Cassini Sends Back Postcards of Saturn Moons

On Jan. 31, 2011, NASA's Cassini spacecraft passed by several of Saturn's intriguing moons, snapping images along the way. Cassini passed within about 60,000 kilometers (37,282 miles) of Enceladus and 28,000 kilometers (17,398 miles) of Helene. It also caught a glimpse of Mimas in front of Saturn's rings. In one of the images, Cassini is looking at the famous jets erupting from the south polar terrain of Enceladus.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute in Boulder, Colo.

Cassini Celebrates 10 Years Since Jupiter Encounter

Ten years ago, on Dec. 30, 2000, NASA's Cassini spacecraft made its closest approach to Jupiter on its way to orbiting Saturn. The main purpose was to use the gravity of the largest planet in our solar system to slingshot Cassini towards Saturn, its ultimate destination. But the encounter with Jupiter, Saturn's gas-giant big brother, also gave the Cassini project a perfect lab for testing its instruments and evaluating its operations plans for its tour of the ringed planet, which began in 2004.

"The Jupiter flyby allowed the Cassini spacecraft to stretch its wings, rehearsing for its prime time show, orbiting Saturn," said Linda Spilker, Cassini project scientist based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Ten years later, findings from the Jupiter flyby still continue to shape our understanding of similar processes in the Saturn system."

Cassini spent about six months - from October 2000 to March 2001 - exploring the Jupiter system. The closest approach brought Cassini to within about 9.7 million kilometers (6 million miles) of Jupiter's cloud tops at 2:05 a.m. Pacific Time, or 10:05 a.m. UTC, on Dec. 30, 2000.

Cassini Marks Holidays With Dramatic Views of Rhea

Newly released for the holidays, images of Saturn's second largest moon Rhea obtained by NASA's Cassini spacecraft show dramatic views of fractures cutting through craters on the moon's surface, revealing a history of tectonic rumbling. The images are among the highest-resolution views ever obtained of Rhea.

The images, captured on flybys on Nov. 21, 2009 and March 2, 2010, can be found at http://www.nasa.gov/cassini, http://saturn.jpl.nasa.gov and http://ciclops.org .

"These recent, high-resolution Cassini images help us put Saturn's moon in the context of the moons' geological family tree," said Paul Helfenstein, Cassini imaging team associate, based at Cornell University, Ithaca, N.Y. "Since NASA's Voyager mission visited Saturn, scientists have thought of Rhea and Dione as close cousins, with some differences in size and density. The new images show us they're more like fraternal twins, where the resemblance is more than skin deep. This probably comes from their nearness to each other in orbit."

Cassini Bags Enceladus 'Tigers'

NASA's Cassini spacecraft has successfully completed its flyby over the "tiger stripes" in the south polar region of Saturn's moon Enceladus and has sent back images of its passage. The spacecraft also targeted the moon Tethys.

The tiger stripes are actually giant fissures that spew jets of water vapor and organic particles hundreds of kilometers, or miles, out into space. While the winter is darkening the moon's southern hemisphere, Cassini has its own version of "night vision goggles" -- the composite infrared spectrometer instrument - to track heat even when visible light is low. It will take time for scientists to assemble the data into temperature maps of the fissures.

The camera was pointing toward Enceladus at approximately 348,913 kilometers (216,805 miles) away, and the image was taken using the CL1 and GRN filters. This image has not been validated or calibrated. A validated/calibrated image will be archived with the NASA Planetary Data System in 2011. Image Credit: NASA/JPL/Space Science Institute

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.

More raw images from the Enceladus flyby, dubbed "E11," are available at: http://saturn.jpl.nasa.gov/photos/raw/

More information about the Cassini-Huygens mission is at: http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100814.html

Blowing in the Wind: Cassini Helps with Dune Whodunit

The answer to the mystery of dune patterns on Saturn's moon Titan did turn out to be blowing in the wind. It just wasn't from the direction many scientists expected.

Basic principles describing the rotation of planetary atmospheres and data from the European Space Agency's Huygens probe led to circulation models that showed surface winds streaming generally east-to-west around Titan's equatorial belt. But when NASA's Cassini spacecraft obtained the first images of dunes on Titan in 2005, the dunes' orientation suggested the sands – and therefore the winds – were moving from the opposite direction, or west to east.

A new paper by Tetsuya Tokano in press with the journal Aeolian Research seeks to explain the paradox. It explains that seasonal changes appear to reverse wind patterns on Titan for a short period. These gusts, which occur intermittently for perhaps two years, sweep west to east and are so strong they do a better job of transporting sand than the usual east-to-west surface winds. Those east-to-west winds do not appear to gather enough strength to move significant amounts of sand.

A related perspective article about Tokano's work by Cassini radar scientist Ralph Lorenz, the lead author on a 2009 paper mapping the dunes, appears in this week's issue of the journal Science.

"It was hard to believe that there would be permanent west-to-east winds, as suggested by the dune appearance," said Tokano, of the University of Cologne, Germany. "The dramatic, monsoon-type wind reversal around equinox turns out to be the key."

Cassini radar sees sand dunes on Saturn's giant moon Titan (upper photo) that are sculpted like Namibian sand dunes on Earth (lower photo). The bright features in the upper radar photo are not clouds but topographic features among the dunes. Image credit: NASA/JPL - upper photo; NASA/JSC - lower photo

The dunes track across the vast sand seas of Titan only in latitudes within 30 degrees of the equator. They are about a kilometer (half a mile) wide and tens to hundreds of kilometers (miles) long. They can rise more than 100 meters (300 feet) high. The sands that make up the dunes appear to be made of organic, hydrocarbon particles. The dunes' ridges generally run west-to-east, as wind here generally sheds sand along lines parallel to the equator.

Scientists predicted winds in the low latitudes around Titan's equator would blow east-to-west because at higher latitudes the average wind blows west-to-east. The wind forces should balance out, based on basic principles of rotating atmospheres.

Tokano re-analyzed a computer-based global circulation model for Titan he put together in 2008. That model, like others for Titan, was adapted from ones developed for Earth and Mars. Tokano added in new data on Titan topography and shape based on Cassini radar and gravity data. In his new analysis, Tokano also looked more closely at variations in the wind at different points in time rather than the averages. Equinox periods jumped out.

Equinoxes occur twice a Titan year, which is about 29 Earth years. During equinox, the sun shines directly over the equator, and heat from the sun creates upwelling in the atmosphere. The turbulent mixing causes the winds to reverse and accelerate. On Earth, this rare kind of wind reversal happens over the Indian Ocean in transitional seasons between monsoons.

