NASA's Cassini spacecraft and Deep Space Network have uncovered evidence Saturn's moon Enceladus harbors a large underground ocean of liquid water, furthering scientific interest in the moon as a potential home to extraterrestrial microbes.

Researchers theorized the presence of an interior reservoir of water in 2005 when Cassini discovered water vapor and ice spewing from vents near the moon's south pole. The new data provide the first geophysical measurements of the internal structure of Enceladus, consistent with the existence of a hidden ocean inside the moon. Findings from the gravity measurements are in the Friday April 4 edition of the journal Science.

"The way we deduce gravity variations is a concept in physics called the Doppler Effect, the same principle used with a speed-measuring radar gun," said Sami Asmar of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., a coauthor of the paper. "As the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we're trying to measure. We see the change in velocity as a change in radio frequency, received at our ground stations here all the way across the solar system."

The gravity measurements suggest a large, possibly regional, ocean about 6 miles (10 kilometers) deep, beneath an ice shell about 19 to 25 miles (30 to 40 kilometers) thick. The subsurface ocean evidence supports the inclusion of Enceladus among the most likely places in our solar system to host microbial life. Before Cassini reached Saturn in July 2004, no version of that short list included this icy moon, barely 300 miles (500 kilometers) in diameter.

"This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole," said David Stevenson of the California Institute of Technology, Pasadena, one of the paper's co-authors.

Cassini has flown near Enceladus 19 times. Three flybys, from 2010 to 2012, yielded precise trajectory measurements. The gravitational tug of a planetary body, such as Enceladus, alters a spacecraft's flight path. Variations in the gravity field, such as those caused by mountains on the surface or differences in underground composition, can be detected as changes in the spacecraft's velocity, measured from Earth.

The technique of analyzing a radio signal between Cassini and the Deep Space Network can detect changes in velocity as small as less than one foot per hour (90 microns per second). With this precision, the flyby data yielded evidence of a zone inside the southern end of the moon with higher density than other portions of the interior.

The south pole area has a surface depression that causes a dip in the local tug of gravity. However, the magnitude of the dip is less than expected given the size of the depression, leading researchers to conclude the depression's effect is partially offset by a high-density feature in the region, beneath the surface.

"The Cassini gravity measurements show a negative gravity anomaly at the south pole that however is not as large as expected from the deep depression detected by the onboard camera," said the paper's lead author, Luciano Iess of Sapienza University of Rome. "Hence the conclusion that there must be a denser material at depth that compensates the missing mass: very likely liquid water, which is seven percent denser than ice. The magnitude of the anomaly gave us the size of the water reservoir."

There is no certainty the subsurface ocean supplies the water plume spraying out of surface fractures near the south pole of Enceladus, however, scientists reason it is a real possibility. The fractures may lead down to a part of the moon that is tidally heated by the moon's repeated flexing, as it follows an eccentric orbit around Saturn.

Much of the excitement about the Cassini mission's discovery of the Enceladus water plume stems from the possibility that it originates from a wet environment that could be a favorable environment for microbial life.

"Material from Enceladus’ south polar jets contains salty water and organic molecules, the basic chemical ingredients for life," said Linda Spilker, Cassini's project scientist at JPL. "Their discovery expanded our view of the 'habitable zone' within our solar system and in planetary systems of other stars. This new validation that an ocean of water underlies the jets furthers understanding about this intriguing environment."  

Quelle: NASA

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Before Saturn's Limb

Jets of water ice particles spew from Saturn's moon Enceladus in this image obtained by NASA's Cassini spacecraft on Aug. 13, 2010. A crescent of the moon appears dimly illuminated in front of the bright limb of Saturn. This view looks toward the night side of Saturn, which occupies the lower half of the image. Enceladus, in the center of the image, is closer to the spacecraft than the planet is in this view. Sunlight scatters through the planet's atmosphere and forms the bright diagonal line running from the left to right of the image. Lit terrain seen on Enceladus (504 kilometers, 313 miles across) is on the leading hemisphere of the moon. North on Enceladus is up. The jets erupting from the south polar region appear faint here, but can be seen at the bottom of the crescent of the moon. See PIA11688 to learn more.

The image was taken in visible light with the Cassini spacecraft wide-angle camera. The view was acquired at a distance of approximately 61,000 kilometers (38,000 miles) from Enceladus and at a sun-Enceladus-spacecraft, or phase, angle of 155 degrees. Image scale is 4 kilometers (2 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 in 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, Boulder, Colo.

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Looming Enceladus

Small water ice particles fly from fissures in the south polar region of Saturn's moon Enceladus in this image taken during the Aug. 13, 2010, flyby of the moon by NASA's Cassini spacecraft. This view looks toward the night side of Saturn, which is in the lower left of the image. Enceladus, in the top right, is closer to the spacecraft than the planet is in this view. Sunlight scatters through the planet's atmosphere and forms the bright diagonal line running from the left to bottom right of the image. The atmosphere appears layered here. Scientists think the different layers on the limb are real and not an artifact of the camera's exposure. The famous jets, imaged by Cassini's cameras for the first time in 2005, are faintly seen here erupting from the fractures that cross the south polar region of the moon.

