Saturn's Moons (Other Satellites)
Mimas,Enceladus, Tethys, Dione, and Rhea are approximately spherical in shape and appear to be composed mostly of water ice. Enceladus reflects almost 100 percent of the sunlight that strikes it. All five satellites represent a size range that had not been explored before.
Mimas (pronounced MY muss or MEE muss, adjective Mimantean) (left and right), looks somewhat like a bull's eye if viewed from a certain angle. The feature that causes this is the huge 140-kilometer-wide (88-mile) Herschel Crater, which is one-third the diameter of Mimas. If the object striking Mimas had been larger or been moving faster, Mimas would probably have been "disrupted" into pieces that might have collapsed back into a new moon or might have scattered into another ring of Saturn. The walls of Herschel Crater are approximately 5 kilometers (3 miles) high, parts of the floor are approximately 10 kilometers (6 miles) deep, and the central peak towers are almost 6 kilometers (4 miles) above the floor of the crater. A comparable crater on Earth would be 4,000 kilometers (2,500 miles) in diameter.
Mimas is an inner moon of Saturn (the innermost of the major moons) that averages 396 kilometers (246 miles) in diameter. Shock waves from the Herschel impact may have caused the fractures, also called chasmata, on the opposite side of Mimas. This is not quite big enough to hold a round shape; the shape is somewhat ovoid with dimensions of 209 x 196 x 191 kilometers (130 x 122 x 119 miles, respectively). Mimas orbits at a range of 185,520 kilometers (115,280 miles) from Saturn in a time of 22 hours and 37 minutes. This orbit makes Mimas the closest major moon of Saturn. Mimas is tidally locked to Saturn with one side always facing in towarditsparent. Mimas' close orbit means that Mimas probably receives several times the rate of collisions as the other moons of Saturn.
Mimas and another Saturn moon, Rhea, have been called "the most heavily cratered body in the Solar System." Mimas would probably have been more heavily cratered except, being closer to Saturn, Mimas was warmer (and consequently softer) for a longer time so early features have faded away. However, with so many impacts the youngest craters have tended to obliterate the older ones, and these moons are cratered about as much as they can get.
Most of the Mimas surface is saturated with impact craters ranging in size up to greater than 40 kilometers (25 miles)in diameter, although none are anywhere near the size of Herschel. However, the craters in the South Pole region of Mimas are generally 20 kilometers (12.4 miles) in diameter or less. This suggests that some melting or other resurfacing processes occurred there later than on the rest of the moon. (Interestingly, the South Pole area of Enceladus appears to be the source of that moon's geysers.)
Mimas, Tethys, Dione, and Rhea are all cratered; Enceladus appears to have by far the most active surface of any satellite in the system (with the possible exception of Titan, whose surface was not photographed).
At least five types of terrain have been identified on Enceladus. Although craters can be seen across portions of its surface, the lack of craters in other areas implies an age less than a few hundred million years for the youngest regions. It seems likely that parts of the surface are still undergoing change, since some areas are covered by ridged plains with no evidence of cratering down to the limit of resolution of Voyager 2's cameras (2 kilometers or 1.2 miles). A pattern of linear faults crisscrosses other areas. It is not likely that a satellite as small as Enceladus could have enough radioactive material to produce the modification. A more likely source of heating appears to be tidal interaction with Saturn, caused by perturbations in Enceladus' orbit by Dione (like Jupiter's satellite Io). Theories of tidal heating do not predict generation of enough energy to explain all the heating that must have occurred. Because it reflects so much sunlight, Enceladus' current surface temperature is only 72 Kelvins (-330 degrees Fahrenheit).
NASA's Cassini spacecraft may have found evidence of liquid water reservoirs that erupt in Yellowstone-like geysers on Saturn's moon Enceladus The rare occurrence of liquid water so near the surface raises many new questions about the mysterious moon.
Enceladus Jets -- Are They Wet or Just Wild?
Nov. 26, 2008
Image Credit: Copyright 2008 Karl Kofoed.
Scientists continue to search for the cause of the geysers on Saturn's moon Enceladus. The geysers are visible as a large plume of water vapor and ice particles escaping the moon. Inside the plume are jets of dust and gas. What causes and controls the jets is a mystery. The Cassini spacecraft continues to collect new data to look for clues.
At the heart of the search is the question of whether the jets originate from an underground source of liquid water. Some theories offer models where the jets could be caused by mechanisms that do not require liquid water. Painstaking detective work by Cassini scientists is testing the possibilities to get closer to an answer.
What generates Enceladus' jets is a burning question in planetary science, because if liquid water is involved, Enceladus would be shown to have everything it needs, in theory, to provide a habitable environment.
One recent model offered the possibility that the jets could be violent bursts of volatile ices freshly exposed to space when Saturn's tidal forces would open vents inside the "tiger stripe" region of the moon's south pole.