The episodic reverse winds on Titan appear to blow around 1 to 1.8 meters per second (2 to 4 mph). The threshold for sand movement appears to be about 1 meter per second (2 mph), a speed that the typical east-to-west winds never appear to surpass. Dune patterns sculpted by strong, short episodes of wind can be found on Earth in the northern Namib sand seas in Namibia, Africa.

Scientists have used data from the Cassini radar mapper to map the global wind pattern on Saturn's moon Titan using data collected over a four-year period, as depicted in this image. Image credit: NASA/JPL/Space Science Institute

"This is a subtle discovery -- only by delving into the statistics of the winds in the model could this rather distressing paradox be resolved," said Ralph Lorenz, a Cassini radar scientist based at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "This work is also reassuring for preparations for proposed future missions to Titan, in that we can become more confident in predicting the winds which can affect the delivery accuracy of landers, or the drift of balloons."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the Cassini-Huygens mission for NASA's Science Mission Directorate. The Cassini orbiter was designed, developed and assembled at JPL. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the United States and several European countries. JPL is a division of the California Institute of Technology in Pasadena.

More Cassini information is available, at http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100729.html

See Beautiful Ontario Lacus: Cassini's Guided Tour

Ontario Lacus, the largest lake in the southern hemisphere of Saturn's moon Titan, turns out to be a perfect exotic vacation spot, provided you can handle the frosty, subzero temperatures and enjoy soaking in liquid hydrocarbon.

Several recent papers by scientists working with NASA's Cassini spacecraft describe evidence of beaches for sunbathing in Titan's low light, sheltered bays for mooring boats, and pretty deltas for wading out in the shallows. They also describe seasonal changes in the lake's size and depth, giving vacationers an opportunity to visit over and over without seeing the same lake twice. (Travel agents, of course, will have to help you figure out how to breathe in an atmosphere devoid of oxygen.)

Using data that give us the most detailed picture yet of a lake on another world, scientists and animators have collaborated on a new video tour of Ontario Lacus based on radar data from Cassini's Titan flybys on June 22, 2009, July 8, 2009, and Jan. 12, 2010. A Web video explaining how scientists look to Earth's Death Valley to understand places like Titan's Ontario Lacus is available at: http://www.nasa.gov/multimedia/videogallery/index.html?collection_id=14658&media_id=16290407

"With such frigid temperatures and meager sunlight, you wouldn't think Titan has a lot in common with our own Earth," said Steve Wall, deputy team lead for the Cassini radar team, based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "But Titan continues to surprise us with activity and seasonal processes that look marvelously, eerily familiar."

Cassini arrived at Saturn in 2004 when the southern hemisphere of the planet and its moons were experiencing summer. The seasons have started to change toward autumn, with winter solstice darkening the southern hemisphere of Titan in 2017. A year on Titan is the equivalent of about 29 Earth years.

This image of Ontario Lacus, the largest lake on the southern hemisphere of Saturn's moon Titan, was obtained by NASA's Cassini spacecraft on Jan. 12, 2010. Image Credit: NASA/JPL-Caltech

Titan is the only other world in our solar system known to have standing bodies of liquid on its surface. Because surface temperatures at the poles average a chilly 90 Kelvin (about minus 300 degrees Fahrenheit), the liquid is a combination of methane, ethane and propane, rather than water. Ontario Lacus has a surface area of about 15,000 square kilometers (6,000 square miles), slightly smaller than its terrestrial namesake Lake Ontario.

Cassini first obtained an image of Ontario Lacus with its imaging camera in 2004. A paper submitted to the journal Icarus by Alex Hayes, a Cassini radar team associate at the California Institute of Technology in Pasadena, and colleagues finds that the lake's shoreline has receded by about 10 kilometers (6 miles). This has resulted in a liquid level reduction of about 1 meter (3 feet) per year over a four–year period.

The shoreline appears to be receding because of liquid methane evaporating from the lake, with a total amount of evaporation that would significantly exceed the yearly methane gas output of all the cows on Earth, Hayes said. Some of the liquid could also seep into porous ground material. Hayes said the changes in the lake are likely occurring as part of Titan's seasonal methane cycle, and would be expected to reverse during southern winter.

This seasonal filling and receding is similar to what occurs at the shallow lakebed known as Racetrack Playa in Death Valley National Park, Hayes said. In fact, from the air, the topography and shape of Racetrack Playa and Ontario Lacus are quite similar, although Ontario Lacus is about 60 times larger.

"We are very excited about these results, because we did not expect Cassini to be able to detect changes of this magnitude in Titan's lakes," Hayes said. "It is only through the continued monitoring of seasonal variation during Cassini's extended mission that these discoveries have been made possible."

Other parts of the Ontario Lacus' shoreline, as described in the paper published in Geophysical Research Letters in March 2010 by Wall, Hayes and other colleagues, show flooded valleys and coasts, further proof that the lake level has changed.

The delta revealed by Cassini radar data on the western shore of Ontario Lacus is also the first well-developed delta observed on Titan, Wall said. He explained that the shape of the land there shows liquid flowing down from a higher plain switching channels on its way into the lake, forming at least two lobes.

Examples of this kind of channel switching and wave-modified deltas can be found on Earth at the southern end of Lake Albert between Uganda and the Democratic Republic of Congo in Africa, and the remains of an ancient lake known as Megachad in the African country Chad, Wall said.

The radar data also show a smooth beach on the northwestern shore of Ontario Lacus. Smooth lines parallel to the current shoreline could be formed by low waves over time, which were likely driven by winds sweeping in from the west or southwest. The pattern at Ontario Lacus resembles what might be seen on the southeastern side of Lake Michigan, where waves sculpt the shoreline in a similar fashion.

"Cassini continues to take our breath away as it fills in the details on the surfaces of these far-off moons," said Linda Spilker, Cassini project scientist based at JPL. "It's exhilarating to ride along as it takes us on the ultimate cold-weather adventure."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the Cassini-Huygens mission for NASA's Science Mission Directorate. The Cassini orbiter was designed, developed and assembled at JPL. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the United States and several European countries.

More Cassini information is available, at http://www.nasa.gov/cassini, http://saturn.jpl.nasa.gov.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100715.html

Saturn System Moves Oxygen From Enceladus to Titan

Complex interactions between Saturn and its satellites have led scientists using NASA's Cassini spacecraft to a comprehensive model that could explain how oxygen may end up on the surface of Saturn's icy moon Titan. The presence of these oxygen atoms could potentially provide the basis for pre-biological chemistry.

