[meteorite-list] Cassini Data Suggest Enceladus' Ocean May Harbor Hydrothermal Activity

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=4507

Spacecraft Data Suggest Saturn Moon's Ocean May Harbor Hydrothermal Activity
Jet Propulsion Laboratory
March 11, 2015

Fast Facts:

* Cassini finds first evidence of active hot-water chemistry beyond planet 
Earth

* Findings in two separate papers support the notion

* The results have important implications for the habitability of icy 
worlds

NASA's Cassini spacecraft has provided scientists the first clear evidence 
that Saturn's moon Enceladus exhibits signs of present-day hydrothermal 
activity which may resemble that seen in the deep oceans on Earth. The 
implications of such activity on a world other than our planet open up 
unprecedented scientific possibilities.

"These findings add to the possibility that Enceladus, which contains 
a subsurface ocean and displays remarkable geologic activity, could contain 
environments suitable for living organisms," said John Grunsfeld, astronaut 
and associate administrator of NASA's Science Mission Directorate in Washington. 
"The locations in our solar system where extreme environments occur in 
which life might exist may bring us closer to answering the question: 
are we alone in the universe."

Hydrothermal activity occurs when seawater infiltrates and reacts with 
a rocky crust and emerges as a heated, mineral-laden solution, a natural 
occurrence in Earth's oceans. According to two science papers, the results 
are the first clear indications an icy moon may have similar ongoing active 
processes.

The first paper, published this week in the journal Nature, relates to 
microscopic grains of rock detected by Cassini in the Saturn system. An 
extensive, four-year analysis of data from the spacecraft, computer simulations 
and laboratory experiments led researchers to the conclusion the tiny 
grains most likely form when hot water containing dissolved minerals from 
the moon's rocky interior travels upward, coming into contact with cooler 
water. Temperatures required for the interactions that produce the tiny 
rock grains would be at least 194 degrees Fahrenheit (90 degrees Celsius).

"It's very exciting that we can use these tiny grains of rock, spewed 
into space by geysers, to tell us about conditions on -- and beneath -- 
the ocean floor of an icy moon," said the paper's lead author Sean Hsu, 
a postdoctoral researcher at the University of Colorado at Boulder.

Cassini's cosmic dust analyzer (CDA) instrument repeatedly detected miniscule 
rock particles rich in silicon, even before Cassini entered Saturn's orbit 
in 2004. By process of elimination, the CDA team concluded these particles 
must be grains of silica, which is found in sand and the mineral quartz 
on Earth. The consistent size of the grains observed by Cassini, the largest 
of which were 6 to 9 nanometers, was the clue that told the researchers 
a specific process likely was responsible.

On Earth, the most common way to form silica grains of this size is hydrothermal 
activity under a specific range of conditions; namely, when slightly alkaline 
and salty water that is super-saturated with silica undergoes a big drop 
in temperature.

"We methodically searched for alternate explanations for the nanosilica 
grains, but every new result pointed to a single, most likely origin," 
said co-author Frank Postberg, a Cassini CDA team scientist at Heidelberg 
University in Germany.

Hsu and Postberg worked closely with colleagues at the University of Tokyo 
who performed the detailed laboratory experiments that validated the hydrothermal 
activity hypothesis. The Japanese team, led by Yasuhito Sekine, verified 
the conditions under which silica grains form at the same size Cassini 
detected. The researchers think these conditions may exist on the seafloor 
of Enceladus, where hot water from the interior meets the relatively cold 
water at the ocean bottom.

The extremely small size of the silica particles also suggests they travel 
upward relatively quickly from their hydrothermal origin to the near-surface 
sources of the moon's geysers. From seafloor to outer space, a distance 
of about 30 miles (50 kilometers), the grains spend a few months to a 
few years in transit, otherwise they would grow much larger.

The authors point out that Cassini's gravity measurements suggest Enceladus' 
rocky core is quite porous, which would allow water from the ocean to 
percolate into the interior. This would provide a huge surface area where 
rock and water could interact.

The second paper, recently published in Geophysical Research Letters, 
suggests hydrothermal activity as one of two likely sources of methane 
in the plume of gas and ice particles that erupts from the south polar 
region of Enceladus. The finding is the result of extensive modeling by 
French and American scientists to address why methane, as previously sampled 
by Cassini, is curiously abundant in the plume.

The team found that, at the high pressures expected in the moon's ocean, 
icy materials called clathrates could form that imprison methane molecules 
within a crystal structure of water ice. Their models indicate that this 
process is so efficient at depleting the ocean of methane that the researchers 
still needed an explanation for its abundance in the plume.

In one scenario, hydrothermal processes super-saturate the ocean with 
methane. This could occur if methane is produced faster than it is converted 
into clathrates. A second possibility is that methane clathrates from 
the ocean are dragged along into the erupting plumes and release their 
methane as they rise, like bubbles forming in a popped bottle of champagne.

The authors agree both scenarios are likely occurring to some degree, 
but they note that the presence of nanosilica grains, as documented by 
the other paper, favors the hydrothermal scenario.

"We didn't expect that our study of clathrates in the Enceladus ocean 
would lead us to the idea that methane is actively being produced by hydrothermal 
processes," said lead author Alexis Bouquet, a graduate student at the 
University of Texas at San Antonio. Bouquet worked with co-author Hunter 
Waite, who leads the Cassini Ion and Neutral Mass Spectrometer (INMS) 
team at Southwest Research Institute in San Antonio.

Cassini first revealed active geological processes on Enceladus in 2005 
with evidence of an icy spray issuing from the moon's south polar region 
and higher-than-expected temperatures in the icy surface there. With its 
powerful suite of complementary science instruments, the mission soon 
revealed a towering plume of water ice and vapor, salts and organic materials 
that issues from relatively warm fractures on the wrinkled surface. Gravity 
science results published in 2014 strongly suggested the presence of a 
6-mile- (10-kilometer-) deep ocean beneath an ice shell about 19 to 25 
miles (30 to 40 kilometers) thick.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European 
Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory 
in Pasadena, California, manages the mission for the agency's Science 
Mission Directorate in Washington. The Cassini CDA instrument was provided 
by the German Aerospace Center. The instrument team, led by Ralf Srama, 
is based at the University of Stuttgart in Germany. JPL is a division 
of the California Institute of Technology in Pasadena.

More information about Cassini, visit:

http://www.nasa.gov/cassini

and

http://saturn.jpl.nasa.gov

Updated on March 11, 2015 at 2:50pm(PST) to include information on the 
role of French and American scientists


Media Contact

Preston Dyches
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-7013
preston.dyches at jpl.nasa.gov 

Dwayne Brown
NASA Headquarters, Washington
202-358-1726
dwayne.c.brown at nasa.gov 

2015-085