CASSINI PHOTOGRAPHS TITAN WITH ITS DEEP BLUE ATMOSPHERE ON 2011 SEPTEMBER 11

News Releases about SATURN

THE ASTRONOMY PICTURE OF THE DAY FOR 2011 February 27

Saturn's Hyperion: A Moon with Odd Craters

Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Explanation: What lies at the bottom of Hyperion's strange craters? Nobody's sure. To help find out, the robot Cassini spacecraft now orbiting Saturn swooped past the sponge-textured moon in 2005 and 2010 and took images of unprecedented detail. An image from the 2005 pass, shown above in false color, shows a remarkable world strewn with strange craters and a generally odd surface. The slight differences in color likely show differences in surface composition. At the bottom of most craters lies some type of unknown dark material. Inspection of the image shows bright features indicating that the dark material might be only tens of meters thick in some places. Hyperion is about 250 kilometers across, rotates chaotically, and has a density so low that it might house a vast system of caverns inside.

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Extended (Equinox) Mission Flybys

Solstice Mission Flybys

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May 17th, 2010 from www.UniverseToday.com

Cassini's Cruise: Close Flybys of Two Moons in Less Than Two Days

Written by Nancy Atkinson

It's a space navigator's dream! The Cassini spacecraft will perform close flybys of two of Saturn's most enigmatic moons all within less than 48 hours

, and with no maneuvers in between. Enceladus and Titan are aligned just right so that Cassini can catch glimpses of these two contrasting moons – one a geyser world and the other an analog to early Earth.

Cassini will make its closest approach to Enceladus late at night on May 17 Pacific time, which is in the early hours of May 18 UTC. The spacecraft will pass within about 435 kilometers (270 miles) of the moon's surface.

The main scientific goal at Enceladus will be to watch the sun play peekaboo behind the water-rich plume emanating from the moon's south polar region. Scientists using the ultraviolet imaging spectrograph will be able to use the flickering light to measure whether there is molecular nitrogen in the plume. Ammonia has already been detected in the plume and scientists know heat can decompose ammonia into nitrogen molecules. Determining the amount of molecular nitrogen in the plume will give scientists clues about thermal processing in the moon's interior.

Then on to Titan: the closest approach will take place in the late evening May 19 Pacific time, which is in the early hours of May 20 UTC. The spacecraft will fly to within 1,400 kilometers (750 miles) of the surface.

Cassini will primarily be doing radio science during this pass to detect the subtle variations in the gravitational tug on the spacecraft by Titan, which is 25 percent larger in volume than the planet Mercury. Analyzing the data will help scientists learn whether Titan has a liquid ocean under its surface and get a better picture of its internal structure. The composite infrared spectrometer will also get its southernmost pass for thermal data to fill out its temperature

map of the smoggy moon.

Cassini has made four previous double flybys and one more is planned in the years ahead.

For more information on the Enceladus flyby, dubbed "E10," see this link.

For more information on the Titan flyby, dubbed "T68," see this link.

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THE ASTRONOMY PICTURE OF THE DAY FOR 2010 March 10

Saturn's Moon Helene from Cassini

Credit: NASA/JPL/SSI

Explanation: What's happening on the surface of Saturn's moon Helene? The moon was imaged in unprecedented detail last week as the robotic Cassini spacecraft orbiting Saturn swooped to within two Earth diameters of the diminutive moon. Although conventional craters and hills appear, the above raw and unprocessed image also shows terrain that appears unusually smooth and streaked. Planetary astronomers will be inspecting these detailed images of Helene to glean clues about the origin and evolution of the 30-km across floating iceberg. Helene is also unusual because it circles Saturn just ahead of the large moon Dione, making it one of only four known Saturnian moons

to occupy a gravitational well known as a stable Lagrange Point.

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2010 FEB 3

RELEASE : 10-030

NASA Extends Cassini's Tour of Saturn, Continuing International Cooperation for World Class Science

WASHINGTON -- NASA will extend the international Cassini-Huygens mission to explore Saturn and its moons to 2017. The agency's fiscal year 2011 budget provides a $60 million per year extension for continued study of the ringed planet.

"This is a mission that never stops providing us surprising scientific results and showing us eye popping new vistas," said Jim Green, director of NASA's planetary science division at NASA Headquarters in Washington. "The historic traveler's stunning discoveries and images have revolutionized our knowledge of Saturn and its moons."

Cassini launched in October 1997 with the European Space Agency's Huygens probe. The spacecraft arrived at Saturn in 2004. The probe was equipped with six instruments to study Titan, Saturn's largest moon. Cassini's 12 instruments have returned a daily stream of data from Saturn's system for nearly six years. The project was scheduled to end in 2008, but the mission received a 27-month extension to Sept. 2010.

"The extension presents a unique opportunity to follow seasonal changes of an outer planet system all the way from its winter to its summer," said Bob Pappalardo, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Some of Cassini's most exciting discoveries still lie ahead."

This second extension, called the Cassini Solstice Mission, enables scientists to study seasonal and other long-term weather changes on the planet and its moons. Cassini arrived just after Saturn's northern winter solstice, and this extension continues until a few months past northern summer solstice in May 2017. The northern summer solstice marks the beginning of summer in the northern hemisphere and winter in the southern hemisphere.

A complete seasonal period on Saturn has never been studied at this level of detail. The Solstice mission schedule calls for an additional 155 orbits around the planet, 54 flybys of Titan and 11 flybys of the icy moon Enceladus.

The mission extension also will allow scientists to continue observations of Saturn's rings and the magnetic bubble around the planet known as the magnetosphere. The spacecraft will make repeated dives between Saturn and its rings to obtain in depth knowledge of the gas giant. During these dives, the spacecraft will study the internal structure of Saturn, its magnetic fluctuations and ring mass.

The mission will be evaluated periodically to ensure the spacecraft has the ability to achieve new science objectives for the entire extension.

"The spacecraft is doing remarkably well, even as we endure the expected effects of age after logging 2.6 billion miles on its odometer," said Bob Mitchell, Cassini program manager at JPL. "This extension is important because there is so much still to be learned at Saturn. The planet is full of secrets, and it doesn't give them up easily."

Cassini's travel scrapbook includes more than 210,000 images; information gathered during more than 125 revolutions around Saturn; 67 flybys of Titan and eight close flybys of Enceladus. Cassini has revealed unexpected details in the planet's signature rings, and observations of Titan have given scientists a glimpse of what Earth might have been like before life evolved.

Scientists hope to learn answers to many questions that have developed during the course of the mission, including why Saturn seems to have an inconsistent rotation rate and how a probable subsurface ocean feeds the Enceladus' jets.

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

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Equinox Arrived in August 2009!

The 15-year wait is over! At 00:15 Universal Coordinated Time on August 11 (8:15 PM EDT on August 10th), the moment of equinox arrived at Saturn, and Cassini was on hand to witness this spectacle of sunlight and shadow. A series of raw, unprocessed images has just beamed back from the spacecraft, and a few are posted here.

Equinox occurs every half-Saturn-year which is equivalent to about 15 Earth years. The illumination geometry that accompanies equinox lowers the sun's angle to the ringplane and causes out-of-plane structures and some moons to cast long shadows across the rings. The ring shadows themselves have become a rapidly narrowing band cast onto the planet.

Revelations in Saturn's rings continue as Saturn's Equinox approached on August 11

August 7, 2009

Cassini-Huygens Mission Status Report -- Feb. 2, 2009

PASADENA, Calif. – The Cassini spacecraft will swap to a backup set of propulsion thrusters in mid-March due to degradation in the performance of the current set of thrusters.

The thrusters are used for making small corrections to the spacecraft's course, for some attitude control functions, and for making angular momentum adjustments in the reaction wheels, which also are used for attitude control.

The current set of eight thrusters, referred to as branch A, has been in use since Cassini's launch more than 11 years ago. The redundant set, branch B, is an identical set of eight thrusters.

