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. Full image and caption
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 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 and http://saturn.jpl.nasa.gov.
Jane Platt
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0880
jane.platt@jpl.nasa.gov
Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown@nasa.gov
Brian Bell
California Institute of Technology, Pasadena
626-395-5832
bpbell@caltech.edu