The interactions are captured in two papers, one led by John Cooper and another led by Edward Sittler, published in the journal Planetary and Space Science in late 2009. Cooper and Sittler are Cassini plasma spectrometer team scientists at NASA's Goddard Space Flight Center in Greenbelt, Md.

"Titan and Enceladus, another icy moon of Saturn, are chemically connected by the flow of material through the Saturn system," Cooper said.

In one paper, Cooper and colleagues provide an explanation for forces that could generate the Enceladus geysers that spew water vapor into space. In the other, published in the same issue, Sittler and colleagues describe a unique new process in which oxygen that circulates in the upper atmosphere of Titan can be carried all the way to the surface without further chemical contamination by being encased in carbon cages called fullerenes.

The work draws upon previous work by Sittler and others that model the dynamics of how particles, including water molecules, travel from Enceladus to Titan. At Enceladus the flow process begins with what they call the "Old Faithful" model, after the Old Faithful geyser in Yellowstone National Park. In this model, gas pressure slowly builds up inside Enceladus, then gets released occasionally in geyser-like eruptions.

Unlike terrestrial geysers, or even geyser-like forces on Jupiter's moon Io, the model proposed by Cooper shows that charged particle radiation raining down from Saturn’s magnetosphere can create the forces from below the surface that are required to eject gaseous jets.

Energetic particles raining down from Saturn's magnetosphere – at Enceladus, mostly electrons from Saturn's radiation belts -- can break up molecules within the surface. This process is called radiolysis. Like a process called photolysis, in which sunlight can break apart molecules in the atmosphere, energetic radiation from charged particles that hit an icy surface, like that of Enceladus, can cause damage to molecules within the ice. These damaged molecules can get buried deeper and deeper under the surface by the perpetual churning forces that can repave the icy surface. Meteorites constantly crashing into the surface and splashing out material might also be burying the molecules.

This annotated image collage features Saturn and the moons Titan, Enceladus, Dione, Rhea and Helene, which are being studied by the Cassini mission. Image credit: NASA/JPL/Space Science Institute

When chemically altered icy grains come into contact beneath the surface with icy contaminants such as ammonia, methane and other hydrocarbons, they can produce volatile gases that can explode outward. Such gases can create plumes of the size seen by Cassini. Cooper and colleagues call such icy volatile mechanics "cryovolcanism."

What's unique about the "Old Faithful" model is that it "is a model for cryovolcanism that is based on not only liquid water, but also requires the production of gases by the radiolytic chemistry observed at Enceladus," said Sittler.

The plumes that emanate from Enceladus' south polar region consist of water, ammonia and other compounds. Scientists have known since the 1980s that Saturn's magnetosphere is inexplicably filled with neutral particles. In the intervening decades, particularly since the discovery of plumes jetting out from the south pole of Enceladus, work has shown how some of the water molecules that escape from Enceladus get split up into neutral and charged particles and are transported throughout Saturn's magnetosphere.

Sittler's new model indicates that as these broken water molecules enter Titan's atmosphere, they may be captured by fullerenes—hollow, soccer-ball shaped shells made of carbon atoms. Although the heavy molecules Cassini has detected in the upper atmosphere of Titan may be other molecules, Sittler suggests they are likely fullerenes.

In Sittler's model, the fullerenes then condense into larger clusters that can attach to polycyclic aromatic hydrocarbons—chemical compounds also found on Earth in oil, coal and tar deposits, and as the byproducts of burning fossil fuels. The fullerene clusters form even larger aerosols that travel down to Titan's surface.

This process protects the trapped oxygen from Titan's atmosphere, which is saturated with hydrogen atoms and compounds that are capable of breaking down other molecules. Otherwise, the oxygen would combine with methane in Titan's atmosphere and form carbon monoxide or carbon dioxide. Until now, scientists have not been able to explain how oxygen fits into the picture of the dynamics and chemistry of Saturn and its moons.

As the oxygen-rich aerosols fall to Titan's surface, they are further bombarded by products of galactic cosmic ray interactions with Titan's atmosphere. Cosmic rays bombarding the oxygen-stuffed fullerenes could produce more complex organic materials, such as amino acids, in the carbon-rich and oxygen-loaded fullerenes. Amino acids are considered important for pre-biological chemistry.

Scientists have been able to couple the new models that describe the generation of plumes at Enceladus and oxygen ion capture in fullerenes near the top of Titan's atmosphere to existing theories of the transport of oxygen across the magnetosphere. Taken together, Sittler and Cooper suggest a chemical pathway that allows the oxygen to be introduced to Titan's surface chemistry.

"Cooper and Sittler's work helps us understand more about the potential for chemical interactions among Saturn's moons," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

"The Saturn system is indeed a dynamic place, with the Enceladus plumes creating the E ring and loading the magnetosphere with water which interacts with Titan and the other moons," Spilker said.

The Cassini mission is a joint effort of NASA, the European Space Agency, and the Italian space agency Agenzia Spaziale Italiana. The mission is managed for NASA by the Jet Propulsion Laboratory, a division of the California Institute of Technology. Partners include the U.S. Air Force, Department of Energy, and academic and industrial participants from 19 countries.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/saturn20100701.html

Cassini Getting the Lowdown on Titan This Weekend

NASA's Cassini spacecraft will take its lowest dip through the hazy atmosphere of Saturn's moon Titan in the early morning of June 21 UTC, which is the evening of June 20 Pacific time. This weekend's flyby, which is the 71st Titan flyby of the mission even though it is known as "T70," takes Cassini 70 kilometers (43 miles) lower than it has ever been at Titan before.

Titan's atmosphere applies torque to objects flying through it, much the same way the flow of air would wiggle your hand around if you stuck it outside a moving car window. Cassini mission planners and the NASA Engineering and Safety Center in Hampton, Va., have analyzed the torque applied by the atmosphere in detail to make sure the spacecraft can fly safely at an altitude of 880 kilometers (547 miles) above the surface.

Artist's concept of Cassini's flyby of Saturn's moon Titan. The spacecraft flies to within 880 kilometers (547 miles) of Titan's surface during its 71st flyby of Titan, known as "T70," the lowest in the entire mission. Image credit: NASA/JPL-Caltech

When engineers calculated the most stable angle for the spacecraft to fly, they found it was almost the same as the angle that would enable Cassini to point its high-gain antenna to Earth. So they cocked the spacecraft a fraction of a degree, enabling them to track the spacecraft in real-time during its closest approach. Thrusters will fire throughout the flyby to maintain pointing automatically.

