The recent discovery of plumes containing water vapor
erupting from the south pole of the frigid Saturnian moon Enceladus set off a
firestorm of debate.
Many scientists thought the geysers
of gaseous water must boil out of liquid water stored under the moon's
surface, which would make Enceladus a promising candidate for life.
But a new study challenges that conclusion, arguing that the
plumes could just as easily come from ice through the process of sublimation -
the direct leap from the solid to gaseous state.
This conclusion may dampen Enceladus' astrobiology
hopes, though it does not exclude the possibility of liquid water, and thus
life, on the moon.
Cold Faithful
Enceladus' geysers were first spotted in 2005 by NASA's Cassini
spacecraft, which was launched in 1997 on a mission to orbit Saturn.
Shortly after the discovery, a research group led by Carolyn Porco, head of
Cassini's imaging science team, calculated the ratio of water ice to vapor in
the plumes. Cassini wasn't able to directly measure the masses of ice and vapor
in the plumes, but the researchers estimated the mass of ice from the observed brightness
of the plumes (because ice reflects light, so the more ice, the brighter the
plumes). They deduced the mass of water vapor from a measurement of the molecular
signature of vapor in the wavelengths of light Cassini observed from the
plumes.
The scientists found that there were almost equal amounts of
ice
and water vapor. If this is the case, the researchers argued, the plumes
cannot result from the sublimation of ice. The ice crystals are believed to be
re-condensed from the vapor, rather than directly ejected from the surface or
subsurface of the moon. A lot of vapor is needed to produce that 50/50 ice and
vapor ratio, but the laws of thermodynamics prevent that much vapor from being
produced solely by sublimation. Therefore, some of the vapor must be produced
by evaporation rather than sublimation. This means a vast reserve of liquid
water must exist on Enceladus, perhaps as shallow as 23 feet (7 m) under the
surface. Under this theory, the researchers dubbed Enceladus' plumes "Cold
Faithful" after the Old Faithful geyser in Yellowstone National Park. Their paper was published in 2006 in the journal Science.
But a new study led by Susan Kieffer of the University of Illinois challenges
that work. She and her colleagues recalculated the ice-to-vapor ratio and
produced vastly different results. Under their calculations, vapor is much more
abundant than ice, with their final ratio less than 2:10, compared to the
previous team's estimate of 1:1.
"We simply redid the calculations described in the [previous]
paper and found that we could not reproduce the results quoted for either the
abundance of vapor or the abundance of ice," Kieffer said. "Our
conclusion was that sublimation should not have been excluded from
consideration as a process that could produce the measured quantities, and thus
should be considered as thoroughly as the liquid water
hypothesis. Unfortunately, it has not been."
Kieffer and team published their findings in the May 2009
issue of the journal Icarus.
Conflicting interpretations
The two contradicting conclusions paint a vastly different
picture of Enceladus. Under the first, the moon has an icy veneer that masks a
dynamic region of flowing water under the surface that could possibly host life.
But another interpretation sees Enceladus as a frigid world, solid with ice and
rock all the way through.
"Water is certainly present in Enceladus, at least as solid and
vapor, but the presence of vapor does not necessarily imply that liquid
water is present," Kieffer wrote with Brucke Jakosky, a scientist at the University of Colorado in Boulder, in an essay in the June 13, 2008 issue of Science.
"A definitive answer about the potential for life may have to await a
follow-on spacecraft mission that can make specific high-resolution
observations that could distinguish between the competing models."
The authors of the original paper, with the more hopeful
outlook on the possibility of liquid water on the moon, agree that more work is
needed to solve the conundrum and understand the conflicting estimates of the
plumes' ice-to-vapor ratio.
"Serious analysis of the Enceladus plume images are
only just beginning on the imaging team, and the jury is still very much out on
this subject," Porco said in response to the new paper.
Andrew Ingersoll, a planetary scientist at Caltech, was
involved with the original Porco et al. estimate, but said he doesn't believe
that either ratio is truly accurate.
"Right now, I am not convinced by anyone's estimates of
the ice/vapor ratio," he said. Ingersoll is currently working on a new,
revised estimate.
Cassini is also scheduled to make future flybys of the moon,
which could help refine the estimates.
"There are a number of reasons that the ice/vapor ratio
cannot be used to prove or disprove either model," Kieffer said. "Nevertheless,
it was the ice/vapor ratio from the original paper that gave the liquid water
hypothesis its momentum. The point of our Icarus paper is that this
value was miscalculated, that two competing hypotheses should have been set in
motion, and both should still be considered by the scientific community."
The intriguing possibilities and conflicting interpretations
of Enceladus are reasons why many scientists are pushing for a dedicated
spacecraft mission to the moon. Though no official plans exist, a number of
proposals are currently being considered for future NASA and ESA (European
Space Agency) missions.
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