New Cassini findings reported in the Nov. 27 issue of the journal Nature, however, cast doubt on that hypothesis. When Enceladus is farther away from Saturn, the theory goes, the vents would compress, reducing or shutting off the jets.
"Our observations do not agree with the predicted timing of the faults opening and closing due to tidal tension and compression," said JPL scientist Candice Hansen of Cassini's ultraviolet imaging spectrograph team.
At the same time, Hansen said, the new findings support at least one theory that attributes the jets to a liquid water source inside Enceladus.
Hansen and her team conducted experiments in 2005 and 2007 to observe starlight passing through Enceladus' plume. During this so-called "stellar occultation," the spectrometer measured the water vapor content and density of the jets. The experiment tested the prediction that a greater amount of material would be measured coming from open fissures in 2005, and less material in 2007 when the fissures would be closing.
Instead, reports Hansen, the opposite was found to be true. The observations showed that the plume was almost two times as dense in 2007 as in 2005, contradicting the model that holds tidal squeezing is in control of the plumes. "We don't rule it out entirely because of the different geometries of our two occultation, but we also definitely do not substantiate this hypothesis," said Hansen.
Hansen said the new Cassini observations, however, do support a mathematical model developed in 2007, which treats the vents as nozzles that channel water vapor from a warm, probably liquid source, to the surface at supersonic speeds.
The authors of that model theorize that only high temperatures close to the melting point of water ice could account for the large number of ice particles present in steady state in Enceladus' jets. A liquid water source inside Enceladus, they said, could be similar to Earth's Lake Vostok, beneath Antarctica, where liquid water exists beneath the ice. In Enceladus' case, the ice grains would then condense from the vapor escaping from the water source and stream through cracks in the ice crust to the surface and out into space.
What causes and controls the jets, and whether there is liquid water remain uncertain, but there may be more clues soon, because Enceladus is a prime target for Cassini to study in its extended Equinox Mission. The presence of liquid water inside Enceladus would have major implications for future astrobiological studies on the possibility of life within icy bodies of the outer solar system.
High-resolution Cassini images show icy jets and towering plumes ejecting large quantities of particles at high speed. Scientists examined several models to explain the process. They ruled out the idea that the particles are produced by or blown off the moon's surface by vapor created when warm water ice converts to a gas. Instead, scientists have found evidence for a much more exciting possibility -- the jets might be erupting from near-surface pockets of liquid water above 0 degrees Celsius (32 degrees Fahrenheit), like cold versions of the Old Faithful geyser in Yellowstone.
Mission scientists report these and other Enceladus findings in this week's issue of Science.
"We previously knew of at most three places where active volcanism exists: Jupiter's moon Io, Earth, and possibly Neptune's moon Triton. Cassini changed all that, making Enceladus the latest member of this very exclusive club, and one of the most exciting places in the solar system," said Dr. John Spencer, Cassini scientist, Southwest Research Institute, Boulder, Colo.
"Other moons in the solar system have liquid-water oceans covered by kilometers of icy crust," said Dr. Andrew Ingersoll, imaging team member and atmospheric scientist at the California Institute of Technology, Pasadena, Calif. "What's different here is that pockets of liquid water may be no more than tens of meters below the surface."
Other unexplained oddities now make sense. "As Cassini approached Saturn, we discovered that the Saturnian system is filled with oxygen atoms. At the time we had no idea where the oxygen was coming from," said Dr. Candy Hansen, Cassini scientist at NASA's Jet Propulsion Laboratory in Pasadena. "Now we know that Enceladus is spewing out water molecules, which break down into oxygen and hydrogen."
This face-on colour view of Enceladus was taken by the international Cassini spacecraft on 31 January 2011, from a distance of 81 000 km, and processed by amateur astronomer Gordan Ugarkovic.
Credit: NASA/JPL-Caltech/SSI/G. Ugarkovic
Ocean Inside Saturn's Moon Enceladus
This diagram illustrates the possible interior of Saturn's moon Enceladus based on a gravity investigation by NASA's Cassini spacecraft and NASA's Deep Space Network, reported in April 2014. The gravity measurements suggest an ice outer shell and a low density, rocky core with a regional water ocean sandwiched in between at high southern latitudes.
Views from Cassini's imaging science subsystem were used to depict the surface geology of Enceladus and the plume of water jets gushing from fractures near the moon's south pole.
Enceladus is 313 miles (504 kilometers) in diameter.
April 13, 2017
NASA Missions Provide New Insights into
'Ocean Worlds' in Our Solar System
Two veteran NASA missions are providing new details about icy, ocean-bearing moons of Jupiter and Saturn, further heightening the scientific interest of these and other "ocean worlds" in our solar system and beyond. The findings are presented in papers published Thursday by researchers with NASA’s Cassini mission to Saturn and Hubble Space Telescope.