But why does Cassini need to get so low? Read on for the perspective of one Cassini team scientist, César Bertucci: http://blogs.nasa.gov/cm/blog/cassini/posts/post_1276817553169.html

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100617.html

Next Stop, Titan: Looking at the Land o' Lakes

NASA's Cassini spacecraft will be eyeing the north polar region of Saturn's moon Titan this weekend, scanning the moon's land o' lakes.

At closest approach on early morning Saturday, June 5 UTC, which is Friday afternoon, June 4 Pacific time, Cassini will glide to within about 2,000 kilometers (1,300 miles) of the Titan surface.

Cassini will make infrared scans of the north polar region, which was in darkness for the first several years of Cassini's tour around the Saturn system. The lighting has improved as northern spring has started to dawn over the area.

Artist's concept of Cassini's June 4, 2010, flyby of Saturn's moon Titan. Image credit: NASA/JPL

The visual and infrared spectrometer will be prime during closest approach, but the imaging science subsystem cameras will also be taking pictures. Among the scientific bounties, Cassini team members are hoping to get another good look at Kraken Mare, the largest lake on Titan, which covers a greater area than the Caspian Sea on Earth.

Although this latest flyby is dubbed "T69," planning changes early in the orbital tour made this the 70th targeted flyby of Titan.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100603.html

What is Consuming Hydrogen and Acetylene on Titan?

PASADENA, Calif. - Two new papers based on data from NASA's Cassini spacecraft scrutinize the complex chemical activity on the surface of Saturn's moon Titan. While non-biological chemistry offers one possible explanation, some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan's surface. According to one theory put forth by astrobiologists, the signatures fulfill two important conditions necessary for a hypothesized "methane-based life."

One key finding comes from a paper online now in the journal Icarus that shows hydrogen molecules flowing down through Titan's atmosphere and disappearing at the surface. Another paper online now in the Journal of Geophysical Research maps hydrocarbons on the Titan surface and finds a lack of acetylene.

This lack of acetylene is important because that chemical would likely be the best energy source for a methane-based life on Titan, said Chris McKay, an astrobiologist at NASA Ames Research Center, Moffett Field, Calif., who proposed a set of conditions necessary for this kind of methane-based life on Titan in 2005. One interpretation of the acetylene data is that the hydrocarbon is being consumed as food. But McKay said the flow of hydrogen is even more critical because all of their proposed mechanisms involved the consumption of hydrogen.

"We suggested hydrogen consumption because it's the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth," McKay said. "If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth."

This artist concept shows a mirror-smooth lake on the surface of the smoggy moon Titan. Image credit: NASA/JPL

To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere, though there are liquid-water-based microbes on Earth that thrive on methane or produce it as a waste product. On Titan, where temperatures are around 90 Kelvin (minus 290 degrees Fahrenheit), a methane-based organism would have to use a substance that is liquid as its medium for living processes, but not water itself. Water is frozen solid on Titan's surface and much too cold to support life as we know it.

The list of liquid candidates is very short: liquid methane and related molecules like ethane. While liquid water is widely regarded as necessary for life, there has been extensive speculation published in the scientific literature that this is not a strict requirement.

The new hydrogen findings are consistent with conditions that could produce an exotic, methane-based life form, but do not definitively prove its existence, said Darrell Strobel, a Cassini interdisciplinary scientist based at Johns Hopkins University in Baltimore, Md., who authored the paper on hydrogen.

Strobel, who studies the upper atmospheres of Saturn and Titan, analyzed data from Cassini's composite infrared spectrometer and ion and neutral mass spectrometer in his new paper. The paper describes densities of hydrogen in different parts of the atmosphere and the surface. Previous models had predicted that hydrogen molecules, a byproduct of ultraviolet sunlight breaking apart acetylene and methane molecules in the upper atmosphere, should be distributed fairly evenly throughout the atmospheric layers.

Strobel found a disparity in the hydrogen densities that lead to a flow down to the surface at a rate of about 10,000 trillion trillion hydrogen molecules per second. This is about the same rate at which the molecules escape out of the upper atmosphere.

"It's as if you have a hose and you're squirting hydrogen onto the ground, but it's disappearing," Strobel said. "I didn't expect this result, because molecular hydrogen is extremely chemically inert in the atmosphere, very light and buoyant. It should 'float' to the top of the atmosphere and escape."

Strobel said it is not likely that hydrogen is being stored in a cave or underground space on Titan. The Titan surface is also so cold that a chemical process that involved a catalyst would be needed to convert hydrogen molecules and acetylene back to methane, even though overall there would be a net release of energy. The energy barrier could be overcome if there were an unknown mineral acting as the catalyst on Titan's surface.

The hydrocarbon mapping research, led by Roger Clark, a Cassini team scientist based at the U.S. Geological Survey in Denver, examines data from Cassini's visual and infrared mapping spectrometer. Scientists had expected the sun's interactions with chemicals in the atmosphere to produce acetylene that falls down to coat the Titan surface. But Cassini detected no acetylene on the surface.

In addition Cassini's spectrometer detected an absence of water ice on the Titan surface, but loads of benzene and another material, which appears to be an organic compound that scientists have not yet been able to identify. The findings lead scientists to believe that the organic compounds are shellacking over the water ice that makes up Titan's bedrock with a film of hydrocarbons at least a few millimeters to centimeters thick, but possibly much deeper in some places. The ice remains covered up even as liquid methane and ethane flow all over Titan's surface and fill up lakes and seas much as liquid water does on Earth.

"Titan's atmospheric chemistry is cranking out organic compounds that rain down on the surface so fast that even as streams of liquid methane and ethane at the surface wash the organics off, the ice gets quickly covered again," Clark said. "All that implies Titan is a dynamic place where organic chemistry is happening now."

The absence of detectable acetylene on the Titan surface can very well have a non-biological explanation, said Mark Allen, principal investigator with the NASA Astrobiology Institute Titan team. Allen is based at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Allen said one possibility is that sunlight or cosmic rays are transforming the acetylene in icy aerosols in the atmosphere into more complex molecules that would fall to the ground with no acetylene signature.

"Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed," Allen said. "We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results - for example, reactions involving mineral catalysts."

"These new results are surprising and exciting," said Linda Spilker, Cassini project scientist at JPL. "Cassini has many more flybys of Titan that might help us sort out just what is happening at the surface."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL.