Illuminated terrain seen on Enceladus is on the leading hemisphere of the moon, or the side facing forward in the moon's orbit around Saturn. North on Enceladus (504 kilometers, 313 miles across) is up. The jets appear faint here, but can be seen near the center of the image. See PIA11688 to learn more. The image was taken in visible light with the Cassini spacecraft narrow-angle camera. The view was obtained at a distance of approximately 59,000 kilometers (37,000 miles) from Enceladus and at a sun-Enceladus-spacecraft, or phase, angle of 155 degrees. Image scale on Enceladus is 353 meters (1,157 feet) per pixel.

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Jet Blue

Cassini imaging scientists used views like this one to help them identify the source locations for individual jets spurting ice particles, water vapor and trace organic compounds from the surface of Saturn's moon Enceladus.

Their study -- published in the Oct. 11, 2007, issue of the journal Nature -- identifies eight source locations, all on the prominent tiger stripe fractures, or sulci, in the moon's south polar region. Some of the sources occur in regions not yet observed by Cassini's composite infrared spectrometer, and the researchers predict that future Cassini observations of those locations will find elevated temperatures. This false-color view was created by combining three clear filter images taken at nearly the same time as Fountains of Enceladus - Image 2. This image product was then specially processed to enhance the individual jets that compose the plume. (Fountains of Enceladus - Image 2 was instead processed to reveal subtleties in the brightness of the overall plume that comprises the jets.) Some artifacts due to the processing are present in the image. The final product was colored as blue for dramatic effect. The images were acquired with the Cassini spacecraft narrow-angle camera on Nov. 27, 2005 at a distance of approximately 148,000 kilometers (92,000 miles) from Enceladus and at a sun-Enceladus-spacecraft, or phase, angle of 161 degrees. Scale in the original images is about 880 meters (0.5 mile) per pixel. This view has been magnified by a factor of two from the original images.

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Enceladus the Storyteller

A masterpiece of deep time and wrenching gravity, the tortured surface of Saturn's moon Enceladus and its fascinating ongoing geologic activity tell the story of the ancient and present struggles of one tiny world. This is a story that is recounted by imaging scientists in a paper published in the journal Science on March 10, 2006. The enhanced color view of Enceladus seen here is largely of the southern hemisphere and includes the south polar terrain at the bottom of the image.

Ancient craters remain somewhat pristine in some locales, but have clearly relaxed in others. Northward-trending fractures, likely caused by a change in the moon's rate of rotation and the consequent flattening of the moon's shape, rip across the southern hemisphere. The south polar terrain is marked by a striking set of `blue' fractures and encircled by a conspicuous and continuous chain of folds and ridges, testament to the forces within Enceladus that have yet to be silenced.

The mosaic was created from 21 false-color frames taken during the Cassini spacecraft's close approaches to Enceladus on March 9 and July 14, 2005. Images taken using filters sensitive to ultraviolet, visible and infrared light (spanning wavelengths from 338 to 930 nanometers) were combined to create the individual frames.

The mosaic is an orthographic projection centered at 46.8 degrees south latitude, 188 degrees west longitude, and has an image scale of 67 meters (220 feet) per pixel. The original images ranged in resolution from 67 meters per pixel to 350 meters (1,150 feet) per pixel and were taken at distances ranging from 11,100 to 61,300 kilometers (6,900 to miles) from Enceladus.

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Highlighting Plumes

At least four distinct plumes of water ice spew out from the south polar region of Saturn's moon Enceladus in this dramatically illuminated image.

Light reflected off Saturn is illuminating the surface of the moon while the sun, almost directly behind Enceladus, is backlighting the plumes. See PIA11688 to learn more about Enceladus and its plumes.

This view looks toward the Saturn-facing side of Enceladus (504 kilometers, or 313 miles across). North is up.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 25, 2009. The view was obtained at a distance of approximately 617,000 kilometers (383,000 miles) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 174 degrees. Image scale is 4 kilometers (2 miles) per pixel.

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Tiger Stripe Split Ends

This image shows a high-resolution heat intensity map of part of the south polar region of Saturn's moon Enceladus, made from data obtained by NASA's Cassini spacecraft.

The map reveals never-before-seen details of warm fractures that branch off like split ends from the ends of the main trenches of two "tiger stripes." The features nicknamed "tiger stripes" are long fissures that spray water vapor and icy particles. These two fissures, Cairo Sulcus (left) and Alexandria Sulcus (right), extend to the lower right, off the bottom of the image. The map also shows an intriguing isolated warm spot, shown in purple-red in the upper left of the image, that is separated from other active fissures.

The thermal data came from Cassini's composite infrared spectrometer during an Aug. 13, 2010, flyby of Enceladus. Scientists overlaid the data on a background map of that region made from Cassini images taken in July 2005. The intensity of thermal radiation, measured at wavelengths from 12 to 16 microns, is color-coded, with dark blue, purple, red and orange denoting progressively more intense radiation, due to higher temperatures and/or larger expanses of warm material. The pale blue color indicates regions that were mapped but that were too cold to emit significant radiation. Alignment of the thermal map with the underlying base map is approximate. The map shows a region approximately 130 kilometers (80 miles) across.