In the papers, Cassini scientists announce that a form of chemical energy that life can feed on appears to exist on Saturn's moon Enceladus, and Hubble researchers report additional evidence of plumes erupting from Jupiter's moon Europa.
“This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment,” said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate at Headquarters in Washington. ”These results demonstrate the interconnected nature of NASA's science missions that are getting us closer to answering whether we are indeed alone or not.”
The paper from researchers with the Cassini mission, published in the journal Science, indicates hydrogen gas, which could potentially provide a chemical energy source for life, is pouring into the subsurface ocean of Enceladus from hydrothermal activity on the seafloor.
The presence of ample hydrogen in the moon's ocean means that microbes – if any exist there – could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in the water. This chemical reaction, known as "methanogenesis" because it produces methane as a byproduct, is at the root of the tree of life on Earth, and could even have been critical to the origin of life on our planet.
Life as we know it requires three primary ingredients: liquid water; a source of energy for metabolism; and the right chemical ingredients, primarily carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. With this finding, Cassini has shown that Enceladus – a small, icy moon a billion miles farther from the sun than Earth – has nearly all of these ingredients for habitability. Cassini has not yet shown phosphorus and sulfur are present in the ocean, but scientists suspect them to be, since the rocky core of Enceladus is thought to be chemically similar to meteorites that contain the two elements.
"Confirmation that the chemical energy for life exists within the ocean of a small moon of Saturn is an important milestone in our search for habitable worlds beyond Earth," said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
The Cassini spacecraft detected the hydrogen in the plume of gas and icy material spraying from Enceladus during its last, and deepest, dive through the plume on Oct. 28, 2015. Cassini also sampled the plume's composition during flybys earlier in the mission. From these observations scientists have determined that nearly 98 percent of the gas in the plume is water, about 1 percent is hydrogen and the rest is a mixture of other molecules including carbon dioxide, methane and ammonia.
The measurement was made using Cassini's Ion and Neutral Mass Spectrometer (INMS) instrument, which sniffs gases to determine their composition. INMS was designed to sample the upper atmosphere of Saturn's moon Titan. After Cassini's surprising discovery of a towering plume of icy spray in 2005, emanating from hot cracks near the south pole, scientists turned its detectors toward the small moon.
Cassini wasn't designed to detect signs of life in the Enceladus plume – indeed, scientists didn't know the plume existed until after the spacecraft arrived at Saturn.
"Although we can't detect life, we've found that there's a food source there for it. It would be like a candy store for microbes," said Hunter Waite, lead author of the Cassini study.
The new findings are an independent line of evidence that hydrothermal activity is taking place in the Enceladus ocean. Previous results, published in March 2015, suggested hot water is interacting with rock beneath the sea; the new findings support that conclusion and add that the rock appears to be reacting chemically to produce the hydrogen.
The paper detailing new Hubble Space Telescope findings, published in The Astrophysical Journal Letters, reports on observations of Europa from 2016 in which a probable plume of material was seen erupting from the moon’s surface at the same location where Hubble saw evidence of a plume in 2014. These images bolster evidence that the Europa plumes could be a real phenomenon, flaring up intermittently in the same region on the moon's surface.
The newly imaged plume rises about 62 miles (100 kilometers) above Europa’s surface, while the one observed in 2014 was estimated to be about 30 miles (50 kilometers) high. Both correspond to the location of an unusually warm region that contains features that appear to be cracks in the moon’s icy crust, seen in the late 1990s by NASA's Galileo spacecraft. Researchers speculate that, like Enceladus, this could be evidence of water erupting from the moon’s interior.
“The plumes on Enceladus are associated with hotter regions, so after Hubble imaged this new plume-like feature on Europa, we looked at that location on the Galileo thermal map. We discovered that Europa’s plume candidate is sitting right on the thermal anomaly," said William Sparks of the Space Telescope Science Institute in Baltimore, Maryland. Sparks led the Hubble plume studies in both 2014 and 2016.
The researchers say if the plumes and the warm spot are linked, it could mean water being vented from beneath the moon's icy crust is warming the surrounding surface. Another idea is that water ejected by the plume falls onto the surface as a fine mist, changing the structure of the surface grains and allowing them to retain heat longer than the surrounding landscape.
For both the 2014 and 2016 observations, the team used Hubble's Space Telescope Imaging Spectrograph (STIS) to spot the plumes in ultraviolet light. As Europa passes in front of Jupiter, any atmospheric features around the edge of the moon block some of Jupiter’s light, allowing STIS to see the features in silhouette. Sparks and his team are continuing to use Hubble to monitor Europa for additional examples of plume candidates and hope to determine the frequency with which they appear.
NASA's future exploration of ocean worlds is enabled by Hubble's monitoring of Europa's putative plume activity and Cassini's long-term investigation of the Enceladus plume. In particular, both investigations are laying the groundwork for NASA's Europa Clipper mission, which is planned for launch in the 2020s.