For more information about the Cassini-Huygens mission visit http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

For more information visit http://www.nasa.gov/topics/solarsystem/features/titan20100603.html

Cassini Returning Enceladus Gravity Data

NASA's Cassini spacecraft successfully completed its 26-hour gravity observation at Saturn's moon Enceladus this week, sending back data scientists will use to understand the moon's interior composition and structure.

The flyby took Cassini through the water-rich plume flaring out from Enceladus' south polar region, with a closest approach of about 100 kilometers (60 miles) occurring in the late afternoon of April 27, 2010, Pacific Time, or just after midnight April 28 UTC.

A steady radio link to NASA's Deep Space Network on Earth enabled Cassini's scientists to use the radio science instrument to measure the variations in the gravitational pull of Enceladus. Analyzing the wiggles will help scientists understand whether an ocean, pond or great lake lies under the famous "tiger stripe" fractures that spew water vapor and organic particles from the south polar region.

NASA’s Casini spacecraft obtained this raw image of Enceladus on April 26, 2010. Image credit: NASA/JPL/Space Science Institute

Results from the experiment will also tell scientists if bubbles of warmer ice in the interior rise toward that region's surface like an underground lava lamp.

Radio science was prime during the flyby and controlled spacecraft pointing. The optical instruments were not pointed at Enceladus during most of the flyby, so the imaging camera obtained some more distant pictures.

Cassini often relies on thrusters to control attitude during flybys such as this one, but this time it turned the thrusters off and relied on its reaction wheels. Using thrusters adds acceleration effects to the spacecraft, complicating the precise measurements needed for the radio science experiment.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.

More information about the Cassini-Huygens mission is at: http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/enceladus20100430.html

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Cassini Finishes Saturnian Doubleheader

NASA's Cassini spacecraft completed its double flyby this week, swinging by Saturn's moons Titan and Dione with no maneuver in between. The spacecraft has beamed back stunning raw images of fractured terrain and craters big and small on Dione, a moon that had only been visited once before by Cassini.

This image was taken on April 7, 2010 by NASA's Cassini spacecraft. The camera was pointing at Saturn. But, by appropriate orientation of the spacecraft, the cameras were able to capture Dione in the sights. Image Credit: NASA/JPL/Space Science Institute

The Titan flyby took place April 5, and the Dione flyby took place April 7 in the UTC time zone, and April 6 Pacific time. During the Titan flyby, an unexpected autonomous reset occurred and Cassini obtained fewer images of Titan than expected. But the cameras were reset before reaching Dione, which was the primary target on this double flyby.

Scientists are poring over data from Dione to discern whether the moon could be a source of charged particles to the environment around Saturn and material to one of its rings. They are also trying to understand the history of dark material found on Dione.

A fortuitous alignment of these moons allowed Cassini to attempt this doubleheader. Cassini had made three previous double flybys and another two are planned in the years ahead. The mission is nearing the end of its first extension, known as the Equinox Mission. It will begin its second mission extension, known as the Solstice Mission, in October 2010.

Saturn's moon Dione. While pointed at Saturn, Cassini's cameras were able, by appropriate orientation, to spy the icy moon. Image Credit: NASA/JPL/Space Science Institute

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.

More information about the Titan flyby, dubbed "T67," is available at: http://saturn.jpl.nasa.gov/mission/flybys/titan20100405/.
More information about the Dione flyby, dubbed "D2," is available at: http://saturn.jpl.nasa.gov/mission/flybys/dione20100407/.
For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100412.html

Cassini Shows Saturnian Roller Derby, Strange Weather

From our vantage point on Earth, Saturn may look like a peaceful orb with rings worthy of a carefully raked Zen garden, but NASA's Cassini spacecraft has been shadowing the gas giant long enough to see that the rings are a rough and tumble roller derby. It has also revealed that the planet itself roils with strange weather and shifting patterns of charged particles. Two review papers to be published in the March 19 issue of the journal Science synthesize Cassini's findings since arriving at Saturn in 2004.

"This rambunctious system gives us a new feel for how an early solar system might have behaved," said Linda Spilker, a planetary scientist and the new Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "This kind of deep, rich data can only be collected by an orbiting spacecraft, and we look forward to the next seven years around Saturn bringing even more surprises."

In the paper describing the elegant mess of activity in the rings, lead author Jeff Cuzzi, Cassini's interdisciplinary scientist for rings and dust who is based at NASA Ames Research Center, Moffett Field, Calif., describes how Cassini has shown us that collisions are routine and chunks of ice leave trails of debris in their wakes. Spacecraft data have also revealed how small moons play tug-of-war with ring material and how bits of rubble that would otherwise join together to become moons are ultimately ripped apart by the gravitational pull that Saturn exerts.

During equinox, the period when sunlight hits the rings exactly edge-on, Cassini witnessed rings that are normally flat - about tens of meters (yards) thick - being flipped up as high as the Rocky Mountains.

The spacecraft has also shown that the rings are composed mostly of water ice, with a mysterious reddish contaminant that could be rust or small organic molecules similar to those found in red vegetables on Earth.

"It has been amazing to see the rings come to life before our very eyes, changing even as we watch, being colorful and taking on a tangible, 3-D nature," Cuzzi said. "The rings were still a nearly unstructured object in even the best telescopes when I was a grad student, but Cassini has brought us an intimate familiarity with them."

This natural color view from the Cassini spacecraft highlights the myriad gradations in the transparency of Saturn's inner rings. Image credit: NASA/JPL/Space Science Institute

Cuzzi said Cassini scientists were surprised to find such fine-scale structure nearly everywhere in the rings, forcing them to be very careful about generalizing their findings across the entire ring disk. The discovery that the rings are clumpy has also called into question some of the previous estimates for the mass of the rings because there might be clusters of material hidden inside of the clumps that have not yet been measured.

In the review paper on Saturn's atmosphere, ionosphere and magnetosphere, lead author Tamas Gombosi, Cassini's interdisciplinary scientist for magnetosphere and plasma science who is based at the University of Michigan in Ann Arbor, describes how Cassini helped scientists understand a south polar vortex that has a diameter 20 to 40 times that of a terrestrial hurricane, and the bizarrely stable hexagon-shaped jet stream at the planet's north pole. Cassini scientists have also calculated a variation in Saturn's wind speeds at different altitudes and latitudes that is 10 times greater than the wind speed variation on Earth.

According to Gombosi's paper, Cassini has also shown us that the small moon Enceladus, not the sun or Saturn's largest moon Titan, is the biggest contributor of charged particles to Saturn's magnetic environment. The charged particles from Enceladus, a moon that features a plume of water vapor and other gases spraying from its south polar region, also contribute to the auroras around the poles of the planet.