Propulsion engineers began to see a lower performance from one of the thrusters on branch A in October. A second branch A thruster is also now showing some degraded performance.

An extensive review with the propulsion system contractor, Lockheed Martin Space Systems, Denver, Colo., the thruster manufacturer, Aerojet, Sacramento, Calif., and propulsion experts at NASA's Jet Propulsion Laboratory, Pasadena, Calif., was completed last week. The recommendation was made to swap to side B as soon as is practical.

Mid-March is the earliest practical opportunity to make the swap. This allows time for the team to properly test and prepare the sequence of commands that will be sent to the spacecraft. Science planners have identified a period where no high-priority science will be lost during the switch, which will be done over a seven-day window. It also is a time when no navigation maneuvers are required to maintain the spacecraft's trajectory.

The swap involves commanding a latch valve to open hydrazine flow to the B side, and powering on some thruster control electronics. No pyrotechnic devices are involved in the swap, and the action is fully reversible if necessary.

Almost all Cassini engineering subsystems have redundant backup capability. This is only the second time in Cassini's 11 years of flight that the engineering teams have gone to a backup system. The backup reaction wheel was brought online a few years ago and is currently functioning as one of the three prime wheels.

Cassini successfully completed its four-year planned tour and is now in extended mission operations.

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

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 Cassini 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.

Carolina Martinez 818-354-9382

Jet Propulsion Laboratory, Pasadena, Calif.

carolina.martinez@jpl.nasa.gov

February 27th, 2009

Titan Dunes Turn Climate Models Upside Down

Written by Anne Minard

Scientists have mapped vast dune fields on Titan that may align with the wind on Saturn’s biggest moon — flowing opposite the way climate models had predicted.

The maps, as above, represent four years of radar data collected by the Cassini spacecraft. They reveal rippled dunes that are generally oriented east-west, which means Titan’s winds probably blow toward the east instead of the west. If so, Titan’s surface winds blow opposite the direction suggested by previous global circulation models. On the example above, the arrows indicate the general wind direction. The dark areas without arrows might have dunes but have not yet been imaged.

“At Titan there are very few clouds, so determining which way the wind blows is not an easy thing, but by tracking the direction in which Titan’s sand dunes form, we get some insight into the global wind pattern,” says Ralph Lorenz, Cassini radar scientist at Johns Hopkins University in Maryland. “Think of the dunes sort of like a weather vane, pointing us to the direction the winds are blowing.”

Titan’s dunes are believed to be made up of hydrocarbon sand grains likely derived from organic chemicals in Titan’s smoggy skies. The dunes wrap around high terrain, which provides some idea of their height. They accumulate near the equator, and may pile up there because drier conditions allow for easy transport of the particles by the wind. Titan’s higher latitudes contain lakes and may be “wetter” with more liquid hydrocarbons, not ideal conditions for creating dunes.

“Titan’s dunes are young, dynamic features that interact with topographic obstacles and give us clues about the wind regimes,” said Jani Radebaugh, from Brigham Young University in Utah. “Winds come at these dunes from at least a couple of different directions, but then combine to create the overall dune orientation.”

Researchers say the wind pattern is important for planning future Titan explorations that might involve balloon-borne experiments. Some 16,000 dune segments were mapped out from about 20 radar images, digitized and combined to produce the new map, which is available at http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini. A paper based on the new findings appeared in the Feb. 11 issue of Geophysical Research Letters.

Cassini, which launched in 1997 and is now in extended mission operations, continues to blaze its trail around the Saturn system and will visit Titan again on March 27. Seventeen Titan flybys are planned this year.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet propulsion Laboratory (JPL) in Pasadena, California manages the Cassini-Huygens mission. 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. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

LEAD IMAGE CREDIT: NASA/JPL/Space Science Institute (Boulder, Colorado)

Source: NASA

Cassini Finds Hydrocarbon Rains May Fill Titan Lakes

January 29, 2009

Labtayt Sulci on Saturn's Enceladus

Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Explanation: Do some surface features on Enceladus roll like a conveyor belt? A leading interpretation of recent images taken of Saturn's most explosive moon indicate that they do. This form of asymmetric tectonic activity, very unusual on Earth, likely holds clues to the internal structure of Enceladus, which may contain subsurface seas where life might be able to develop. Pictured above is a composite of 28 images taken by the robotic Cassini spacecraft in October just after swooping by the ice-spewing orb. Inspection of these images show clear tectonic displacements where large portions of the surface all appear to move all in one direction. Near the top of the image appears one of the most prominent tectonic divides: Labtayt Sulci, a canyon about one kilometer deep.

Note : APOD Editor to Speak in New York on Jan. 2

SATURN's NORTH POLAR AURORA - 2008 NOV 12

This image of the northern polar region of Saturn shows both the aurora and underlying atmosphere, seen at two different wavelengths of infrared light as captured by NASA’s Cassini spacecraft.

Energetic particles, crashing into the upper atmosphere cause the aurora, shown in blue, to glow brightly at 4 microns (six times the wavelength visible to the human eye). The image shows both a bright ring, as seen from Earth, as well as an example of bright auroral emission within the polar cap that had been undetected until the advent of Cassini. This aurora, which defies past predictions of what was expected, has been observed to grow even brighter than is shown here. Silhouetted by the glow (cast here to the color red) of the hot interior of Saturn (clearly seen at a wavelength of 5 microns, or seven times the wavelength visible to the human eye) are the clouds and haze that underlie this auroral region. For a similar view of the region beneath the aurora see http://photojournal.jpl.nasa.gov/catalog/PIA09185 .

This image is a composite captured with Cassini’s visual and infrared mapping spectrometer. The aurora image was taken in the near-infrared on Nov. 10, 2006, from a distance of 1,061,000 kilometers (659,000 miles), with a phase angle of 157 degrees and a sub-spacecraft planetocentric latitude of 52 degrees north. The image of the clouds was obtained by Cassini on June 15, 2008, from a distance of 602,000 kilometers (374,000 miles) and a sub-spacecraft planetocentric latitude of 73 degrees north.

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 mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The Visual and Infrared Mapping Spectrometer team is based at the University of Arizona.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Credit:NASA/JPL/University of Arizona

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A VIEW OF TITAN THAT FACES SATURN - 2008 Nov 17

The Cassini spacecraft looks through Titan's thick atmosphere to reveal bright and dark terrains on the Saturn-facing side of the planet's largest moon. North is up.

The image was taken with the Cassini spacecraft narrow-angle camera on Oct. 11, 2008 using a spectral filter sensitive to wavelengths of infrared light centered at 938 nanometers. The view was obtained at a distance of approximately 2.222 million kilometers (1.381 million miles) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 10 degrees. Image scale is 13 kilometers (8 miles) per pixel.

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 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.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Credit: NASA/JPL/Space Science Institute

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Astronomy Picture of the Day for 2008 October 14

Image above: This diagram shows the trajectory of Cassini's closest Enceladus flybys from the Prime and Equinox missions, relative to the south polar plume, which is shown in false color based on an image taken by the Cassini cameras in April 2007. Flybys on orbits 3 and 121 do not come quite close enough to Enceladus to appear on this graphic.

Credit: NASA/JPL/SwRI/SSI

Our own instrument, the Composite Infrared Spectrometer (CIRS) will once again be mapping the heat radiation from the warm tiger stripe fractures, trying to understand how hot the fractures are, and exactly where the heat is coming from. We only had time to map a small fraction of the active fractures from close range in August, so this time we'll be looking at different regions, in cooperation with the imaging camera and the ultraviolet spectrometer which will be observing simultaneously for much of the time.