“If there are plumes on Europa, as we now strongly suspect, with the Europa Clipper we will be ready for them,” said Jim Green, Director of Planetary Science, at NASA Headquarters.
Editor: Karen Northon
Tethys has two overpowering features, a giant impact crater and a great valley. Odysseus Crater (named for a Greek warrior king in Homer's two great works, The Iliad and The Odyssey) dominates the Tethyan western hemisphere. Odysseus Crater is 400 kilometers in-diameter (almost 250 miles). That diameter is nearly two-fifths of Tethys itself. Such an impact could have shattered a solid body, which suggests that the internal composition of Tethys was still partially molten. The crater's rim and central peak have largely collapsed, leaving a shallow crater, and this also suggests a terrain that was elastic enough to change shape. The subdued features of Odysseus are in contrast to many steep cliffs elsewhere. This again suggests that the ancient terrain that was still elastic enough to change shape.
Photos of Tethys show an even larger impact crater, named Odysseus, nearly one-third the diameter of Tethys and larger than Mimas. That such an impact didn't shatter Tethys completely indicates that it may have been liquid or at least not very solid at the time. The crater is now quite flat (or more precisely, it conforms to Tethys' spherical shape), like the craters on Callisto, without the high ring mountains and central peaks commonly seen on the Moon and Mercury.
The second major feature seen on Tethys is a huge valley (called Ithaca Chasma) 100 km wide and 3 to 5 km deep which runs 2000 km or 3/4 of the way around Tethys' circumference (above).
Clearly then, Tethys has not always been frozen solid. At some point in its past it was probably liquid. The impact craters from that era have been smoothed out. As it froze and expanded, the surface must have cracked to accommodate the extra volume producing Ithaca Chasma. The smaller impact craters we see today are more recent. Discovered by Cassini in 1684.
Tethys sits within Saturn's shadow, but not in complete darkness. While in eclipse the moon is illuminated by feeble ringshine reflected from the planet's night side and by sunlight scattered through the rings.
Credit: NASA/JPL/Space Science Institute
Hyperion [pronounced hi-PEER-ee-en; adjective form: Hyperionian] is the largest known irregular (nonspherical) body in the Solar System. Hyperion's average diameter is 270 kilometers (168 miles), but since Hyperion is rather potato-shaped, its shape can be described in terms of its diameter along its three axes: 410 x 260 x220 kilometers (255 x163 x137 miles, respectively). Considering its odd shape, Hyperion is probably a remnant of a larger moon that was destroyed by a major impact.
Hyperion shows no evidence of internal activity. Its irregular shape causes an unusual phenomenon: Each time Hyperion passes Titan, the larger satellite's gravity gives Hyperion a tug and it tumbles erratically, changing orientation. The irregular shape of Hyperion and evidence of bombardment by meteors make it appear to be the oldest surface in the Saturn system.
Hyperion has a notably reddish tint when viewed in natural color. The red color was toned down in this false-color view (left), and the other hues were enhanced, in order to make more subtle color variations across Hyperion's surface more apparent.
Cassini scientists think that Hyperion's unusual appearance can be attributed to the fact that it has an unusually low density for such a large object, giving it weak surface gravity and high porosity. These characteristics help preserve the original shapes of Hyperion's craters by limiting the amount of impact ejecta coating the moon's surface. Impactors tend to make craters by compressing the surface material, rather than blasting it out. Further, Hyperion's weak gravity, and correspondingly low escape velocity, means that what little ejecta is produced has a good chance of escaping the moon altogether.
Credit: NASA/JPL-Caltech/Space Science Institute
June 2 2015
Cassini Sends Final Close Views of Odd Moon Hyperion
NASA's Cassini spacecraft has returned images from its final close approach to Saturn's oddball moon Hyperion, upholding the moon's reputation as one of the most bizarre objects in the solar system. The views show Hyperion's deeply impact-scarred surface, with many craters displaying dark material on their floors.
During this flyby, Cassini passed Hyperion at a distance of about 21,000 miles (34,000 kilometers) at closest approach. Cassini's closest-ever Hyperion flyby took place on Sept. 26, 2005, at a distance of 314 miles (505 kilometers).
Hyperion is the largest of Saturn’s irregular, or potato-shaped, moons and may be the remnant of a violent collision that shattered a larger object into pieces. Cassini scientists attribute Hyperion's peculiar, sponge-like appearance to the fact that it has an unusually low density for such a large object -- about half that of water. Its low density indicates Hyperion is quite porous, with weak surface gravity. These characteristics mean impactors tend to compress the surface, rather than excavating it, and most material that is blown off the surface never returns.
Cassini will make several more close flybys of Saturn's moons this year before departing the planet's equatorial plane to begin a year-long setup of the mission's daring final act. For its grand finale, set for 2017, Cassini will repeatedly dive through the space between Saturn and its rings.
