Titan's Seas of Sand

TITAN'S SEAS ARE SAND
(From Lori Stiles, University Communications, 520-621-1877)

- Thursday, May 04, 2006

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Contact Information
Ralph D. Lorenz 520-621-4933 (use e-mail this week)


Related Web sites
http://www.lpl.arizona.edu/~rlorenz
http://www.nasa.gov/cassini
http://saturn.jpl.nasa.gov
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Until a couple of years ago, scientists thought the dark equatorial
regions
of Titan might be liquid oceans.

New radar evidence shows they are seas -- but seas of sand dunes like
those
in the Arabian or Namibian Deserts, a University of Arizona member of
the
Cassini radar team and colleagues report in Science (May 5).

Radar images taken when the Cassini spacecraft flew by Titan last
October
show dunes 330 feet (100 meters) high that run parallel to each other
for
hundreds of miles at Titan's equator. One dune field runs more than 930
miles (1500 km) long, said Ralph Lorenz of UA's Lunar and Planetary
Laboratory.

"It's bizarre," Lorenz said. "These images from a moon of Saturn look
just
like radar images of Namibia or Arabia. Titan's atmosphere is thicker
than
Earth's, its gravity is lower, its sand is certainly different --
everything
is different except for the physical process that forms the dunes and
resulting landscape."

Ten years ago, scientists believed that Saturn's moon Titan is too far
from
the sun to have solar-driven surface winds powerful enough to sculpt
sand
dunes. They also theorized that the dark regions at Titan's equator
might be
liquid ethane oceans that would trap sand.

But researchers have since learned that Saturn's powerful gravity
creates
significant tides in Titan's atmosphere. Saturn's tidal effect on Titan
is
roughly 400 times greater than our moon's tidal pull on Earth.

As first seen in circulation models a couple of years ago, Lorenz said,
"Tides apparently dominate the near-surface winds because they're so
strong
throughout the atmosphere, top to bottom. Solar-driven winds are strong
only
high up."

The dunes seen by Cassini radar are a particular linear or longitudinal
type that is characteristic of dunes formed by winds blowing from
different
directions. The tides cause wind to change direction as they drive
winds
toward the equator, Lorenz said.

And when the tidal wind combines with Titan's west-to-east zonal wind,
as
the radar images show, it creates dunes aligned nearly west-east except
near
mountains that influence local wind direction.

"When we saw these dunes in radar it started to make sense," he said.
"If
you look at the dunes, you see tidal winds might be blowing sand around
the
moon several times and working it into dunes at the equator. It's
possible
that tidal winds are carrying dark sediments from higher latitudes to
the
equator, forming Titan's dark belt."

The researchers' model of Titan suggests tides can create surface winds
that reach about one mile per hour (a half-meter per second). "Even
though
this is a very gentle wind, this is enough to blow grains along the
ground
in Titan's thick atmosphere and low gravity," Lorenz said. Titan's sand
is a
little coarser but less dense than typical sand on Earth or Mars.
"These
grains might resemble coffee grounds."

The variable tidal wind combines with Titan's west-to-east zonal wind
to
create surface winds that average about one mile per hour (a half meter
per
second). Average wind speed is a bit deceptive, because sand dunes
wouldn't
form on Earth or Mars at their average wind speeds.

Whether the grains are made of organic solids, water ice, or a mixture
of
both is a mystery. Cassini's Visual and Infrared Mapping Spectrometer,
led
by UA's Robert Brown, may get results on sand dune composition.

How the sand formed is another peculiar story.

Sand may have formed when liquid methane rain eroded particles from ice
bedrock. Researchers previously thought that it doesn't rain enough on
Titan
to erode much bedrock, but they thought in terms of average rainfall.

Observations and models of Titan show that clouds and rain are rare.
That
means that individual storms could be large and still yield a low
average
rainfall, Lorenz explained.

When the UA-led Descent Imager/Spectral Radiometer (DISR) team produced
images taken during the Huygens probe landing on Titan in January 2005,
the
world saw gullies, streambeds and canyons in the landscape. These same
features on Titan have been seen with radar.

These features show that when it does rain on Titan, it rains in very
energetic events, just as it does in the Arizona desert, Lorenz said.

Energetic rain that triggers flash floods may be a mechanism for making
sand, he added.

Alternatively, the sand may come from organic solids produced by
photochemical reactions in Titan's atmosphere.

"It's exciting that the radar, which is mainly to study the surface of
Titan, is telling us so much about how winds on Titan work," Lorenz
said.
"This will be important information for when we return to Titan in the
future, perhaps with a balloon."

An international group of scientists are co-authors on the Science
article,
"The Sand Seas of Titan: Cassini Observations of Longitudinal Dunes."
They
are from the Jet Propulsion Laboratory, California Institute of
Technology,
U.S. Geological Survey - Flagstaff, Planetary Science Institute,
Wheeling
Jesuit College, Proxemy Research of Bowie, Md., Stanford University,
Goddard
Institute for Space Studies, Observatoire de Paris, International
Research
School of Planetary Sciences, Universita' d'Annunzio, Facolt di
Ingegneria,
Universit La Sapienza, Politecnico di Bari and Agenzia Spaziale
Italiana.
Jani Radebaugh and Jonathan Lunine of UA's Lunar and Planetary
Laboratory
are among the co-authors.

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, manages
the
mission for NASA's Science Mission Directorate, Washington. The Cassini
orbiter was designed, developed and assembled at JPL.