Cassini's VIMS Detects Vast Polar Ethane Cloud on Titan

Cassini's VIMS Detects Vast Polar Ethane Cloud on Titan
Lori Stiles
University of Arizona
September 14, 2006

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Science Contact:
Caitlin Griffith
520-626-3806
griffith@xxxxxxxxxxxxxxx

Related Web sites
NASA Cassini
http://www.nasa.gov/cassini

JPL Cassini
http://saturn.jpl.nasa.gov <http://saturn.jpl.nasa.gov/>

VIMS
http://wwwvims.lpl.arizona.edu <http://wwwvims.lpl.arizona.edu/>
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Cassini's Visual and Infrared Mapping Spectrometer (VIMS) has detected
what
appears to be a massive ethane cloud surrounding Titan's north pole.
The
cloud might be snowing ethane snowflakes into methane lakes below.

The cloud may be the clue needed in solving a puzzle that has
confounded
scientists who so far have seen little evidence of a veil of ethane
clouds
and surface liquids originally thought extensive enough to cover the
entire
surface of Titan with a 300-meter-deep ocean.

Before the Cassini-Huygens mission began visiting Titan in 2004, "We
expected to see lots of ethane -- vast ethane clouds at all latitudes
and
extensive seas on the surface of Saturn's giant moon Titan," University
of
Arizona planetary scientist Caitlin Griffith said.

That's because solar ultraviolet light irreversibly breaks down methane
in
Titan's mostly nitrogen atmosphere. Ethane is by far the most plentiful
byproduct when methane breaks down. If methane has been a constituent
of the
atmosphere throughout Titan's 4.5-billion-year lifetime -- and there
was no
reason to suspect it had not -- the large moon would be awash with seas
of
ethane, scientists theorized.

NASA's Cassini spacecraft radar found lakes in Titan's north arctic
latitudes on a flyby last July 22. However, "We now know that Titan's
surface is largely devoid of lakes and oceans," Griffith said. She is a
member of the UA-based Cassini VIMS team, headed by Professor Robert
Brown
of UA's Lunar and Planetary Lab.

The missing ethane is all the more mysterious because Cassini images
suggest
that other less abundant solid precipitates from the photochemical
reactions
in Titan's atmosphere have formed dunes and covered craters on its
surface,
Griffith said.

VIMS made the first detection of Titan's vast polar ethane cloud when
it
probed Titan's high northern latitudes on Cassini flybys in December
2004,
August 2005, and September 2005.

VIMS detected the cirrus cloud as a bright band at altitudes from
between 30
km and 60 km at the edge of Titan's arctic circle, between 51 degrees
and 69
degrees north latitude. VIMS saw only part of the cloud because most of
the
northern polar region is in winter's shadow and won't be fully
illuminated
until 2010, Griffith noted.

"Our observations imply that surface deposits of ethane should be found
specifically at the poles, rather than globally distributed across
Titan's
disk as previously assumed," Griffith said. "That may partially explain
the
lack of liquid ethane oceans and clouds at Titan's middle and lower
latitudes."

"We think that ethane is raining or, if temperatures are cool enough,
snowing on the north pole right now. When the seasons switch, we expect
ethane to condense at the south pole during its winter," Griffith said.
If
polar conditions are as cool as predictions say, ethane could
accumulate as
polar ice.

Ethane dissolves in methane, which scientists predict is raining from
the
atmosphere at the north pole during its cool winter. "During the polar
winters, we expect the lowlands to cradle methane lakes that are rich
with
ethane," Griffith noted. "Perhaps these are the lakes recently imaged
by
Cassini."

If ethane was produced at today's rate over Titan's entire lifetime, a
total
of two kilometers of ethane would have precipitated over the poles. But
that
seems unlikely, Griffith said.

Scientists have no direct evidence for polar caps of ethane ice.
Titan's
north pole is in winter darkness, and Cassini cameras have yet to see
it in
reflected light. Cassini cameras have imaged Titan's south pole. "The
morphology seen in those images doesn't suggest a two kilometer polar
ice
cap, but the images do show flow features," Griffith said.

"We're going to start making more polar passes in the upcoming months,"
she
added. "By the end of next year Cassini will have recorded the first
polar
temperature profile of Titan, which will tell us how cold conditions
are at
the pole."

Griffith is first author on the article, "Evidence for a Polar Ethane
Cloud
on Titan," published in the current (Sept.15) issue of Science. Paulo
Pinteado and VIMS team leader Robert Brown of the UA and researchers
from
France, the Jet Propulsion Laboratory in Pasadena, Calif., the U.S.
Geological Survey, Cornell University, NASA Ames Research Center,
Portugal
and Germany are co-authors.

Griffith, Pinteado and Robert Kursinski of UA collaborated earlier in
studies of the thousand-mile-long methane clouds that band Titan at
southern
latitudes. They concluded from analyzing VIMS images that these highly
localized, convective clouds, which are composed of methane, result
from
summer heating much as thunderstorms form on Earth.

The VIMS instrument is an imaging spectrometer that produces a special
data
set called an image cube. It takes an image of an object in many colors
simultaneously. An ordinary video camera takes images in three primary
colors (red, green, and blue) and combines them to produce images as
seen by
the human eye. The VIMS instrument takes images in 352 separate
wavelengths,
or colors, spanning a realm of colors far beyond those visible to
humans.
All materials reflect light in a unique way. So molecules of any
element or
compound can be identified by the wavelengths they reflect or absorb,
their
"signature" spectra.

The Cassini-Huygens mission is a cooperative project of NASA, the
European
Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a
division of the California Institute of Technology in Pasadena, Calif.,
manages the mission for NASA's Science Mission Directorate, Washington,
D.C.
The Cassini orbiter and its two onboard cameras were designed,
developed and
assembled at JPL. The Visual and Infrared Mapping Spectrometer team is
based
at The University of Arizona in Tucson.

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