We'll be following up on some of our August 11th discoveries. Back then, we got a beautiful spectrum of one of the most active regions of the fracture Damascus Sulcus, allowing us to make some precise temperature measurements. We're really interested in the temperatures, because the warmer the fractures are on the surface, the more likely it is that liquid water lurks somewhere below. The highest temperatures we saw, 167 Kelvin or -159 Fahrenheit, were a bit lower than we had estimated from our less precise data from the March 12th flyby (at least 180 Kelvin or -136 Fahrenheit), though both numbers are dramatically warmer than we would expect at this time of year if internal heat wasn't leaking out of Enceladus (roughly 60 Kelvin or -352 Fahrenheit). Whether the Damascus fracture has really cooled since March, or whether our March measurement was an overestimate, or some other factor explains the discrepancy, is one of the things we're still puzzling over, and maybe this week's data can help us sort that out.

Another thing we'll be doing is measuring the total amount of heat energy coming out of the south pole. Knowing the total horsepower of Enceladus' heat engine is crucial for understanding what's driving the activity there. We're pretty sure the deformation of Enceladus by Saturn's tides is the ultimate power source, but so far none of the theorists have been able to explain in detail how that source can continuously supply the roughly 6 Gigawatts of power that CIRS is seeing. So we'll be trying to refine that power estimate on this flyby. Whether the new observations will make life easier or more difficult for the theorists’ remains to be seen!

SUPER CLOSEUP OF ENCELADUS BELOW

ASTRONOMY PICTURE OF THE DAY FOR 2008 November 5

Seventeen Hundred Kilometers Above Enceladus

Credit: Cassini Imaging Team, ISS, JPL, ESA, NASA

Explanation: Above is one of the closest pictures yet obtained of Saturn's ice-spewing moon Enceladus. The image was taken from about 1,700 kilometers up as the robotic Cassini spacecraft zoomed by the fractured ice ball last week. Features the size of a bus are resolvable in this highly detailed image taken of Enceladus' active tiger stripe region. Very different from most other moons and planets, grooves and hills dot an alien moonscape devoid of craters. Space pioneers might wonder where, on such a highly textured surface, a future probe might land in search of freshly deposited ice, subsurface seas, or even indicators of life. Although appearing dark in the above contrast-enhanced image, the surface of Enceladus is covered with some of the brightest ice in the entire Solar System, reflecting about 99 percent of the light it receives. To help better understand this enigmatic world, Cassini is scheduled to swoop by Enceladus at least five more times.

Cassini Mission Timeline for 2008

• 2008 Saturn Tour Highlights:

• (Dates listed in Spacecraft Event Time [SCET] -- the time the something happens at the spacecraft based on Coordinated Universal Time [UTC].)

• Jan. 5, 2008: Titan flyby (T40) -- the Visual and Infrared Mapping Spectrometer (VIMS) maps the Huygens landing site with a highly illuminated surface. This flyby features two separate stellar occultations to study the structure of Titan's atmosphere. VIMS watches the star Alpha Bootes, and the Ultraviolet Imaging Spectrograph (UVIS) watches Alpha Lyra.

• Feb. 22, 2008: Titan flyby (T41) -- features a RADAR Synthetic Aperture Radar Imager (SAR) swath of Titan's southeast quadrant and the Huygens landing site. This flyby also has the mission's "longest" stellar occultation of Titan's atmosphere, occurring about a day after closest approach.

• March 12, 2008: Enceladus flyby -- This is the third and final targeted flyby of Enceladus in the prime mission. It is an inclined flyby with a closest approach distance near the equator of about 50 kilometers, enabling the first good views of the northern hemisphere. The trajectory will take Cassini through the plume at a grazing angle (deeper into the plume than during the July 2005 flyby), allowing for fields-and-particles measurements within the plume. Enceladus will enter eclipse (Saturn's shadow) shortly after closest approach, allowing for thermal measurements of the south pole at high resolution.

• March 25, 2008: Titan flyby (T42) -- it is "High Noon" for Cassini during this encounter, with the Sun high in Titan's sky. Just prior to closest approach, the Ion and Neutral Mass Spectrometer (INMS) examines Titan's upper atmosphere. Immediately after closest approach, VIMS captures high resolution imaging of the probe landing site.

• May 12, 2008: Titan flyby (T43) -- RADAR captures SAR imaging of Titan's bright region known as Xanadu. The only other RADAR coverage of this area was back in April 2006 (T13).

• May 28, 2008: Titan flyby (T44) -- this unique and highly important flyby features a RADAR pass across Xanadu, making T44 and T43 coverage of the area the only RADAR pass other than the T13 flyby (in April 2006).

• June through August 2008: The Cassini spacecraft enters the highest inclination orbits of the tour. In these high inclined orbits scientists will have their best opportunities to use Stellar Occultations to penetrate the B ring. And the spacecraft will be able to show views of Saturn as no one has seen it before! The rings will be spread out like a giant halo around Saturn.

• These high inclined orbits also present the Magnetosphere and Plasma Science (MAPS) scientists with many science opportunities that have been long awaited.

• June 30, 2008: End of Prime Mission.

• Extended Mission beginning in July 2008 will be known as the Cassini Equinox Mission.

• July 31, 2008: Titan flyby (T45) -- Radio Science Subsystem (RSS) uses this pass to measure Titan's gravity field, allowing us to explore the moon's interior. This is the fourth of four fly-bys needed to determine if Titan has an internal ocean.

• Aug. 11, 2008: Enceladus flyby -- This is the first targeted flyby of Enceladus in the Extended Mission. The flyby geometry is very similar to that of the March 2008 flyby, however the spacecraft will be oriented to optimize the flyby for viewing by cameras and spectrometers, to obtain the highest resolution views of the active south pole region.

• Oct. 9, 2008: Enceladus flyby -- This is the second of seven targeted Enceladus fly-bys in the Extended Mission. The flyby geometry is very similar to that of the March 2008 flyby: an inclined trajectory allowing Cassini to pass through the plumes for fields-and-particles measurements near closest approach.

• Oct. 31, 2008: Enceladus flyby -- This is the final inclined flyby of Enceladus, and at about 200 kilometers, is more distant than the earlier fly-bys in the year. This flyby will be dedicated to remote sensing measurements by cameras and spectrometers, to obtain images as well as compositional and thermal information on the north and south pole regions.

• Nov. 3, 2008: Titan flyby (T46) -- Radio Science watches as the Earth slips behind Titan, using the spacecraft's signal to probe Titan's atmosphere in the north mid latitude, with the occultation at 26 degrees North. In another RSS experiment, a signal is bounced off of Titan's surface and then returned to Earth.

• Nov. 19, 2008: Titan flyby (T47) -- VIMS observes the Huygens probe landing site, and UVIS observes a star (Beta Cma) through Titan's atmosphere, using the star as a "flashlight" to study Titan's atmospheric structure and composition. This is one of the best "low phase" fly-bys for the VIMS instrument in the extended mission.

• Dec. 5, 2008: Titan flyby (T48) -- Cassini swoops down to 960 kilometers (about 580 miles) over Titan's surface. The Ion and Neutral Mass Spectrometer (INMS) takes advantage of this low altitude, sampling Titan's ionosphere on the dayside. In fact, this is the best dayside pass for INMS during the extended mission. RADAR gets to capture a SAR swath over the Tui Regio area of Xanadu, a very bright feature in VIMS observations.

• Dec. 21, 2008: Titan flyby (T49) -- Closing out the year, RADAR will carry out altimetry observations over the area known as Ontario Lacus, in Titan's Southern hemisphere.

• Dec. 26, 2008: 100th orbit periapsis.

SATURN'S MOON ENCELADUS

From Wikipedia, the free encyclopedia

This article is about the moon of Saturn; for the mythological giant, see Enceladus (mythology).

Enceladus (pronounced /ɛnˈsɛlədəs/ en-SEL-ə-dəs, or as in Greek Εγκέλαδος), is the sixth-largest moon of Saturn.^[12] It was discovered in 1789 by William Herschel.^[13] Until the two Voyager spacecraft passed near it in the early 1980s, very little was known about this small moon besides the identification of water ice on its surface. The Voyagers showed that Enceladus is only 500 km in diameter and reflects almost 100% of the sunlight that strikes it. Voyager 1 found that Enceladus orbited in the densest part of Saturn's diffuse E ring, indicating a possible association between the two, while Voyager 2 revealed that despite the moon's small size, it had a wide range of terrains ranging from old, heavily cratered surfaces to young, tectonically deformed terrain, with some regions with surface ages as young as 100 million years old.

