[meteorite-list] NASA Space Assets Detect Ocean Inside Saturn Moon Enceladus

[Ron Baalke]

April 3, 2014
     
NASA Space Assets Detect Ocean inside Saturn Moon

[Image]
Gravity measurements by NASA's Cassini spacecraft and Deep Space Network  
suggest that Saturn's moon Enceladus, which has jets of water vapor and ice  
gushing from its south pole, also harbors a large interior ocean beneath an  
ice shell, as this illustration depicts.
Image Credit: NASA/JPL-Caltech

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."

The Cassini-Huygens mission is a cooperative project of NASA, the European  
Space Agency and the Italian Space Agency. JPL manages the mission for NASA's  
Science Mission Directorate in Washington. For more information about  
Cassini, visit:

http://www.nasa.gov/cassini 

-end-

Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown at nasa.gov 

Jane Platt
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0880
jane.platt at jpl.nasa.gov 

Brian Bell
California Institute of Technology, Pasadena
626-395-5832
bpbell at caltech.edu