"We learned from Cassini that the Saturnian magnetosphere is swimming in water," Gombosi said. "This is unique in the solar system and makes Saturn's plasma environment particularly fascinating."

Of course, Cassini's intense investigation has opened up a host of new mysteries. For example, Cassini has shown us images of occasional cannon-ball-like objects that rocket across one of the outer rings known as the F ring, without many clues about where they came from or why they quickly disappear.

Learning more about a kind of radio emission known as "kilometric radiation" at Saturn has unsettled debates about the planet's rotation rate rather than settled them. While the regular periods of kilometric radiation have given scientists a sense of the rotation rate at Jupiter, Saturn has clocked different periods for the radiation during NASA's Voyager flybys in 1980 and 1981 and the nearly six years of Cassini's investigations. The modulations vary by about 30 seconds to a minute, but they shouldn't be varying at all. The inconsistency may be related to a source in the magnetic bubble around the planet rather than the core of the gas giant, but scientists are still debating.

"Cassini has answered questions we were not even smart enough to ask when the mission was planned and raised a lot of new ones," Cuzzi said. "We are hot on the trail, though."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the project for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.

More Cassini information is available at http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100318.html

Cassini Data Show Ice and Rock Mixture Inside Titan

PASADENA, Calif. -- By precisely tracking NASA's Cassini spacecraft on its low swoops over Saturn's moon Titan, scientists have determined the distribution of materials in the moon's interior. The subtle gravitational tugs they measured suggest the interior has been too cold and sluggish to split completely into separate layers of ice and rock.

The finding, to be published in the March 12 issue of the journal Science, shows how Titan evolved in a different fashion from inner planets such as Earth, or icy moons such as Jupiter's Ganymede, whose interiors have split into distinctive layers.

"These results are fundamental to understanding the history of moons of the outer solar system," said Cassini Project Scientist Bob Pappalardo, commenting on his colleagues' research. Pappalardo is with NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We can now better understand Titan's place among the range of icy satellites in our solar system."

Scientists have known that Titan, Saturn's largest moon, is about half ice and half rock, but they needed the gravity data to figure out how the materials were distributed. It turns out Titan's interior is a sorbet of ice studded with rocks that probably never heated up beyond a relatively lukewarm temperature. Only in the outermost 500 kilometers (300 miles) is Titan's ice devoid of any rock, while ice and rock are mixed to various extents at greater depth.

This artist's illustration shows the likely interior structure of Saturn's moon Titan deduced from gravity field data collected by NASA's Cassini spacecraft. Image credit: NASA/JPL

"To avoid separating the ice and the rock, you must avoid heating the ice too much," said David J. Stevenson, one of the paper's co-authors and a professor of planetary science at the California Institute of Technology in Pasadena. "This means that Titan was built rather slowly for a moon, in perhaps around a million years or so, back soon after the formation of the solar system."

This incomplete separation of ice and rock makes Titan less like Jupiter's moon Ganymede, where ice and rock have fully separated, and perhaps more like another Jovian moon, Callisto, which is believed to have a mixed ice and rock interior. Though the moons are all about the same size, they clearly have diverse histories.

The Cassini measurements help construct a gravity map, which may help explain why Titan has a stunted topography, since interior ice must be warm enough to flow slowly in response to the weight of heavy geologic structures, such as mountains.

Creating the gravity map required tracking minute changes in Cassini's speed along a line of sight from Earth to the spacecraft as it flew four close flybys of Titan between February 2006 and July 2008. The spacecraft took paths between about 1,300 to 1,900 kilometers (800 to 1,200 miles) above Titan.

"The ripples of Titan's gravity gently push and pull Cassini along its orbit as it passes by the moon and all these changes were accurately recorded by the ground antennas of the Deep Space Network within 5 thousandths of a millimeter per second [0.2 thousandths of an inch per second] even as the spacecraft was over a billion kilometers [more than 600 million miles] away," said Luciano Iess, a Cassini radio science team member at Sapienza University of Rome in Italy, and the paper's lead author. "It was a tricky experiment."

The results don't speak to whether Titan has an ocean beneath the surface, but scientists say this hypothesis is very plausible and they intend to keep investigating. Detecting tides induced by Saturn, a goal of the radio science team, would provide the clearest evidence for such a hidden water layer.

A Cassini interdisciplinary investigator, Jonathan Lunine, said of his colleagues' findings, "Additional flybys may tell us whether the crust is thick or thin today." Lunine is with the University of Rome, Tor Vergata, Italy, and the University of Arizona, Tucson. "With that information we may have a better understanding of how methane, the ephemeral working fluid of Titan's rivers, lakes and clouds, has been resupplied over geologic time. Like the history of water on Earth, this is fundamental to a deep picture of the nature of Titan through time."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of Caltech, manages the project for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL. Cassini's radio science subsystem has been jointly developed by NASA and the Italian Space Agency (ASI).

More Cassini information is available, at http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100311.html

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Deep Space Network Antenna Starts Major Surgery

Like a hard-driving athlete whose joints need help, the giant "Mars antenna" at NASA's Deep Space Network site in Goldstone, Calif. has begun major, delicate surgery. The operation on the historic 70-meter-wide (230-foot) antenna, which has received data and sent commands to deep space missions for over 40 years, will replace a portion of the hydrostatic bearing assembly. This assembly enables the antenna to rotate horizontally.

The rigorous engineering plans call for lifting about 4 million kilograms (9 million pounds) of finely tuned scientific instruments a height of about 5 millimeters (0.2 inches) so workers can replace the steel runner, walls and supporting grout. This is the first time the runner has been replaced on the Mars antenna.

The operation, which will cost about $1.25 million, has a design life of 20 years.

"This antenna has been a workhorse for NASA/JPL for over 40 years," said Alaudin Bhanji, Deep Space Network Project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "It has provided a critical lifeline to dozens of missions, while enabling scientific results that have enriched the hearts and minds of generations. We want it to continue doing so."

The repair will be done slowly because of the scale of the task, with an expected completion in early November. During that time, workers will also be replacing the elevation bearings, which enable the antenna to track up and down from the horizon. The network will still be able to provide full coverage for deep space missions by maximizing use of the two other 70-meter antennas at Deep Space complexes near Madrid, Spain, and Canberra, Australia, and arraying several smaller 34-meter (110-foot) antennas together.