The Cassini spacecraft of the mid- to late 2000s acquired additional data on Enceladus, answering a number of the mysteries opened by the Voyager spacecraft and starting a few new ones. Cassini performed several close flybys of Enceladus in 2005, revealing the moon's surface and environment in greater detail. In particular, the probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few (if any) impact craters in the south polar region, shows that Enceladus is geologically active today. Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity; proximity to the planet can then lead to tidal heating of the satellite's interior, offering a possible explanation for the activity.

Enceladus is one of only three outer solar system bodies (along with Jupiter's moon Io and Neptune's moon Triton) where active eruptions have been observed. Analysis of the outgassing suggests that it originates from a body of sub-surface liquid water, which along with the unique chemistry found in the plume, has fueled speculations that Enceladus may be important in the study of astrobiology.^[14] The discovery of the plume has added further weight to the argument that material released from Enceladus is the source of the E-ring.

Name

See also: List of geological features on Enceladus

Enceladus is named after the Giant Enceladus of Greek mythology. It is also designated Saturn II or S II Enceladus. The name Enceladus – like the names of each of the first seven satellites of Saturn to be discovered– was suggested by William Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.^[15] He chose these names because Saturn, known in Greek mythology as Cronus, was the leader of the Titans. The adjectival form of the name is either Enceladean or Enceladan (both are used with roughly equal frequency).

Features on Enceladus are named by the International Astronomical Union (IAU) after characters and places from the Arabian Nights.^[16] Impact craters are named after characters, while other feature types, such as fossae (long, narrow depressions), dorsa (ridges), planitia (plains), and sulci (long parallel grooves), are named after places. 57 features have been officially named by the IAU; 22 features were named in 1982 based on the results of the Voyager flybys, and 35 features were approved in November 2006 based on the results of Cassini's three flybys in 2005.^[17] Examples of approved names include Samarkand Sulci, Aladdin crater, Daryabar Fossa, and Sarandib Planitia.

Exploration

Enceladus is one of the major inner satellites of Saturn. It is the fourteenth satellite when ordered by distance from Saturn, and orbits within the densest part of the E Ring, the outermost of Saturn's rings, an extremely wide but very diffuse disk of microscopic icy or dusty material, beginning at the orbit of Mimas and ending somewhere around the orbit of Rhea.

Enceladus orbits Saturn at a distance of 238 000 km from the planet's center and 180 000 km from its cloudtops, between the orbits of Mimas and Tethys, requiring 32.9 hours to revolve once (fast enough for its motion to be observed over a single night of observation). Enceladus is currently in a 2:1 mean motion orbital resonance with Dione, completing two orbits of Saturn for every one orbit completed by Dione. This resonance helps maintain Enceladus's orbital eccentricity (0.0047) and provides a heating source for Enceladus's geologic activity.^[2]

Like most of the larger satellites of Saturn, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Unlike the Earth's moon, Enceladus does not appear to librate about its spin axis (more than 1.5°). However, analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin-orbit libration.^[2] This libration, like the resonance with Dione, could have provided Enceladus with an additional heat source.

Interaction with E Ring

The E Ring is the widest and outermost ring of Saturn. It is an extremely wide but very diffuse disk of microscopic icy or dusty material, beginning at the orbit of Mimas and ending somewhere around the orbit of Rhea, though some observations suggest that it extends beyond the orbit of Titan, making it 1 000 000 km wide. However, numerous mathematical models show that such a ring is unstable, with a lifespan between 10 000 and 1 000 000 years. Therefore, particles composing it must be constantly replenished. Enceladus is orbiting inside this ring, in a place where it is narrowest but present in its highest density. Therefore, several theories suspected Enceladus to be the main source of particles for the E Ring. This hypothesis was supported by Cassini's flyby.

There are actually two distinct mechanisms feeding the ring with particles.^[29] The first, and probably the most important, source of particles comes from the cryovolcanic plume in the South polar region of Enceladus. While a majority of particles fall back to the surface, some of them escape Enceladus's gravity and enter orbit around Saturn, since Enceladus's escape velocity is only 866 km/h. The second mechanism comes from meteoric bombardment of Enceladus, raising dust particles from the surface. This mechanism is not unique to Enceladus, but is valid for all Saturn's moons orbiting inside the E Ring.

Size and shape

Enceladus is a relatively small satellite, with a mean diameter of 505 km, only one-seventh the diameter of Earth's own Moon. It is small enough to fit within the length of the United Kingdom; in fact, it is barely the size of England alone (see picture). It could also fit comfortably within the states of Arizona or Colorado, although as a spherical object its surface area is much greater, just over 800 000 km², almost the same as Mozambique, or 15% larger than Texas.

Its mass and diameter make Enceladus the sixth most massive and largest satellite of Saturn, after Titan (5150 km), Rhea (1530 km), Iapetus (1440 km), Dione (1120 km) and Tethys (1050 km). It is also one of the smallest of Saturn's spherical satellites, since all smaller satellites except Mimas (390 km) have an irregular shape.

Enceladus has a shape of a flattened ellipsoid; its dimensions, calculated from pictures taken by Cassini's ISS instrument, are of 513(a)×503(b)×497(c) km,^[2] with (a) corresponding to the diameter between sub- and anti-Saturnian poles, (b) to the diameter between the leading and trailing poles, and (c) to the distance between the north and south poles. This is the most stable orientation, with the moon's rotation along the short axis, and the long axis aligned radially away from Saturn.

Surface

Voyager 2, in August 1981, was the first spacecraft to observe the surface in detail. Examination of the resulting highest resolution mosaic reveals at least five different types of terrain, including several regions of cratered terrain, regions of smooth (young) terrain, and lanes of ridged terrain often bordering the smooth areas.^[24] In addition, extensive linear cracks^[30] and scarps were observed. Given the relative lack of craters on the smooth plains, these regions are probably less than a few hundred million years old. Accordingly, Enceladus must have been recently active with "water volcanism" or other processes that renew the surface. The fresh, clean ice that dominates its surface gives Enceladus probably the most reflective surface of any body in the solar system with a visual geometric albedo of 1.38.^[6] Because it reflects so much sunlight, the mean surface temperature at noon only reaches −198 °C (somewhat colder than other Saturnian satellites).^[8]

Observations during three flybys by Cassini on February 17, March 9, and July 14 of 2005 revealed Enceladus's surface features in much greater detail than the Voyager 2 observations. For example, the smooth plains observed by Voyager 2 resolved into relatively crater-free regions filled with numerous small ridges and scarps. In addition, numerous fractures were found within the older, cratered terrain, suggesting that the surface has been subjected to extensive deformation since the craters were formed.^[31] Finally, several additional regions of young terrain were discovered in areas not well-imaged by either Voyager spacecraft, such as the bizarre terrain near the south pole.^[2]

Impact craters

Impact cratering is a common occurrence on many solar system bodies. Much of Enceladus's surface is covered with craters at various densities and levels of degradation. From Voyager 2 observations, three different units of cratered topography were identified on the basis of their crater densities, from ct₁ and ct₂, both containing numerous 10–20 km-wide craters though differing in the degree of deformation, to cp consisting of lightly cratered plains.^[32] This subdivision of cratered terrains on the basis of crater density (and thus surface age), believes that Enceladus has been resurfaced in multiple stages.

