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.
