Rock and Roll: Titan's Gem Tumbler

Rock and Roll: Titan's Gem Tumbler

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-156&cid=release_2010-156

It appears flash flooding has paved streambeds in the Xanadu region of Saturn's moon
Titan with thousands of sparkling crystal balls of ice, according to scientists with
NASA's Cassini spacecraft. By analyzing the way the terrain has scattered radar beams,
scientists deduce the spheres measure at least a few centimeters (inches) and maybe up to
a couple of meters (yards) in diameter. The spheres likely originated as part of water-ice
bedrock in higher terrain in Xanadu.

"What we believe happened in this area is a lot like what creates polished river rocks on
Earth," said Alice Le Gall, a postdoctoral fellow at NASA's Jet Propulsion Laboratory,
Pasadena, Calif., and the lead author of the study, which used the Cassini radar
instrument. "Bouncing downstream smoothes out the edges of rocks."

As foothill residents know in southern California and other areas, sudden rains can trigger
mudslides and flooding at the mountainous fringes of desert areas. Those flows can pick
up boulders and debris and tumble them downstream. On Titan, the flows appear to have
occurred periodically for eons, on a catastrophic scale. The process on Titan, however,
involves rain made of liquid methane and ethane, rather than Earth's water rain. Titan's
rocks are believed to be made primarily of water ice frozen into a hard mass about minus
180 degrees Celsius (minus 290 degrees Fahrenheit), rather than Earth's mineral rocks.

Earth-like river rocks have already been observed on Titan at the landing site of the
European Space Agency's Huygens probe, near the equator in the borderland between the
Adiri and Shangri-la regions. The landing site also showed signs of flash flooding that
deposited cobblestones about 2 to 20 centimeters (1 to 8 inches) in diameter.

But the spidery channels in this southern lowland part of Xanadu looked brighter to
Cassini's radar instrument than the Huygens landing area. In fact, the channels, which
were scanned by Cassini in May 2008, are among the brightest features ever seen on Titan
by the radar instrument.

In a paper now available online in the journal Icarus, Le Gall and colleagues concluded
that the most plausible explanation for the extreme brightness of the Xanadu channels
was a collection of transparent spherical sediments, packed more tightly together than the
cobblestones at the Huygens landing site. The effect would be similar to bejeweling an
area with light-catching rhinestones.

The spheres appear to be made of water ice – possibly doped with ammonia – that would
look bright to the microwaves used by Cassini's radar. Spheres are good at sending light
back in the direction it came from. This property has actually led manufacturers to use
plastic spheres in reflective paints and tape, Le Gall said.

Xanadu may be an especially good gem grinder because of its broad expanse and gentle
southward slope. Flows could have traveled long distances there and tumbled the chunks
for hundreds of kilometers (miles). The subtle work to shape them into spheres could have
come from fine grit rubbing against the rocks in the flowing methane. Or, ice may be
malleable in Titan's cold temperatures, deforming plastically during the collisions rather
than fracturing. The flows that transported these icy spheres probably traveled around 1
meter per second (2 mph).

"It's been really hard for a long time for people to understand why Xanadu is so bright,"
said Steve Wall, a radar team member at JPL. "You might not expect these kinds of
geometries in a natural setting, but we believe this can explain the enigma."

The radar team plans to continue looking for other instances of small, smooth spheres in
nature to increase their confidence about the explanation. They also said more study is
needed on the mechanical properties of water ice at such cold temperatures.

"Here is yet another example of Titan as a world with Earth-like processes," said Linda
Spilker, Cassini project scientist at JPL. "As the seasons change on Titan, maybe we'll get
a chance to see methane flow through some of the river channels."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space
Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of
the California Institute of Technology in Pasadena, manages the mission for NASA's
Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed,
developed and assembled at JPL. The radar instrument was built by JPL and the Italian
Space Agency, working with team members from the United States and several European
countries.

For more information about the Cassini-Huygens mission visit
http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

#2010-156

-end-

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook@jpl.nasa.gov

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