Recent Cassini observations have provided a much closer look at the ct₂ and cp cratered units. These high-resolution observations, like Figure 6, reveal that many of Enceladus's craters are heavily deformed through viscous relaxation and fracturing.^[33] Viscous relaxation causes craters and other topographic features formed in water ice to deform over geologic time scales due to the effects of gravity, reducing the amount of topography over time. The rate at which this occurs is dependent on the temperature of the ice: warmer ice is easier to deform than colder, stiffer ice. Viscously relaxed craters tend to have domed floors, or are recognized as craters only by a raised, circular rim (seen at center just below the terminator in Figure 6). Dunyazad, the large crater seen in Figure 8 just left of top center, is a prime example of a viscously relaxed crater on Enceladus, with a prominent domed floor. In addition, many craters on Enceladus have been heavily modified by tectonic fractures. The 10-km-wide crater right of bottom center in Figure 8 is a prime example: thin fractures, several hundred metres to a kilometre wide, have heavily altered the crater's rim and floor. Nearly all craters on Enceladus thus far imaged by Cassini in the Ct2 unit show signs of tectonic deformation. These two deformation styles—viscous relaxation and fracturing—demonstrate that, while cratered terrains are the oldest regions on Enceladus due to their high crater retention, nearly all craters on Enceladus are in some stage of degradation.

Tectonics

Voyager 2 found several types of tectonic features on Enceladus, including troughs, scarps, and belts of grooves and ridges.^[24] Recent results from Cassini suggest that tectonism is the dominant deformation style on Enceladus. One of the more dramatic types of tectonic features found on Enceladus are rifts. These canyons can be up to 200 km long, 5–10 km wide, and one km deep. Figure 7 shows a typical large fracture on Enceladus cutting across older, tectonically deformed terrain. Another example can be seen running along the bottom of the frame in Figure 8. Such features appear relatively young, as they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.

Another example of tectonism on Enceladus is grooved terrain, consisting of lanes of curvilinear grooves and ridges. These bands, first discovered by Voyager 2, often separate smooth plains from cratered regions.^[24] An example of this terrain type can be seen in Figures 6 and 10 (in this case, a feature known as Samarkand Sulci). Grooved terrain such as Samarkand Sulci are reminiscent of grooved terrain on Ganymede. However, unlike those seen on Ganymede, grooved topography on Enceladus is generally much more complex. Rather than parallel sets of grooves, these lanes can often appear as bands of crudely aligned, chevron-shaped features. In other areas, these bands appear to bow upwards with fractures and ridges running the length of the feature. Cassini observations of Samarkand Sulci have revealed intriguing dark spots (125 and 750 m wide), which appear to run parallel to narrow fractures. Currently, these spots are interpreted as collapse pits within these ridged plain belts.^[33]

In addition to deep fractures and grooved lanes, Enceladus has several other types of tectonic terrain. Figure 9 shows sets of narrow fractures (still several hundred metres wide) that were first discovered by the Cassini spacecraft. Many of these fractures are found in bands cutting across cratered terrain. These fractures appear to propagate down only a few hundred metres into the crust. Many appear to have been influenced during their formation by the weakened regolith produced by impact craters, often changing the strike of the propagating fracture.^[33][34] Another example of tectonic features on Enceladus are the linear grooves first found by Voyager 2 and seen at a much higher resolution by Cassini. Examples of linear grooves can be found in the lower left of the figure at top and Figure 10 (lower left), running from north to south from top center before turning to the southwest. These linear grooves can be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they appear to be among the youngest features on Enceladus. However, some linear grooves appear to be softened like the craters nearby, suggesting an older age. Ridges have also been observed on Enceladus, though not nearly to the extent as those seen on Europa. Several examples can be seen in the lower left corner of Figure 7. These ridges are relatively limited in extent and are up to one km tall. One-kilometre high domes have also been observed.^[33] Given the level of tectonic resurfacing found on Enceladus, it is clear that tectonism has been an important driver of geology on this small moon for much of its history.

Smooth plains

Two units of smooth plains were also observed by Voyager 2. These plains generally have low relief and have far fewer craters than in the cratered terrains and plains, indicating a relatively young surface age.^[32] In one of the smooth plain regions, Sarandib Planitia, no impact craters were visible down to the limit of resolution. Another region of smooth plains to the southwest of Sarandib, is criss-crossed by several troughs and scarps. Cassini has since viewed these smooth plains regions, like Sarandib Planitia and Diyar Planitia at much higher resolution. Cassini images show smooth plain regions to be filled with low-relief ridges and fractures. These features are currently interpreted as being caused by shear deformation.^[33] The high resolution images of Sarandib Planitia have revealed a number of small impact craters, which allow for an estimate of the surface age, either 170 million years or 3.7 billion years, depending on assumed impactor population.^[2][35]

The expanded surface coverage provided by Cassini has allowed for the identification of additional regions of smooth plains, particularly on Enceladus's leading hemisphere (the side of Enceladus that faces the direction of motion as the moon orbits Saturn). Rather than being covered in low relief ridges, this region is covered in numerous criss-crossing sets of troughs and ridges, similar to the deformation seen in the south polar region. This area is on the opposite side of the satellite from Sarandib and Diyar Planitiae, suggesting that the placement of these regions is influenced by Saturn's tides on Enceladus.^[36]

South polar region

See also: Tiger Stripes (Enceladus)

Images taken by Cassini during the flyby on July 14, 2005 revealed a distinctive, tectonically-deformed region surrounding Enceladus's south pole. This area, reaching as far north as 60° south latitude, is covered in tectonic fractures and ridges.^[2][37] The area has few sizable impact craters, suggesting that it is the youngest surface on Enceladus and on any of the mid-sized icy satellites; modeling of the cratering rate suggests that the region is less than 10–100 million years old.^[2] Near the center of this terrain are four fractures bounded on either side by ridges, unofficially called "tiger stripes". These fractures appear to be the youngest features in this region and are surrounded by mint-green-colored (in false color, UV-green-near IR images), coarse-grained water ice, seen elsewhere on the surface within outcrops and fracture walls.^[37] Here the "blue" ice is on a flat surface, indicating that the region is young enough not to have been coated by fine-grained water ice from E ring. Results from the Visual and Infrared Spectrometer (VIMS) instrument suggest that the green-colored material surrounding the tiger stripes is chemically distinct from the rest of the surface of Enceladus. VIMS detected crystalline water ice in the stripes, suggesting that they are quite young (likely less than 1000 years old) or the surface ice has been thermally altered in the recent past.^[38] VIMS also detected simple organic compounds in the tiger stripes, chemistry not found anywhere else on the satellite thus far.^[39]

One of these areas of "blue" ice in the south polar region was observed at very high resolution during the July 14 flyby, revealing an area of extreme tectonic deformation and blocky terrain, with some areas covered in boulders 10–100 m across.^[40]

The boundary of the south polar region is marked by a pattern of parallel, Y- and V-shaped ridges and valleys. The shape, orientation, and location of these features indicate that they are caused by changes in the overall shape of Enceladus. Currently, there are two theories for what could cause such a shift in shape. First, the orbit of Enceladus may have migrated inward (from the article: "the lack of any plausible mechanism for increased flattening"), leading to an increase in Enceladus's rotation rate. Such a shift would have led to a flattening of Enceladus's rotation axis.^[2] Another theory suggests that a rising mass of warm, low density material in Enceladus's interior led to a shift in the position of the current south polar terrain from Enceladus's southern mid-latitudes to its south pole.^[36] Consequently, the ellipsoid shape of Enceladus would have adjusted to match the new orientation. One consequence of the axial flattening theory is that both polar regions should have similar tectonic deformation histories.^[2] However, the north polar region is densely cratered, and has a much older surface age than the south pole.^[32] Thickness variations in Enceladus's lithosphere is one explanation for this discrepancy. Variations in lithospheric thickness are supported by the correlation between the Y-shaped discontinuities and the V-shaped cusps along the south polar terrain margin and the relative surface age of the adjacent non-south polar terrain regions. The Y-shaped discontinuities, and the north-south trending tension fractures into which they lead, are correlated with younger terrain with presumably thinner lithospheres. The V-shaped cusps are adjacent to older, more heavily cratered terrains.^[2]

Cryovolcanism

Following the Voyager encounters with Enceladus in the early 1980s, scientists postulated that the moon may be geologically active based on its young, reflective surface and location near the core of the E ring.^[24] Based on the connection between Enceladus and the E ring, it was thought that Enceladus was the source of material from the E ring, perhaps through venting of water vapor from Enceladus's interior. However, the Voyagers failed to provide conclusive evidence that Enceladus is active today.

