Water vapor jets spewing from Saturn’s icy moon, Enceladus. (Image courtesy NASA/JPL/Space Science Institute) |
Samples
of icy spray shooting from Saturn’s moon Enceladus collected during
Cassini spacecraft flybys show the strongest evidence yet for the
existence of a large-scale, subterranean saltwater ocean, says a new
international study led by the Univ. of Heidelberg and involving
the Univ. of Colorado Boulder.
The
new discovery was made during the Cassini-Huygens mission to Saturn, a
collaboration of NASA, the European Space Agency and the Italian Space
Agency. Launched in 1997, the mission spacecraft arrived at the Saturn
system in 2004 and has been touring the giant ringed planet and its vast
moon system ever since.
The
plumes shooting water vapor and tiny grains of ice into space were
originally discovered emanating from Enceladus—one of 19 known moons
of Saturn—by the Cassini spacecraft in 2005. The plumes were
originating from the so-called “tiger stripe” surface fractures at the
moon’s south pole and apparently have created the material for the faint
E Ring that traces the orbit of Enceladus around Saturn.
During
three of Cassini’s passes through the plume in 2008 and 2009, the
Cosmic Dust Analyser, or CDA, on board measured the composition of
freshly ejected plume grains. The icy particles hit the detector’s
target at speeds of up to 11 miles per second, instantly vaporizing
them. The CDA separated the constituents of the resulting vapor clouds,
allowing scientists to analyze them.
The
study shows the ice grains found further out from Enceladus are
relatively small and mostly ice-poor, closely matching the composition
of the E Ring. Closer to the moon, however, the Cassini observations
indicate that relatively large, salt-rich grains dominate.
“There
currently is no plausible way to produce a steady outflow of salt-rich
grains from solid ice across all the tiger stripes other than the salt
water under Enceladus’ icy surface,” said Frank Postberg of the
Univ. of Germany, lead author of a study published in Nature. Other co-authors include Jürgen Schmidt from the Univ.
of Potsdam, Jonathan Hillier from Open Univ. headquartered in
Milton Keynes, England, and Ralf Srama from the Univ. of Stuttgart.
“The
study indicates that ‘salt-poor’ particles are being ejected from the
underground ocean through cracks in the moon at a much higher speed than
the larger, salt-rich particles,” said CU-Boulder faculty member and
study co-author Sascha Kempf of the Laboratory for Atmospheric and Space
Physics.
“The
E Ring is made up predominately of such salt-poor grains, although we
discovered that 99% of the mass of the particles ejected by the
plumes was made up of salt-rich grains, which was an unexpected
finding,” said Kempf. “Since the salt-rich particles were ejected at a
lower speed than the salt-poor particles, they fell back onto the moon’s
icy surface rather than making it to the E Ring.”
|
According
to the researchers, the salt-rich particles have an “ocean-like”
composition that indicates most, if not all, of the expelled ice comes
from the evaporation of liquid salt water rather than from the icy
surface of the moon. When salt water freezes slowly the salt is
“squeezed out,” leaving pure water ice behind. If the plumes were coming
from the surface ice, there should be very little salt in them, which
was not the case, according to the research team.
The
researchers believe that perhaps 50 miles beneath the surface crust of
Enceladus a layer of water exists between the rocky core and the icy
mantle that is kept in a liquid state by gravitationally driven tidal
forces created by Saturn and several neighboring moons, as well as by
heat generated by radioactive decay.
According
to the scientists, roughly 440 pounds of water vapor is lost every
second from the plumes, along with smaller amounts of ice grains.
Calculations show the liquid ocean must have a sizable evaporating
surface or it would easily freeze over, halting the formation of the
plumes. “This study implies that nearly all of the matter in the
Enceladus plumes originates from a saltwater ocean that has a very large
evaporating surface,” said Kempf.
Salt
in the rock dissolves into the water, which accumulates in a liquid
ocean beneath the icy crust, according to the Nature authors. When the
outermost layer of the Enceladus crust cracks open, the reservoir is
exposed to space. The drop in pressure causes the liquid to evaporate
into a vapor, with some of it “flash-freezing” into salty ice grains,
which subsequently creates the plumes, the science team believes.
“Enceladus
is a tiny, icy moon located in a region of the outer Solar System where
no liquid water was expected to exist because of its large distance
from the sun,” said Nicolas Altobelli, ESA’s project scientist for the
Cassini-Huygens mission. “This finding is therefore a crucial new piece
of evidence showing that environmental conditions favorable to the
emergence of life may be sustainable on icy bodies orbiting gas giant
planets.”
The
Huygens probe was released from the main spacecraft and parachuted
through the atmosphere to the surface of Saturn’s largest moon, Titan,
in 2005.
The
Cassini spacecraft is carrying 12 science instruments, including a
$12.5 million CU-Boulder ultraviolet imaging spectrograph designed and
built by a LASP team led by Professor Larry Esposito.
