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.”
0 التعليقات:
إرسال تعليق