Thanks to data from a number of instruments on the Cassini spacecraft in 2005, cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, has been discovered on Enceladus. The first Cassini sighting of a plume of icy particles above Enceladus's south pole came from the Imaging Science Subsystem (ISS) images taken in January and February 2005,^[2] though the possibility of the plume being a camera artifact stalled an official announcement. Data from the magnetometer instrument during the February 17, 2005 encounter provided a hint that the feature might be real when it found evidence for an atmosphere at Enceladus. The magnetometer observed an increase in the power of ion cyclotron waves near Enceladus. These waves are produced by the interaction of ionized particles and magnetic fields, and the frequency of the waves can be used to identify the composition, in this case ionized water vapor.^[9] During the next two encounters, the magnetometer team determined that gases in Enceladus's atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower.^[9] The Ultraviolet Imaging Spectrograph (UVIS) confirmed this result by observing two stellar occultations during the February 17 and July 14 encounters. Unlike the magnetometer, UVIS failed to detect an atmosphere above Enceladus during the February encounter when it looked for evidence for an atmosphere over the equatorial region, but did detect water vapor during an occultation over the south polar region during the July encounter.^[10]

Fortuitously, Cassini flew through this gas cloud during the July 14 encounter, allowing instruments like the Ion and Neutral Mass Spectrometer (INMS) and the Cosmic Dust Analyser (CDA) to directly sample the plume. INMS measured the composition of the gas cloud, detecting mostly water vapor, as well as minor components like molecular nitrogen, methane, and carbon dioxide.^[11] CDA "detected a large increase in the number of particles near Enceladus," confirming the satellite as the primary source for the E ring.^[29] Analysis of the CDA and INMS data suggest that the gas cloud Cassini flew through during the July encounter, and was observed from a distance by the magnetometer and UVIS, was actually a water-rich cryovolcanic plume, originating from vents near the south pole.^[41]

Visual confirmation of venting came in November 2005, when ISS imaged fountain-like jets of icy particles rising from the moon's south polar region.^[2] (As stated above, the plume was imaged before, in January and February 2005, but additional studies of the camera's response at high phase angles, when the sun is almost behind Enceladus, and comparison with equivalent high phase images taken of other Saturnian satellites, were required before the plume could be confirmed.^[42]) The images taken in November 2005 showed the plume's fine structure, revealing numerous jets (perhaps due to numerous distinct vents) within a larger, faint component extending out nearly 500 km from the surface, thus making Enceladus the fourth body in the solar system to have confirmed volcanic activity, along with Earth, Neptune's Triton, and Jupiter's Io.^[41] Cassini's UVIS later observed gas jets coinciding with the dust jets seen by ISS during a non-targeted encounter with Enceladus in October 2007.

Additional observations were acquired during a flyby on March 12, 2008. Data on this flyby revealed additional chemicals in the plume, including simple and complex hydrocarbons such as propane, ethane, and acetylene.^[43] This finding further raises the potential for life beneath the surface of Enceladus.^[44] The composition of Enceladus's plume as measured by the INMS instrument on Cassini is similar to that seen at most comets.^[43]

The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized sub-surface chambers, similar to geysers on Earth.^[2] Because no ammonia was found in the vented material by INMS or UVIS, which could act as an anti-freeze, such a heated, pressurized chamber would consist of nearly pure liquid water with a temperature of at least 270 K, as illustrated in Figure 14. Pure water would require more energy to melt, either from tidal or radiogenic sources, than an ammonia-water mixture. Another possible method for generating a plume is sublimation of warm surface ice. During the July 14, 2005 flyby, the Composite Infrared Spectrometer (CIRS) found a warm region near the South Pole. Temperatures found in this region range from 85–90 K, to small areas with temperatures as high as 157 K, much too warm to be explained by solar heating, indicating that parts of the south polar region are heated from the interior of Enceladus.^[8] Ice at these temperatures is warm enough to sublimate at a much faster rate than the background surface, thus generating a plume. This hypothesis is attractive since the sub-surface layer heating the surface water ice could be an ammonia-water slurry at temperatures as low as 170 K, and thus not as much energy is required to produce the plume activity. However, the abundance of particles in the south polar plume favors the "cold geyser" model, as opposed to an ice sublimation model.^[2]

Alternatively, Kieffer et al. (2006) suggest that Enceladus's geysers originate from clathrate hydrates, where carbon dioxide, methane, and nitrogen are released when exposed to the vacuum of space by the active, tiger stripe fractures.^[45] This hypothesis would not require the amount of heat needed to melt water ice as required by the "Cold Geyser" model, and would explain the lack of ammonia.

Internal structure

Prior to the Cassini mission, relatively little was known about the interior of Enceladus. However, results from recent flybys of Enceladus by the Cassini spacecraft have provided much needed information for models of Enceladus's interior. These include a better determination of the mass and tri-axial ellipsoid shape, high-resolution observations of the surface, and new insights on Enceladus's geochemistry.

Mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice.^[24] However, based on the effects of Enceladus's gravity on Cassini, its mass was determined to be much higher than previously thought, yielding a density of 1.61 g/cm³.^[2] This density is higher than Saturn's other mid-sized icy satellites, indicating that Enceladus contains a greater percentage of silicates and iron. With additional material besides water ice, Enceladus's interior may have experienced comparatively more heating from the decay of radioactive elements.

Castillo et al. 2005 suggested that Iapetus, and the other icy satellites of Saturn, formed relatively quickly after the formation of the Saturnian sub-nebula, and thus were rich in short-lived radionuclides.^[46] These radionuclides, like aluminium-26 and iron-60, have short half-lives and would produce interior heating relatively quickly. Without the short-lived variety, Enceladus's complement of long-lived radionuclides would not have been enough to prevent rapid freezing of the interior, even with Enceladus's comparatively high rock-mass fraction, given Enceladus's small size.^[47] Given Enceladus's relatively high rock-mass fraction, the proposed enhancement in ²⁶Al and ⁶⁰Fe would result in a differentiated body, with an icy mantle and a rocky core.^[48] Subsequent radioactive and tidal heating would raise the temperature of the core to 1000 K, enough to melt the inner mantle. However, for Enceladus to still be active, part of the core must have melted too, forming magma chambers that would flex under the strain of Saturn's tides. Tidal heating, such as from the resonance with Dione or from libration, would then have sustained these hot spots in the core until the present, and would power the current geological activity.^[49]

In addition to its mass and modeled geochemistry, researchers have also examined Enceladus's shape to test whether the satellite is differentiated or not. Porco et al. 2006 used limb measurements to determine that Enceladus's shape, assuming it is in hydrostatic equilibrium, is consistent with an undifferentiated interior, in contradiction to the geological and geochemical evidence.^[2] However, the current shape also supports the possibility that Enceladus is not in hydrostatic equilibrium, and may have rotated faster at some point in the recent past (with a differentiated interior).^[48]

Sky from Enceladus

See also: Extraterrestrial skies

Seen from Enceladus, Saturn would have a visible diameter of almost 30°, sixty times more than the Moon visible from Earth ^[50]. Moreover, since Enceladus rotates synchronously with its orbital period and therefore keeps one face pointed toward Saturn, the planet never moves in Enceladus's sky (albeit with slight variations coming from the orbit's eccentricity), and cannot be seen from the far side of the satellite.

Saturn's rings would be seen from an angle of only 0.019°, and would appear as a very narrow, bright line crossing the disk of Saturn, but their shadow on Saturn's disk would be clearly distinguishable. Like our own Moon from Earth, Saturn itself would show regular phases, cycling from "new" to "full" in about 16 hours. From Enceladus, the Sun would have a diameter of only 3.5 minutes of arc, nine times smaller than that of the Moon as seen from Earth.

