* Astronomy

NASA's Cassini spacecraft will be eyeing the north polar region of Saturn's moon Titan this weekend, scanning the moon's land o' lakes.
At closest approach on early morning Saturday, June 5 UTC, which is Friday afternoon, June 4 Pacific time, Cassini will glide to within about 2,000 kilometres of the Titan surface.
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Credit: NASA/JPL/Space Science Institute

This image of Titan behind Saturn's rings was taken by the Cassini spaceprobe on the 18th May, 2010.
The image was taken using the CL1 and CL2 filters.

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 analysing the way the terrain has scattered radar beams, scientists deduce the spheres measure at least a few centimetres and maybe up to a couple of metres in diameter. The spheres likely originated as part of water-ice bedrock in higher terrain in Xanadu.
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.
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Credit: NASA/JPL/Space Science Institute

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.

Title: Elastic ice shells of synchronous moons: Implications for cracks on Europa and non-synchronous rotation of Titan
Authors: Peter M. Goldreich, Jonathan L. Mitchell
(Version v2)

A number of synchronous moons are thought to harbour water oceans beneath their outer ice shells. A subsurface ocean frictionally decouples the shell from the interior. This has led to proposals that a weak tidal or atmospheric torque might cause the shell to rotate differentially with respect to the synchronously rotating interior. As a result of centrifugal and tidal forces, the ocean would assume an ellipsoidal shape with its long axis aligned toward the parent planet. Any displacement of the shell away from its equilibrium position would induce strains thereby increasing its elastic energy and giving rise to an elastic restoring torque. We compare the elastic torque with the tidal torque acting on Europa and the atmospheric torque acting on Titan. For Europa, the tidal torque is far too weak to produce stresses that could fracture the ice shell, thus refuting a widely advocated idea. Instead, we suggest that cracks arise from time-dependent stresses due to non-hydrostatic gravity anomalies from tidally driven, episodic convection in the interior. Two years of Cassini RADAR observations of Titan's surface are interpreted as implying an angular displacement of ~0.24 degrees relative to synchroneity. Compatibility of the amplitude and phase of the observed non-synchronous rotation with estimates of the atmospheric torque requires that Titan's shell be decoupled from its interior. We find that the elastic torque balances the atmospheric torque at an angular displacement <0.05 degrees, thus coupling the shell to the interior. Moreover, if Titan's surface were spinning faster than synchronous, the tidal torque tending to restore synchronous rotation would certainly be larger than the atmospheric torque. There must either be a problem with the interpretation of the radar observations, or with our understanding of Titan's atmosphere and/or interior.

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