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
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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:
NASA
Guillermo Gonzalo Sánchez Achutegui
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