JPL: Rock and Roll: Titan's Gem Tumbler

[bystander]

Rock and Roll: Titan's Gem Tumbler
NASA JPL 2010-156 - 11 May 2010


The left-hand image, obtained by the European Space Agency's Huygens probe, shows rounded rocks from the surface of Saturn's
moon Titan. Huygens rode with NASA's Cassini spacecraft to the Saturn system. The right-hand image, taken by amateur
photographer Sandra M. Matheson, shows river rocks on Earth. (NASA/JPL/ESA/University of Arizona and S.M. Matheson)

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.

Radar glories in Titan rivers
Planetary Society - 11 May 2010

Wow, this is a cool paper.

Here's the gist: the Cassini RADAR team has spotted some river channels on Titan that shine so brightly in radar images, there must be something special going on to explain that brightness. What we're seeing is the same phenomenon that makes reflective paint on street signs shine brightly in headlights. But instead of tiny plastic beads embedded in reflective paint, we're seeing the internal reflections from pebbles and cobbles of ice, as big as tennis balls or even footballs, lining Titan's river beds.

That's the hypothesis explored in "Radar-bright channels on Titan," a paper just published in Icarus by Alice Le Gall and four coauthors (plus the Cassini RADAR team). I read the paper over lunch, and it was fun: every time I had a question, my question was answered in the very next section of the paper! Le Gall, who's "only" a postdoc, really made a case that convinced me.

The paper is concerned specifically with two river beds observed in the T44 flyby of Titan, when the Cassini RADAR instrument acquired an image swath across the southwestern edge of the bright region on Titan known as Xanadu. Xanadu is a huge area of Titan, 2,500 kilometers across, that's often imagined as a "continent" on Titan, although in truth it's not exactly analogous to Earth continents.

[neufer]

Glory Radar backscattering in a spherical cobble

This diagram explains how a transparent sphere can efficiently scatter incoming radiation back at the radiation source. It doesn't matter what direction the radiation comes from; provided the sphere is reasonably transparent, a significant amount of the radiation will be sent back to its source. Credit: Le Gall et al, Icarus 207 (2010) 948-958

• The left-hand image, obtained by the European Space Agency's Huygens probe, shows rounded rocks from the surface of Saturn's
  moon Titan. Huygens rode with NASA's Cassini spacecraft to the Saturn system. The right-hand image, taken by amateur
  photographer Sandra M. Matheson, shows river rocks on Earth. (NASA/JPL/ESA/University of Arizona and S.M. Matheson)
  [/url]

These Titan rocks don't look terribly spherical to me but they do seem to tend to cluster
in groups of three which might suggest more of a corner reflector than a glory:

The density of rock triplet quasi-corner reflectors
would increase with the cube of the density of rocks

So do Titan river beds have:

• 1) more quasi-transparent quasi-spherical rocks generating glories?
  
  or simply
  
  2) a higher density of reflective rock triplet quasi-corner reflectors?