NASA's Cassini imaging scientists processed this view of Saturn's moon Hyperion, taken during a close flyby on May 31, 2015. This flyby marks the mission's final close approach to Saturn's largest irregularly shaped moon.
Credit: NASA/JPL-Caltech/Space Science Institute
With a density of only 1.1, Iapetus must be composed almost entirely of water ice. Discovered by Cassini in 1671.
The leading and trailing hemispheres of Iapetus are radically different. The albedo of most of the leading hemisphere is about .04, as dark as lampblack, whereas the trailing hemisphere's albedo is .6, as bright as Europa. This difference is so striking that Cassini noted that he could see Iapetus only on one side of Saturn and not on the other.
One explanation of this is that the leading hemisphere is dusted with a coating of material knocked off of Phoebe or some other Saturnian body. However, the color of the leading half of Iapetus and that of Phoebe don't quite match. Another possibility is that some active process within Iapetus is responsible. The puzzle is compounded by the fact that the dividing line between the two sides is inexplicably sharp.
On the last day of 2004, Cassini made its first close encounter with Iapetus. The images show that the dark material overlays the topography, indicating that it is relatively young. And as in the image to the left, along the edge of the dark area there are many craters where only one side is covered by the dark material; the boundary between the two regions isn't so sharp after all. So far the Cassini's data do not resolve the puzzle of the origin of the dark material but there's more to come!
One theory proposes that its dark material may have erupted onto Iapetus's icy surface from the interior. Another theory holds that the dark material represented accumulated debris ejected by impact events on dark, outer satellites of Saturn. Details of this Cassini image mosaic do not definitively rule out either of the theories. However, they do provide important new insights and constraints.
Cassini's first encounter with Iapetus also revealed another striking feature not seen before: a ridge 13 kilometers higher than the surrounding terrain that extends at least 1300 km almost almost exactly parallel with Iapetus's equator. The ridge is conspicuous in the picture as an approximately 20-kilometer wide (12 miles) band that extends from the western side of the disc almost to the day/night boundary on the right. On the left horizon, the peak of the ridge reaches at least 13 kilometers (8 miles) above the surrounding terrain. Along the roughly 1,300 kilometer (800 mile) length over which it can be traced in this picture, it remains almost exactly parallel to the equator within a couple of degrees. The physical origin of the ridge has yet to be explained. It is not yet clear whether the ridge is a mountain belt that has folded upward, or an extensional crack in the surface through which material from inside Iapetus erupted onto the surface and accumulated locally, forming the ridge.
All of Saturn's moons except for Iapetus and Phoebe are very nearly in the plane of Saturn's equator. Iapetus is inclined almost 15 degrees.
This high-resolution view shows a vast range of crater sizes in the dark terrain of the leading hemisphere of Saturn's moon Iapetus.
Across the scene, a few small bright spots indicate fresh, rayed craters where impactors have punched through the thin blanket of dark material to the cleaner ice beneath.
The slight elevation on the bottom half of the image is part of the giant equatorial ridge that spans a wide fraction of Iapetus' circumference. The numerous craters on top of the ridge indicate that it is an old surface feature.
This stunning close-up view shows mountainous terrain that reaches about 10 kilometers (6 miles) high along the unique equatorial ridge of Iapetus. The view was acquired during Cassini's only close flyby of the two-toned Saturn moon.
Above the middle of the image can be seen a place where an impact has exposed the bright ice beneath the dark overlying material.
Credit: NASA/JPL/Space Science Institute
Phoebe (FEE-bee) is one of Saturn's most intriguing satellites, orbiting at a distance of 12,952,000 kilometers (8,049,668 miles) from the planet, almost four times the distance from Saturn than its nearest neighbor, the moon Iapetus. Phoebe and Iapetus are the only major moons in the Saturnian system that do not orbit closely to the plane of Saturn's equator.
Phoebe is roughly spherical and has a diameter of about 220 kilometers (about 132 miles), about one-fifteenth the diameter of Earth's moon. Phoebe rotates on its axis every nine hours, and it completes a full orbit around Saturn in about 18 months. Its irregular, elliptical orbit is inclined about 30 degrees to Saturn's equator. Phoebe's orbit is also retrograde, which means it goes around Saturn the opposite direction than most other moons -- as well as most objects in the solar system.
Unlike most major moons orbiting Saturn, Phoebe is very dark and reflects only 6 percent of the sunlight it receives. Its darkness and irregular, retrograde orbit suggest Phoebe is most likey a captured object. A captured object is a celestial body that is trapped by the gravitational pull of a much bigger body, generally a planet. Phoebe's darkness, in particular, suggests that the small moon comes from the outer solar system, an area where there is plenty of dark material.