An observer located on Enceladus could also observe Mimas (the biggest satellite located inside Enceladus's orbit) transit in front of Saturn every 72 hours on average. Its apparent size would be at most 26 minutes of arc, about the same size as the Moon seen from Earth. Pallene and Methone would appear nearly star-like. Tethys would reach a maximum apparent size just above one degree of arc, about twice the Moon as seen from the Earth, but is visible only from Enceladus's anti-Saturnian side when it is at closest approach.

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Giant Cyclones at Saturn's Poles Create a Swirl of Mystery

October 13, 2008

(Source: Jet Propulsion Laboratory)

New images from NASA’s Cassini spacecraft reveal a giant cyclone at Saturn’s north pole, and show that a similarly monstrous cyclone churning at Saturn’s south pole is powered by Earth-like storm patterns. The new-found cyclone at Saturn’s north pole is only visible in the near-infrared wavelengths because the north pole is in winter, thus in darkness to visible-light cameras. At these wavelengths, about seven times greater than light seen by the human eye, the clouds deep inside Saturn’s atmosphere are seen in silhouette against the background glow of Saturn’s internal heat.

The entire north pole of Saturn is now mapped in detail in infrared, with features as small as 120 kilometers (75 miles) visible in the images. Time-lapse movies of the clouds circling the north pole show the whirlpool-like cyclone there is rotating at 530 kilometers per hour (325 miles per hour), more than twice as fast as the highest winds measured in cyclonic features on Earth. This cyclone is surrounded by an odd, honeycombed-shaped hexagon, which itself does not seem to move while the clouds within it whip around at high speeds, also greater than 500 kilometers per hour (300 miles per hour). Oddly, neither the fast-moving clouds inside the hexagon nor this new cyclone seem to disrupt the six-sided hexagon. New Cassini imagery of Saturn’s south pole shows complementary aspects of the region through the eyes of two different instruments. Near-infrared images from the visual and infrared mapping spectrometer instrument show the whole region is pockmarked with storms, while the imaging cameras show close-up details.

The new views are available online at: http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

Unlike Earth-bound hurricanes, powered by the ocean’s heat and water, Saturn's cyclones have no body of water at their bases, yet the eye-walls of Saturn’s and Earth’s storms look strikingly similar. Saturn's hurricanes are locked to the planet's poles, whereas terrestrial hurricanes drift across the ocean. "These are truly massive cyclones, hundreds of times stronger than the most giant hurricanes on Earth," said Kevin Baines, Cassini scientist on the visual and infrared mapping spectrometer at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Dozens of puffy, convectively formed cumulus clouds swirl around both poles, betraying the presence of giant thunderstorms lurking beneath. Thunderstorms are the likely engine for these giant weather systems," said Baines.

Just as condensing water in clouds on Earth powers hurricane vortices, the heat released from the condensing water in Saturnian thunderstorms deep down in the atmosphere may be the primary power source energizing the vortex.

In the south, the new infrared images of the pole, under the daylight conditions of southern summer, show the entire region is marked by hundreds of dark cloud spots. The clouds, like those at the north pole, are likely a manifestation of convective, thunderstorm-like processes extending some 100 kilometers (62 miles) below the clouds. They are likely composed of ammonium hydrosulfide with possibly a mixture of materials dredged up from the depths. By contrast, most of the hazes and clouds seen on Saturn are thought to be composed of ammonia, which condenses at high, visible altitudes.

Complementary images of the south pole from Cassini’s imaging cameras, obtained in mid-July, are 10 times more detailed than any seen before. "What looked like puffy clouds in lower resolution images are turning out to be deep convective structures seen through the atmospheric haze," said Cassini imaging team member Tony DelGenio of NASA’s Goddard Institute for Space Studies in New York. "One of them has punched through to a higher altitude and created its own little vortex."

The "eye" of the vortex is surrounded by an outer ring of high clouds. The new images also hint at an inner ring of clouds about half the diameter of the main ring, and so the actual clear "eye" region is smaller than it appears in earlier low-resolution images.

"It’s like seeing into the eye of a hurricane," said Andrew Ingersoll, a member of Cassini's imaging team at the California Institute of Technology, Pasadena. "It’s surprising. Convection is an important part of the planet’s energy budget because the warm upwelling air carries heat from the interior. In a terrestrial hurricane, the convection occurs in the eyewall; the eye is a region of downwelling. Here convection seems to occur in the eye as well."

Further observations are planned to see how the features at both poles evolve as the seasons change from southern summer to fall in August 2009.

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 mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The visual and infrared mapping spectrometer team is based at the University of Arizona. The imaging team is based at the Space Science Institute, Boulder, Colo.

Contacts:

Carolina Martinez 818-354-9382

Jet Propulsion Laboratory, Pasadena, Calif.

Carolina.Martinez@jpl.nasa.gov

NEWS RELEASE: 2008-192

These two images of Saturn show the entire south polar region, not just the little area around the core of the hurricane-like vortex. Earth-like storm patterns seem to be powering this vortex.

These images were taken in the near-infrared on May 11, 2007, from a distance of 416,000 kilometers (258,500 miles), and with a phase angle of 36 degrees.

From this distance, the resolution is 208 kilometers (128 miles) per pixel. The upper image represents 5.04 microns, a near-infrared wavelength some seven times the reddest wavelength visible to the human eye. At this wavelength, the planet's own heat produces an internal glow against which clouds deep within Saturn are seen in silhouette. Thus, dark areas represent thick clouds while bright areas represent clearings in the clouds.

The lower image shows the polar region in false color, with red, green, and blue depicting the appearance of the pole in three different near-infrared colors. Here, red depicts the 5.04 micron image shown in the black and white image above. Green and blue show the polar region, as seen in reflected sunlight at 3.08 and 4.08 micron wavelengths. The aqua color produced by green and blue light together show bright hazes and clouds in the upper atmosphere away from the pole; the lack of an aqua color component over the pole reveals a surprising dearth of upper-level bright hazes and clouds at high latitudes poleward of 73 degrees latitude, perhaps indicative of a general downwelling, heating, and sublimation into gas vapor of aerosol particles there. This clearing of upper-level hazes and clouds then allows - at other wavelengths not depicted here - unusually clear sunlit views of the deep atmosphere of Saturn near the 1-bar level, much deeper than typically seen elsewhere on the planet.

Due to the lack of bright polar hazes, the pole itself shows up only in reddish hues in this color composite. These red hues depict the near-infrared warm glow of Saturn’s interior heat diffusing upward through the clouds, thus revealing clouds at much deeper levels than what can be seen in reflected sunlight. Here, the brightest red coloring indicates clearings between low-lying clouds. These clearings extend downward from just below the haze layer down to about the 5-bar level some 125 kilometers (78 miles) below the upper-level hazes. The eye of the polar vortex is bright, showing that it is nearly cloud free. Dark spots throughout the region reveal the presence of thick convective clouds lurking in the depths of Saturn in the 2 to 5-bar region, about 50 to 125 kilometers (30-80 miles) underneath the hazes. At the edge of the polar region, where the greenish-blue tint of the upper hazes begins, a large ring of thick clouds can be seen in silhouette encircling the planet. Fine strands of streaky cloud material can be seen spiraling into this ring, indicating north/south motions in the planet's deep, dynamic atmosphere.

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 mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The Visual and Infrared Mapping Spectrometer team is based at the University of Arizona.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov/home/index.cfm. The visual and infrared mapping spectrometer team homepage is at http://wwwvims.lpl.arizona.edu.