NASA built the Mars antenna when missions began venturing beyond the orbit of Earth and needed more powerful communications tools. The Mars antenna was the first of the giant antennas designed to receive weak signals and transmit very strong ones far out into space, featuring a 64-meter-wide (210-foot) dish when it became operational in 1966. (The dish was upgraded from 64 to 70 meters in 1988 to enable the antenna to track NASA's Voyager 2 spacecraft as it encountered Neptune and Uranus.)

While officially dubbed Deep Space Station 14, the antenna picked up the Mars name from its first task: tracking the Mariner 4 spacecraft, which had been lost by smaller antennas after its historic flyby of Mars. Through its history, the Mars antenna has supported missions including Pioneer, Cassini and the Mars Exploration Rovers. It received Neil Armstrong's famous communiqué from Apollo 11: "That's one small step for man. One giant leap for mankind." It has also helped with imaging nearby planets, asteroids and comets by bouncing its powerful radar signal off the objects of study.

The 70-meter antenna at the Goldstone Deep Space Communications Complex in the Mohave Desert in California. This complex is one of three comprising NASA's Deep Space Network. Image credit: NASA/JPL

A flat, stable surface is critical for the Mars antenna to rotate slowly as it tracks spacecraft. Three steel pads support the weight of the antenna rotating structure, dish and other communications equipment above the circular steel runner. A film of oil about the thickness of a sheet of paper -- about 0.25 millimeters (0.010 inches) -- is produced by a hydraulic system to float the three pads.

After decades of constant use, oil has seeped through the runner joints, slowly degrading the structural integrity of the cement-based grout that supports it. Rather than continuing on a weekly schedule to adjust shims underneath the runner to keep it flat, Deep Space Network managers decided to replace the whole runner assembly.

"As with any large, rotating structure that has operated almost 24 hours per day, seven days per week for over 40 years, we eventually have to replace major elements," said Wayne Sible, the network's deputy project manager at JPL. "We need to replace those worn parts so we can get another 20 years of valuable service from this national treasure."

Over the next few months, workers will lay a new epoxy grout that is impervious to oil and fit the antenna with a thicker runner with more tightly sealed joints. They will then test that the rotation is smooth before turning the antenna back on again.

"The runner replacement task has been in development for close to two years," said JPL's Peter Hames, who is responsible for maintaining the network's antennas. "We've been testing and evaluating modern epoxy grouts, which were unavailable when the antenna was built, updating the design of the runner and designing a replacement process that has to be performed without completely disassembling the antenna. We've had to make sure we've reviewed it for practicality and safety."

JPL, a division of the California Institute of Technology in Pasadena, manages the Deep Space Network for NASA Headquarters, Washington. More information about the Deep Space Network is online at: http://deepspace.jpl.nasa.gov .

For more information visit http://www.nasa.gov/topics/technology/features/dsn20100308.html

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Smooth Sailing by Rhea and Helene

Cassini's closest-ever flyby of Saturn's moon Rhea went quite smoothly and teams are busy checking out their data! These flybys never fail to amaze me. And the raw images -- which give us an unprocessed first look -- are really cool!

Raw image N00152150 gives us a view of part of the bright, fractured terrain we refer to as "wispy terrain" from about 14,000 kilometers (8,900 miles) away. We know that Rhea's albedo overall is quite high. (When I say "albedo," I basically mean "brightness" or "reflectivity." Studying the albedo can tell a lot about surface composition, geologic processes, and interactions with external environment.) But this image demonstrates how bright these cracks are since they are so shiny that the surrounding terrain looks quite dark. There are also some interesting apparent albedo variations seen in this image, which are really intriguing.

Amanda Hendrix
Scientist, works on the Cassini mission to Saturn's Ultraviolet Imaging Spectograph. She is also co-lead for the Satellite Orbiter Science Team.

This raw image (N00152175) from Cassini's narrow-angle camera image was taken about 40 minutes after closest approacha. The image shows a region adjacent to the wispy terrain --craters, craters everywhere! And wow, are those crater rims bright compared to the surrounding terrain.

Cassini captured a full portrait of the serene moon with its wide-angle camera (raw image W00063107) on the outbound leg of the flyby, about 1.25 hours after closest approach. Keep in mind that the phase angle is quite low here (only about 2.5 degrees), meaning that the sun is almost directly behind Cassini and Rhea is nearly fully illuminated -- so there are no shadows. Large-scale albedo variations are apparent across the surface.

This image was taken on March 02, 2010 and received on Earth March 03, 2010. Image credit: NASA/JPL/Space Science Institute .

The spacecraft also obtained a cool image of little Helene with raw image N00152211 . We're basically looking at the night side of the body -- but it doesn't appear very dark, because it's illuminated by sunlight reflecting off Saturn. During the later image sequence of Helene, this small moon was transiting Saturn - so you can see Saturn in the background.

This image was taken on March 03, 2010 and received on Earth March 03, 2010. Image credit: NASA/JPL/Space Science Institute

Sometimes, pointing at these little guys can be very tricky, especially so close after a targeted flyby. It can be difficult (or impossible!) to get the positions of the spacecraft, the moon and the instruments all lined up -- but boy are these close-up Helene images incredible! The detail on the surface is tremendous, and should go a long way to informing geologists about surface properties and processes.

As the imaging team is taking a closer look at images such as these, other instrument teams -- including those for the radar instrument, composite infrared spectrometer, visual and infrared mapping spectrometer and the ultraviolet imaging spectrograph (the instrument I work on) -- are also busy processing their data. At a science meeting Friday, we talked about a few of the preliminary results. Some of the magnetospheric and plasma science instruments teams reported that they're seeing some really interesting and surprising results! So stay tuned to hear more about those!

Of course, after one successful flyby, we get right to work on another. Coming up next: Dione on April 7!

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100308.html

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Cassini Finds Plethora of Plumes, Hotspots at Enceladus

Newly released images from last November's swoop over Saturn's icy moon Enceladus by NASA's Cassini spacecraft reveal a forest of new jets spraying from prominent fractures crossing the south polar region and yield the most detailed temperature map to date of one fracture.

The new images from the imaging science subsystem and the composite infrared spectrometer teams also include the best 3-D image ever obtained of a "tiger stripe," a fissure that sprays icy particles, water vapor and organic compounds. There are also views of regions not well-mapped previously on Enceladus, including a southern area with crudely circular tectonic patterns.

The images and additional information are online at http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

"Enceladus continues to astound," said Bob Pappalardo, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "With each Cassini flyby, we learn more about its extreme activity and what makes this strange moon tick."