Some scientists think Phoebe could be a captured Centaur. Centaurs are believed to be Kuiper Belt bodies that migrated into the inner solar system. Centaurs are found between the asteroid belt and the Kuiper Belt, and are considered a kind of intermediate type of small body, neither an asteroid nor a Kuiper Belt object. If Phoebe is indeed a captured Centaur, images and scientific data of Phoebe taken by the Cassini spacecraft will give scientists the first opportunity to study a Kuiper Belt object.
Phoebe's true nature is revealed in startling clarity in this mosaic of two images taken during Cassini's flyby on June 11, 2004. The image shows evidence for the emerging view that Phoebe may be an ice-rich body coated with a thin layer of dark material. Small bright craters in the image are probably fairly young features. This phenomenon has been observed on other icy satellites, such as Ganymede at Jupiter.
When impactors slammed into the surface of Phoebe, the collisions excavated fresh, bright material -- probably ice -- underlying the surface layer. Further evidence for this can be seen on some crater walls where the darker material appears to have slid downwards, exposing more light-colored material. Some areas of the image that are particularly bright - especially near lower right - are over-exposed.
Cassini Finds Saturn Moon has Planet-Like Qualities
PASADENA, Calif. -- Data from NASA's Cassini mission reveal Saturn's moon Phoebe has more planet-like qualities than previously thought.
Scientists had their first close-up look at Phoebe when Cassini began exploring the Saturn system in 2004. Using data from multiple spacecraft instruments and a computer model of the moon's chemistry, geophysics and geology, scientists found Phoebe was a so-called planetesimal, or remnant planetary building block. The findings appear in the April issue of the Journal Icarus.
"Unlike primitive bodies such as comets, Phoebe appears to have actively evolved for a time before it stalled out," said Julie Castillo-Rogez, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Objects like Phoebe are thought to have condensed very quickly. Hence, they represent building blocks of planets. They give scientists clues about what conditions were like around the time of the birth of planets and their moons."
Cassini images suggest Phoebe originated in the far-off Kuiper Belt, the region of ancient, icy, rocky bodies beyond Neptune's orbit. Data show Phoebe was spherical and hot early in its history, and has denser rock-rich material concentrated near its center. Its average density is about the same as Pluto, another object in the Kuiper Belt. Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.
Saturn is surrounded by a cloud of irregular moons that circle the planet in orbits tilted from Saturn's orbit around the sun, the so-called equatorial plane. Phoebe is the largest of these irregular moons and also has the distinction of orbiting backward in relation to the other moons. Saturn's large moons appear to have formed from gas and dust orbiting in the planet's equatorial plane. These moons currently orbit Saturn in that same plane.
Analyses suggest that Phoebe was born within the first 3 million years of the birth of the solar system, which occurred 4.5 billion years ago. The moon may originally have been porous but appears to have collapsed in on itself as it warmed up. Phoebe developed a density 40 percent higher than the average inner Saturnian moon.
Objects of Phoebe's size have long been thought to form as "potato-shaped" bodies and remained that way over their lifetimes. If such an object formed early enough in the solar system's history, it could have harbored the kinds of radioactive material that would produce substantial heat over a short timescale. This would warm the interior and reshape the moon.
Phoebe started with a nearly spherical shape, rather than being an irregular shape later smoothed into a sphere by impacts," said co-author Peter Thomas, a Cassini team member at Cornell.
Phoebe likely stayed warm for tens of millions of years before freezing up. The study suggests the heat also would have enabled the moon to host liquid water at one time. This could explain the signature of water-rich material on Phoebe's surface previously detected by Cassini.
The new study also is consistent with the idea that several hundred million years after Phoebe cooled, the moon drifted toward the inner solar system in a solar-system-wide rearrangement. Phoebe was large enough to survive this turbulence.
Image credit: NASA/JPL/Space Science Institute
Though somewhat larger,Rhea is otherwise very similar to Dione. They both have similar compositions, albedo features and varied terrain. Both rotate synchronously and have dissimilar leading and trailing hemispheres.
Both Dione and Rhea have bright, wispy streaks that stand out against an already-bright surface. The streaks are probably the results of ice that evolved from the interior along fractures in the crust.
Rhea [pronounced REE-uh; adjective: Rhean] is the second largest moon of Saturn, but with a diameter of 1,528 kilometers (949 miles) it is less than a third the size of the largest moon, Titan. Rhea is a small, cold, airless body that is very similar to sister moons Dione and Tethys. As with the other two moons, Rhea is tidally locked in phase with its parent -- one side always faces toward Saturn.Rhea's surface temperatures are also similar to Dione and Tethys, being roughly as warm as -174 degrees Celsius (-281 degrees Fahrenheit) in sunlit areas and ranging down to -220 degrees Celsius (-364 degrees Fahrenheit) in shaded areas. Also like them, it has a high reflectivity (or geometric albedo) suggesting a composition largely of water ice, which behaves like rock in Rhea's temperature range.
