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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 -Caltech/SSI/Univ. of Arizona/G. Mitri/University of Nantes. Larger image
Scientists analyzing data from NASA's Cassini mission have firm
evidence the ocean inside Saturn's largest moon, Titan, might be as
salty as 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 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.