These data were obtained as winter darkness began to engulf the south polar region of Enceladus. Away from the warm tiger stripes, which reach temperatures up to 190 Kelvin (minus 120 degrees Fahrenheit), Cassini measured surface temperatures near Enceladus' south pole as low as 52 Kelvin (minus 365 degrees Fahrenheit), and still colder temperatures are expected as winter advances. Scientists are still analyzing the data to calculate a temperature for the cross-cutting fractures and the isolated warm spot.

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Hot and Narrow Tiger Stripe

Data from NASA's Cassini spacecraft have enabled scientists to make the highest-resolution heat intensity maps yet for the hottest part of a "tiger stripe" fissure on Saturn's moon Enceladus. The moon's south polar region features several of these long fissures that spray water and icy particles, and the one in this image is called Damascus Sulcus.

The thermal infrared data, shown in color, come from Cassini's composite infrared spectrometer (CIRS). The grayscale background image, which is illuminated by light reflected from Saturn rather than by direct sunlight, is from Cassini's high-resolution imaging camera (ISS). The CIRS scan gives scientists confidence that the peak temperature along Damascus Sulcus, the most active tiger stripe, was about 190 Kelvin (minus 120 degrees Fahrenheit). This temperature is slightly higher than the previous maximum temperatures measured by CIRS at Damascus, which were around 170 Kelvin (minus 150 degrees Fahrenheit).

The intensity of heat radiation, measured by CIRS at wavelengths from 7 to 9 microns, is color-coded, with blue, purple, red, orange and yellow denoting progressively more intense radiation, due to higher temperatures and/or larger expanses of warm material. The image is centered near 80 degrees south latitude and 315 degrees west longitude, and covers a region about 16 kilometers (10 miles) wide. The smallest details seen in the CIRS overlay are about 800 meters (0.5 miles) in size.

The region of peak temperature is sharply bounded by the sides of the trench. Thanks to its high resolution, the CIRS map also shows for the first time that the regions on either side of the central trench are also radiating heat (shown as blue strips flanking the central multicolored strip in this image). CIRS measured temperatures of about 120 Kelvin (minus 240 degrees Fahrenheit) in the flanking regions about 400 to 1,200 meters (a quarter to three-quarters of a mile) away from the central trench.

These data were obtained on Aug. 13, 2010 as the south pole of Enceladus began to go into winter darkness.

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Enceladus Flyby - Nov. 6, 2011

This raw, unprocessed image of Saturn's moon Enceladus was taken by NASA's Cassini spacecraft on Nov. 6, 2011 and received on Earth November 07, 2011.

The camera was pointing toward Enceladus at approximately 108,044 kilometers away, and the image was taken using the CL1 and CL2 filters.

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Dark Moon, Bright Spray

This raw image of Saturn's moon Enceladus was taken by NASA's Cassini spacecraft on Dec. 20, 2010. The spacecraft was approximately 158,000 kilometers (98,000 miles) away from Enceladus.

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Brilliant Enceladus

The Cassini spacecraft looks at a brightly illuminated Enceladus and examines the surface of the leading hemisphere of this Saturnian moon.

See PIA11685 to learn more about the surface of Enceladus. See PIA11688 to learn about the jets of water ice emanating from the moon's south polar region. North on Enceladus (313 miles across, or 504 kilometers) is up and rotated 21 degrees to the right.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Nov. 6, 2011. The view was obtained at a distance of approximately 67,700 miles (109,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 21 degrees. Image scale is 2,130 feet (649 meters) per pixel.

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Rings and Enceladus

A crescent Enceladus appears with Saturn's rings in this Cassini spacecraft view of the moon.

The famed jets of water ice emanating from the south polar region of the moon are faintly visible here. They appear as a small white blur below the dark south pole, down and to the right of the illuminated part of the moon's surface in the image. The image’s contrast was enhanced to increase the visibility of the jets. See PIA11688 to learn more about the jets.

Lit terrain seen here is on the trailing hemisphere of Enceladus (313 miles, 504 kilometers across). North on Enceladus is up.

This view looks toward the northern, sunlit side of the rings from just above the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Jan. 4, 2012. The view was obtained at a distance of approximately 181,000 miles (291,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 136 degrees. Image scale is 1 mile (2 kilometers) per pixel.

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Dark Moon, Dramatic Plume

Below a darkened Enceladus, a plume of water ice is backlit in this view of one of Saturn's most dramatic moons.

See PIA11688 and PIA08386 to learn more about the jets of water ice emanating from the south polar region of Enceladus. Lit terrain seen here is on the leading hemisphere of Enceladus (313 miles, or 504 kilometers across). North is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Feb. 20, 2012. The view was acquired at a distance of approximately 83,000 miles (134,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 165 degrees. Image scale is 2,628 feet (801 meters) per pixel.

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