Researchers found that Titan's ice shell, which
overlies a very salty ocean, varies in thickness around the moon, suggesting the
crust is in the process of becoming rigid.
Image Credit:
NASA/JPL/SSI/Univ. of Arizona/G. Mitri/University
of Nantes
Scientists
analyzing data from NASA’s Cassini mission have firm evidence the ocean inside
Saturn's largest moon, Titan, might be as salty as the Earth's Dead Sea.
The new results come from a study of gravity and topography data collected
during Cassini's repeated flybys of Titan during the past 10 years. Using the
Cassini data, researchers presented a model structure for Titan, resulting in an
improved understanding of the structure of the moon's outer ice shell. The
findings are published in this week’s edition of the journal Icarus.
"Titan continues to prove itself as an endlessly fascinating world, and with
our long-lived Cassini spacecraft, we’re unlocking new mysteries as fast as we
solve old ones," said Linda Spilker, Cassini project scientist at NASA's Jet
Propulsion Laboratory (JPL) in Pasadena, California, who was not involved in the
study.
Additional findings support previous indications the moon's icy shell is
rigid and in the process of freezing solid. Researchers found that a relatively
high density was required for Titan's ocean in order to explain the gravity
data. This indicates the ocean is probably an extremely salty brine of water
mixed with dissolved salts likely composed of sulfur, sodium and potassium. The
density indicated for this brine would give the ocean a salt content roughly
equal to the saltiest bodies of water on Earth.
"This is an extremely salty ocean by Earth standards," said the paper's lead
author, Giuseppe Mitri of the University of Nantes in France. "Knowing this may
change the way we view this ocean as a possible abode for present-day life, but
conditions might have been very different there in the past."
Cassini data also indicate the thickness of Titan's ice crust varies slightly
from place to place. The researchers said this can best be explained if the
moon's outer shell is stiff, as would be the case if the ocean were slowly
crystalizing, and turning to ice. Otherwise, the moon's shape would tend to even
itself out over time, like warm candle wax. This freezing process would have
important implications for the habitability of Titan's ocean, as it would limit
the ability of materials to exchange between the surface and the ocean.
A further consequence of a rigid ice shell, according to the study, is any
outgassing of methane into Titan's atmosphere must happen at scattered "hot
spots" -- like the hot spot on Earth that gave rise to the Hawaiian Island
chain. Titan's methane does not appear to result from convection or plate
tectonics recycling its ice shell.
How methane gets into the moon's atmosphere has long been of great interest
to researchers, as molecules of this gas are broken apart by sunlight on short
geological timescales. Titan's present atmosphere contains about five percent
methane. This means some process, thought to be geological in nature, must be
replenishing the gas. The study indicates that whatever process is responsible,
the restoration of Titan's methane is localized and intermittent.
"Our work suggests looking for signs of methane outgassing will be difficult
with Cassini, and may require a future mission that can find localized methane
sources," said Jonathan Lunine, a scientist on the Cassini mission at Cornell
University, Ithaca, New York, and one of the paper's co-authors. "As on Mars,
this is a challenging task."
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.
For more information about Cassini, visit
NASA
Guillermo Gonzalo Sánchez Achutegui
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