At 527,040 kilometers (327,490 miles), Rhea is farther away from Saturn than these other two moons, so there is much less tidal attraction from parent Saturn to cause internal heating on Rhea. This has an important effect. The other two moons have more areas of smooth plains than Rhea. Such plains are probably areas where liquid water reached the surface and ponded in depressions such as craters, forming flat surfaces before refreezing and thus erasing existing craters. The lesser internal warmth at Rhea could have resulted in fewer erasures, or there could have been more bombardment on Rhea. Whatever the reason, Rhea is more heavily cratered than Dione and Tethys.
The leading hemisphere is heavily cratered and uniformly bright. Like Callisto, the craters lack the high relief features seen on the Moon and Mercury.
Rhea is composed primarily of water ice with rock making up less than 1/3 of its mass.
Rhea's history is probably very similar to Dione's. Discovered by Cassini in 1672.
Credit: NASA/JPL-Caltech/Space Science Institute
TitanKing of Saturn's Moons
Titan is the largest of Saturn's satellites. It is the second largest satellite in the solar system, and the only one known to have a dense atmosphere.
It may be the most interesting body, from a terrestrial perspective, in the solar system. For almost two decades, space scientists have searched for clues to the primeval Earth. The chemistry in Titan's atmosphere may be similar to what occurred in Earth's atmosphere several billion years ago.
Because of its thick, opaque atmosphere, astronomers believed Titan was the largest satellite in the solar system. Their measurements were necessarily limited to the cloud tops. Voyager 1's close approach and diametric radio occultation show Titan's surface diameter is only 5,150 kilometers (3,200 miles) - - slightly smaller than Ganymede, Jupiter's largest satellite. Both are larger than Mercury. Titan's density appears to be about twice that of water ice; it may be composed of nearly equal amounts of rock and ice.
Titan's surface cannot be seen in any Voyager photos; it is hidden by a dense, photochemical haze whose main layer is about 300 kilometers (200 miles) above Titan's surface. Several distinct, detached haze layers can be seen above the opaque haze layer. The haze layersmerge with the main layer over the north pole of Titan, forming what scientists first thought was a dark hood. The hood was found, under the better viewing conditions of Voyager 2, to be a dark ring around the pole. The southernhemisphere is slightly brighter than the northern, possibly the result of seasonal effects. When the Voyagers flew past, the season on Titan was the equivalent of mid-April and early May on Earth, or early spring in the northern hemisphere and early fall in the south.
Atmospheric pressure near Titan's surface is about 1.6 bars, 60 percent greater than Earth's. The atmosphere is mostly nitrogen, also the major constituent of Earth's atmosphere.
The surface temperature appears to be about 95 Kelvins (-289 degrees Fahrenheit), only 4 Kelvins above the triple-point temperature of methane. Methane, however, appears to be below its saturation pressure near Titan's surface; rivers and lakes of methane probably don't exist, in spite of the tantalizing analogy to water on Earth. On the other hand, scientists believe lakes of ethane exist, and methane is probably dissolved in the ethane. Titan's methane, through continuing photochemistry, is converted to ethane, acetylene, ethylene, and (when combined with nitrogen) hydrogen cyanide. The last is an especially important molecule; it is a building block of amino acids. Titan's low temperature undoubtedly inhibits more complex organic chemistry.
Titan has no intrinsic magnetic field; therefore it has no electrically conducting and convecting liquid core. Its interaction with Saturn's magnetosphere creates a magnetic wake behind Titan. The big satellite also serves as a source for both neutral and charged hydrogen atoms in Saturn's magnetosphere.
This radar image of Titan (left) shows a semi-circular feature that may be part of an impact crater. Very few impact craters have been seen on Titan so far, implying that the surface is young. Each new crater identified on Titan helps scientists to constrain the age of the surface.
Taken by Cassini's radar mapper on Jan. 13, 2007, during a flyby of Titan, the image swath revealed what appeared to be the northernmost half of an impact crater. This crater is roughly 180 kilometers (110 miles) wide. Only three impact craters have been identified on Titan and several others, like this one, are likely to also have been caused by impact. The bright material is interpreted to be part of the crater’s ejecta blanket, and is likely topographically higher than the surrounding plains. The inner part of the crater is dark, and may represent smooth deposits that have covered the inside of the crater.
This side-by-side view shows a newly discovered impact crater (at left) compared with a previously discovered crater (at right). The new crater was just discovered by the Cassini spacecraft's radar instrument during its most recent Titan flyby on May 12, 2008. This makes the fourth feature definitely identified as an impact crater so far on Titan -- fewer than 100 features are regarded as possible impacts. Compared with Saturn's other moons, which have many thousands of craters, Titan's surface is very sparsely cratered. This is in part due to Titan's dense atmosphere, which burns up the smaller impacting bodies before they can hit the surface. Geological processes, such as wind-driven motion of sand and icy volcanism, may also wipe out craters.