Credit: NASA/JPL/University of Arizona

Cassini Buzzed Enceladus on Oct. 9

Written by Nancy Atkinson

The Cassini spacecraft will make two close passes of Saturn's geyser-spewing moon Enceladus this month. The first one on October 9 is the closest flyby yet of any moon of Saturn, at a white-knuckle distance of only 25 kilometers (16 miles) from the surface. The not-quite-as-scary Halloween flyby on Oct. 31 will be farther out, at 196 kilometers (122 miles). The focus of the Oct. 9 is the plume of the moon's geyser, and the spacecraft's fields and particles instruments will venture deeper into the plume than ever before, directly sampling the particles and gases. Scientists are intrigued by the possibility that liquid water, perhaps even an ocean, may exist beneath the surface of Enceladus. Trace amounts of organics have also been detected, raising tantalizing possibilities about the moon's habitability.

While Cassini's cameras and other optical instruments were the focus of an earlier flyby in August, this time the emphasis will be on the composition of the plume rather than imaging the surface.

"We know that Enceladus produces a few hundred kilograms per second of gas and dust and that this material is mainly water vapor and water ice," said Tamas Gambosi, Cassini scientist at the University of Michigan, Ann Arbor. "The water vapor and the evaporation from the ice grains contribute most of the mass found in Saturn's magnetosphere.

"One of the overarching scientific puzzles we are trying to understand is what happens to the gas and dust released from Enceladus, including how some of the gas is transformed to ionized plasma and is disseminated throughout the magnetosphere," said Gambosi.

On Oct. 31, the cameras and other optical remote sensing instruments will be front and center, imaging the fractures that slash across the moon's south polar region like stripes on a tiger.

These two flybys might augment findings from the most recent Enceladus flyby, which hint at possible changes associated with the icy moon. Cassini's Aug. 11 encounter with Enceladus showed temperatures over one of the tiger-stripe fractures were lower than those measured in earlier flybys. The fracture, called Damascus Sulcus, was about 160 to 167 Kelvin (minus 171 to minus 159 degrees Fahrenheit), below the 180 Kelvin (minus 136 degrees Fahrenheit) reported from a flyby in March of this year.

"We don't know yet if this is due to a real cooling of this tiger stripe, or to the fact that we were looking much closer, at a relatively small area, and might have missed the warmest spot," said John Spencer, Cassini scientist on the composite infrared spectrometer, at the Southwest Research Institute, Boulder, Colo.

Results from Cassini's magnetometer instrument during the August flyby suggest a difference in the intensity of the plume compared to earlier encounters. Information from the next two flybys will help scientists understand these observations.

Four more Enceladus flybys are planned in the next two years, bringing the total number to seven during Cassini's extended mission, called the Cassini Equinox Mission.

The Enceladus geysers were discovered by Cassini in 2005. Since then, scientists have been intrigued about what powers them, because the moon is so tiny, roughly the width of Arizona at only 500 kilometers (310 miles) in diameter.

Enceladus Just After the Flyby

We Sniffed the Plume!

Posted on Oct 10, 2008 12:47:15 AM | Saturn News

Todd BarberCassini Lead Propulsion Engineer (bio) Hello again from Pasadena, California! As I type these words, our spacecraft is dutifully executing a dynamic encounter with Saturn's perplexing satellite, Enceladus. Some of you have expressed an interest in the engineering particulars of our daringly close flyby, with respect to its effects on the spacecraft. I'm happy to say that even though scientists should gather plenty of material for chemical and physical analysis, the icy plumes of Enceladus are rather tenuous, even with our very close flyby distance of only 25 kilometers (16 miles). One reason this is true is our dive into the plumes occurs at a much higher altitude than the closest approach, as you can see in the image below. As such, we don't expect Cassini to slow down or change course due to impacts. However, gaseous plume material may place small torques on the spacecraft which will cause our thruster deadbands to be exceeded, a common occurrence while in thruster control. In that case, our small hydrazine thrusters (0.2 pounds or 0.9 Newtons) will fire to correct the pointing of the spacecraft. In fact, our attitude control engineers may be able to estimate the density of the Enceladus plume by studying these rocket firings!

As far as the potential for ice damage to Cassini, fortunately the ejecta from Enceladus consist of very tiny particles only. This, coupled with our micrometeoroid shielding and relatively high altitude while within the plume, will help keep Cassini safe and sound. It's truly the best of both worlds--we'll be close enough to really ingest the smorgasbord offered up by Enceladus, but we'll still be comfortably far from the surface and well away from affecting the spacecraft adversely.

A new Cassini video update was recently released, further explaining our busy October plans for twin Enceladus flybys. You may view the video here

Unfortunately, I won't be here to report our first signal tomorrow after the flyby, as I am leaving for a leaf-peeping vacation to New England this evening. Rest assured I'll be thinking of our intrepid robotic friend and the flood of science data to come. Go, Cassini, go!

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Where No Hoover Has Gone Before

Posted on Oct 09, 2008 06:01:58 PM | Saturn News

Geraint JonesScientist on the Cassini Plasma Spectrometer (bio) Cassini's next white-knuckle Enceladus flyby is quickly approaching, and the excitement is building for those of us working on the Cassini Plasma Spectrometer, CAPS. This instrument team is headed by Dave Young at the Southwest Research Institute, San Antonio, but the team members work in several countries. This includes the UK, where several of us on CAPS work at the Mullard Space Science Laboratory—part of University College London that's located in the countryside to the south of the city. CAPS is part of Cassini's suite of magnetospheric and plasma science, or MAPS experiments. With these, the spacecraft can sense its immediate environment: the gases, plasma, electromagnetic fields, and dust that can tell us a huge amount about the Saturn system.

The results from each of this year's Enceladus flybys has a different flavor. The main reason for this is that not all instruments can be pointed in their respective "best" directions at once, and the flybys are so brief that there's no time to turn Cassini, so every instrument gets a good look or samples the particles they want to... it's a bit like deciding who gets the window and aisle seats! Luckily, because we've got several flybys, the teams can take it in turns to get the best data out of their respective experiments... this time, as it was back on March 12th, the MAPS instrument teams are in their favorite seats. So what are the CAPS team going to be looking at during this daring dive? Enceladus is sitting in a flow of plasma—a mix of ions and electrons that's trapped around Saturn by the planet's magnetic field. We expect Cassini to briefly enter a wake where this magnetospheric plasma can't penetrate, a "shadowed" region where CAPS should sense a big drop in the density of the plasma. Once past closest approach, only 25 kilometers, or 15.5 miles, from Enceladus' surface, Cassini will spend a few minutes actually inside the plume of gas and ice particles being thrown out by the moon. CAPS will be pointed in the right direction to scoop up this material at almost 18 kilometers. or 11 miles a second. as we whiz through the plume. Some of the gases released in the plume are electrically-charged by the time they reach Cassini; CAPS can measure the energies of these and how many are present. From this, we should be able to learn a lot about the plume: how dense it is, its composition, and how this plume affects the plasma flowing past it. This approach worked really well during the March encounter, so we're hoping to repeat that success, but taking a different "cut" through the plume because Cassini will fly past Enceladus from a different direction. We're strapped in, noses against the window, and can't wait for the ride!

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Cassini Is 'GO' for Enceladus-5

Posted on Oct 07, 2008 05:49:03 PM | Saturn News

Todd BarberCassini Lead Propulsion Engineer (bio) Hello again from the realm of Cassini engineering, just 48 hours before another wonderful date with destiny! It is a distinct pleasure to kick off yet another Enceladus flyby blog, particularly one about a flyby so thrilling and daring. I'm happy to report the spacecraft is right on target for this historic encounter with Saturn's icy and active companion. As an engineer, I think I'm most floored by the closest approach distance of 25 kilometers, roughly 16 miles. Cassini hasn't been this close to any solid body since our ascent in a pre-dawn October sky in Florida in 1997! To further put this flyby in perspective, one can convert the minimum altitude into feet--yes, we'll actually be that close! In this system of units, we will buzz this active water-geyser surface at only 82,000 feet, which puts Cassini closer to Enceladus (900 million miles from Earth, mind you) than an SR71 Blackbird can fly above the ground on the third rock! Wow!