In this unique mosaic image combining high-resolution data from the imaging science subsystem and composite infrared spectrometer aboard NASA's Cassini spacecraft, pockets of heat appear along one of the mysterious fractures in the south polar region of Saturn's moon Enceladus. Image credit: NASA/JPL/GSFC/SWRI/SSI

For Cassini's visible-light cameras, the Nov. 21, 2009 flyby provided the last look at Enceladus' south polar surface before that region of the moon goes into 15 years of darkness, and includes the most detailed look yet at the jets.

Scientists planned to use this flyby to look for new or smaller jets not visible in previous images. In one mosaic, scientists count more than 30 individual geysers, including more than 20 that had not been seen before. At least one jet spouting prominently in previous images now appears less powerful.

"This last flyby confirms what we suspected," said Carolyn Porco, imaging team lead based at the Space Science Institute in Boulder, Colo. "The vigor of individual jets can vary with time, and many jets, large and small, erupt all along the tiger stripes."

A new map that combines heat data with visible-light images shows a 40-kilometer (25-mile) segment of the longest tiger stripe, known as Baghdad Sulcus. The map illustrates the correlation, at the highest resolution yet seen, between the geologically youthful surface fractures and the anomalously warm temperatures that have been recorded in the south polar region. The broad swaths of heat previously detected by the infrared spectrometer appear to be confined to a narrow, intense region no more than a kilometer (half a mile) wide along the fracture.

In these measurements, peak temperatures along Baghdad Sulcus exceed 180 Kelvin (minus 135 degrees Fahrenheit), and may be higher than 200 Kelvin (minus 100 degrees Fahrenheit). These warm temperatures probably result from heating of the fracture flanks by the warm, upwelling water vapor that propels the ice-particle jets seen by Cassini's cameras. Cassini scientists will be testing this idea by investigating how well the hot spots correspond with the jet sources.

"The fractures are chilly by Earth standards, but they're a cozy oasis compared to the numbing 50 Kelvin (-370 Fahrenheit) of their surroundings," said John Spencer, a composite infrared spectrometer team member based at Southwest Research Institute in Boulder, Colo. "The huge amount of heat pouring out of the tiger stripe fractures may be enough to melt the ice underground. Results like this make Enceladus one of the most exciting places we've found in the solar system."

Dramatic plumes, both large and small, spray water ice out from many locations along the famed "tiger stripes" near the south pole of Saturn's moon Enceladus. Image credit: NASA/JPL/SSI

Some of Cassini's scientists infer that the warmer the temperatures are at the surface, the greater the likelihood that jets erupt from liquid. "And if true, this makes Enceladus' organic-rich, liquid sub-surface environment the most accessible extraterrestrial watery zone known in the solar system," Porco said.

The Nov. 21 flyby was the eighth targeted encounter with Enceladus. It took the spacecraft to within about 1,600 kilometers (1,000 miles) of the moon's surface, at around 82 degrees south latitude.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo. The composite infrared spectrometer team is based at NASA's Goddard Space Flight Center, Greenbelt, Md., where the instrument was built.

More details are also available at the imaging team's website http://ciclops.org and the composite infrared spectrometer team's website http://cirs.gsfc.nasa.gov.
› See related images

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook@jpl.nasa.gov

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100223.html

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Behold the Violent History of Saturn's White Whale Moon

Like the battered white whale Moby Dick taunting Captain Ahab, Saturn's moon Prometheus surges toward the viewer in a 3-D image from NASA's Cassini spacecraft.

The image exposes the irregular shape and circular surface scars on Prometheus, pointing to a violent history. These craters are probably the remnants from impacts long ago.

Prometheus is one of Saturn's innermost moons. It orbits the gas-giant at a distance of about 140,000 kilometers (86,000 miles) and is 86 kilometers (53 miles) across at its widest point. The porous, icy world was originally discovered in images taken by NASA's Voyager 1 spacecraft back in 1980.

Cassini's narrow-angle camera captured two black-and-white images of the moon on Dec. 26, 2009, and the imaging team combined the images to make this new stereo view. It looks different from the "egg-cellent" raw image of Prometheus obtained on Jan. 27 because that view shows one of the short ends of the oddly shaped moon. In this 3-D image, the sun illuminates Prometheus at a different angle, making the moon's elongated body visible.

Saturn's potato-shaped moon Prometheus is rendered in three dimensions in this close-up from Cassini. Image credit: NASA/JPL/Space Science Institute

The Cassini Equinox Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. For more information about the Cassini Equinox Mission visit: http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100219.html

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Cassini Shoots New Close-Ups of Death Star-like Moon

Blazing through its closest pass of the Saturnian moon Mimas on Feb. 13, Cassini sent back striking close-ups of the moon likened to the Death Star from "Star Wars" and the enormous crater scarring its surface. The flyby also yielded solid data on the moon's thermal signature and surface composition.

Some of the raw, unprocessed images sent back from the flyby show the bright, steep slopes of the giant Herschel Crater, which measures about 140 kilometers (88 miles) wide. The icy slopes appear to be pitched around 24 degrees, which would probably earn them a black- or double-black-diamond rating on Earth. Olympic downhill skiers could probably tear down these runs with ease, but it's clear Mimas is no place for bunny-slope beginners.

Cassini's Feb. 13, 2010, flyby of Saturn's moon Mimas took the spacecraft as close as about 9,500 kilometers (5,900 miles) above Mimas to examine the surface in detail. Image credit: NASA/JPL/Space Science Institute

The images, which have the highest resolution so far, also show jumbled terrain inside the crater and many craters within the crater. These features hint at a long history, which scientists will be working diligently to analyze.

"This flyby has been like looking at a cell or an onion skin under the microscope for the first time," said Bonnie Buratti, one of the leads for the Satellite Orbiter Science Team. "We'd seen the large crater from afar since the early 1980s, but now its small bumps and blemishes are all clearly visible."

Cassini captured this image of Mimas' giant Herschel Crater, which measures about 140 kilometers (88 miles) wide, during its Feb. 13, 2010, flyby of the Death Star-like Saturnian moon. Image credit: NASA/JPL/Space Science Institute

This encounter took the spacecraft as close as about 9,500 kilometers (5,900 miles) above Mimas. Cassini had to maneuver through a dusty region to get in position, but survived the trip unscathed, as expected.

The moon averages 396 kilometers (246 miles) in diameter. The walls of Herschel Crater are about 5 kilometers (3 miles) high, and parts of the floor are approximately 10 kilometers (6 miles) deep.

Unprocessed images of the flyby are available at http://saturn.jpl.nasa.gov/photos/raw/. More information about the Cassini mission is at http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100216.html

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