Cassini radar sees sand dunes on Saturn's giant moon Titan (right-upper photo) that are sculpted like Namibian sand dunes on Earth (right-lower photo). The bright features in the upper radar photo are not clouds but topographic features among the dunes.
Photo credit: NASA/JPL - upper photo; NASA/JSC - lower photo
January 23, 2012
Data from NASA's Cassini spacecraft show that the sizes and patterns of dunes on Saturn's moon Titan vary as a function of altitude and latitude. The dunes in areas that are more elevated or are higher in latitude, such as in the Fensal region pictured at bottom left, tend to be thinner and more widely separated, with gaps that have a thinner covering of sand. Dunes in the Belet region, pictured at top left, are at a lower altitude and latitude. The dunes in Belet are wider, with thicker blankets of sand between them. The Kalahari dunes in South Africa and Namibia, located in a region with limited sediment available and pictured at bottom right, show effects similar to the Fensal dunes. The Belet dunes on Titan resemble Earth's Oman dunes in Yemen and Saudi Arabia, where there is abundant sediment available. The Oman dunes are shown at top right.
The altitude effect suggests that the "sand" (likely composed of hydrocarbons) needed to build the dunes is mostly in the lowlands of Titan. Saturn's elliptical orbit may explain why dunes tend to be thinner, more widely separated and less sand-covered in the areas in between dunes as one moves northward. Summers in the southern hemisphere are shorter and warmer than in the northern hemisphere, possibly leaving the soil in the south less moist because northern areas experience more evaporation and condensation. When soil is moist, it is more difficult to move sand particles because they are sticky and heavier. As a result, it is more difficult to build dunes..
Credit: NASA/JPL-Caltech, and NASA/GSFC/METI/ERSDAC/JAROS and U.S./Japan ASTER Science Team
Titan and Earth Similarities
River Rocks: Titan vs Earth
May 11, 2010
The left-hand image, obtained by the European Space Agency’s Huygens probe, shows rounded rocks from the surface of Saturn’s moon Titan. Huygens rode with NASA’s Cassini spacecraft to the Saturn system. The right-hand image, taken by amateur photographer Sandra M. Matheson, shows river rocks on Earth.
Credit: NASA/JPL/ESA/University of Arizona and S.M. Matheson
A recent study finds that the lake known as Ontario Lacus on Saturn's moon Titan (left) bears striking similarity to a salt pan on Earth known as the Etosha Pan (right). A salt pan is a lake bed that fills with a shallow layer of water from groundwater levels that rise during the rainy season. This layer then evaporates and leaves sediments like tide marks showing the previous extent of the water. Ontario Lacus, seen in an image obtained by the radar instrument aboard NASA's Cassini spacecraft on Jan. 12, 2010, covers an area about 140 by 47 miles (230 by 75 kilometers). The Etosha Pan, seen in an image obtained by a NASA and USGS Landsat satellite on Jan. 21, 2003, covers an area about 75 by 40 miles (120 by 65 kilometers). The north direction is indicated by the arrows.
Credit: NASA/JPL-Caltech and NASA/USGS
The Titanian Seasons Turn, Turn, Turn
PASADENA, Calif. – Images from NASA's Cassini spacecraft show a concentration of high-altitude haze and a vortex materializing at the south pole of Saturn's moon Titan, signs that the seasons are turning on Saturn's largest moon. "The structure inside the vortex is reminiscent of the open cellular convection that is often seen over Earth's oceans," said Tony Del Genio, a Cassini team member at NASA's Goddard Institute for Space Studies, N.Y. "But unlike on Earth, where such layers are just above the surface, this one is at very high altitude, maybe a response of Titan's stratosphere to seasonal cooling as southern winter approaches. But so soon in the game, we're not sure."
Cassini first saw a "hood" of high-altitude haze and a vortex, which is a mass of swirling gas around the pole in the moon's atmosphere, at Titan's north pole when the spacecraft first arrived in the Saturn system in 2004. At the time, it was northern winter. Multiple instruments have been keeping an eye on the Titan atmosphere above the south pole for signs of the coming southern winter.
While the northern hood has remained, the circulation in the upper atmosphere has been moving from the illuminated north pole to the cooling south pole. This movement appears to be causing downwellings over the south pole and the formation of high-altitude haze and a vortex.
Cassini's visible light cameras saw the first signs of hazes starting to concentrate over Titan's south pole in March, and the spacecraft's visual and infrared mapping spectrometer (VIMS) obtained false-color images on May 22 and June 7.
"VIMS has seen a concentration of aerosols forming about 200 miles [300 kilometers] above the surface of Titan's south pole," said Christophe Sotin, a VIMS team member at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We've never seen aerosols here at this level before, so we know this is something new."
During a June 27 distant flyby, Cassini's imaging cameras captured a crow's-eye view of the south polar vortex in visible light. These new images show this detached, high-altitude haze layer in stunning new detail.
Comparing Earth with Titan. (left)
Comparing Saturn with Titan (right)
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