories."

    Cassini was not able to send images back during its final descent, but eight of its scientific instruments continued operating and beaming back data in realtime as the spacecraft, its antenna locked on Earth, slammed into Saturn's discernible atmosphere early Friday morning.

    Traveling at a velocity of 70,000 mph, Cassini's demise was quick. Even so, scientists expect a wealth of data from the probe's final moments.

    "The highest science priority is to sample the atmosphere," Spilker said. "We stand to gain fundamental insights into Saturn's formation and evolution as well as processes that occur in the atmosphere."

    Cassini would have encountered the first wisps of gasses in the extreme upper atmosphere about 1,190 miles above Saturn's visible cloud tops, where atmospheric pressure is equivalent to sea level on Earth.

    Small thrusters were designed to automatically fire to keep Cassini properly oriented, and its antenna locked on Earth, as atmospheric buffeting begins. But within one minute of entry, about 120 miles into the discernible atmosphere, with the thrusters overwhelmed, Cassini was expected to begin tumbling and telemetry to come to an abrupt end.

    A few moments after that, the atmosphere's extreme heating would rip Cassini apart and utterly destroy its components.

    An artist's illustration of Cassini during one of its final orbits. NASA

    "It goes really fast," said spacecraft engineer Julie Webster. "First, the (insulation) blankets will burn off, and then we'll reach the aluminum melting point within about 20 seconds. The iridium will be the last thing to melt, and it will go about 30 seconds after the aluminum. It goes within a minute."

    Cassini's final signal, traveling across the solar system at the speed of light -- 186,000 miles per second -- will reach a huge antenna in Australia 83 minutes later, at 7:55 a.m. That's when flight controllers, engineers and scientists gathered at the Jet Propulsion Laboratory will know Cassini and its $3.4 billion mission well and truly gone.

    "The mission has exceeded all of our expectations, done better than we could have ever dreamed," said Curt Niebur, Cassini program scientist at NASA Headquarters. "The Saturn system is absolutely chock full of amazing worlds of all sizes, and Cassini has been exploring them for the past 13 years.

    "We've watched the particles in the rings around Saturn collide and glide during their gravitational dance and we've confirmed things that we suspected might exist in the Saturn system. But even more pleasantly, we've been shocked by things that we never predicted we would find."


    Like watching a titanic globe-spanning storm develop and move around the entire planet, running into itself like a snake eating its tail. Like discovering a bizarre hexagon-shaped storm around Saturn's north pole that has persisted for decades. And the discovery of methane seas, lakes, rivers and rain on Titan, where conditions mimic those on Earth in the distant past.

    "And we were absolutely shocked to learn that tiny, tiny Enceladus has a global liquid water ocean underneath a relatively thin ice crust that's warmed by hydrothermal activity and has jets of water from that ocean shooting out into space through cracks in the south pole," Niebur said. "Enceladus may have all of the ingredients needed for life as we know it to currently exist, right now, at this very second."

    Over the course of its 13-year mission, Cassini has executed 2.5 million commands, carried out 360 engine burns, completed 162 targeted flybys of Saturn's moons, taken more than 453,000 images and discovered six previously unknown moons, covering 4.9 billion miles since launch in 1997.

    Most important, the spacecraft, built in the early 1990s, collected 635 gigabits of data resulting in nearly 4,000 peer-reviewed scientific papers.

    "The mission has been insanely, wildly, beautifully successful," Niebur said. "And it's coming to an end. . I find great comfort in the fact that Cassini will continue teaching us up to the very last second."

    Launched in October 1997, Cassini arrived at Saturn in July 2004 and dropped off a lander built by the European Space Agency that successfully completed a parachute descent to the surface of Titan the following January.

    Titan is larger than Mercury but its surface is hidden below a thick smog-like atmosphere. The Huygens lander revealed an alien landscape with rounded rocks and boulders under an orange-hued sky while Cassini's cloud-piercing radar imaging system eventually filled in a global map of the moon that revealed methane lakes, rivers and seas.

    "To put a probe onto Titan, capture a signal on the way down, land it softly on the surface and play those images back, I still give myself goosebumps just seeing that first image," Maize said. "I'll never forget it."

    Cassini's final orbits carried it between Saturn's cloud tops and innermost rings, giving scientists an unprecedented opportunity to learn more about the planet's atmosphere and vast ring system. NASA

    Since then, Cassini has flown through a complex set of ever-changing orbits, repeatedly using Titan's gravity to alter its trajectory. Energy from the Titan flybys was the equivalent of 127,000 pounds of propellant, Maize said, enabling views of Saturn and its huge ring system from different perspectives and setting up close flybys of and many of its moons.

    But all good things must come to an end.

    On April 22, Cassini carried out a Titan flyby that kicked off the "Grand Finale," putting the spacecraft on a trajectory that repeatedly carried it between the innermost rings and Saturn's cloud tops and set up a mission-ending impact in the atmosphere on Friday.

    The Grand Finale orbits brought Cassini closer to Saturn and its rings than ever before and gave scientists a unique opportunity to determine the mass of the rings. While those studies are ongoing, it appears the rings may be a relatively young phenomenon and not a relic of Saturn's birth.

    But for many, discoveries about Titan and Enceladus are the icing on the cake, more than justifying the decision to end Cassini's mission with a dramatic plunge into Saturn's atmosphere.

    "These two new worlds, Titan and Enceladus, that were so completely revealed to us by Cassini have changed the idea that ocean worlds like Earth and (Jupiter's moon) Europa are rare in the universe," Niebur said. "This, in turn, is changing our views about how prevalent and common habitable environments and even life beyond Earth might truly be."

    NASA is in the early stages of designing a spacecraft to make repeated flybys of Jupiter's moon Europa in the 2020s and many hope a follow-on mission to Saturn will someday be mounted to explore Enceladus in more detail.

    "Enceladus has no business existing," Niebur said. "And yet there it is, practically screaming at us, 'look at me! I complete invalidate all of your assumptions about the solar system.' It's just been a remarkable opportunity to study Enceladus and unveil the secrets it's been keeping. It's an amazing destination."

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