Cassini Spies Methane Lakes on Titan

Cassini Spies Methane Lakes on Titan
http://sciencenow.sciencemag.org/cgi/content/full/2006/725/1

By Richard A. Kerr
ScienceNOW Daily News
25 July 2006

Team members poring over data returned last Friday by the Cassini
spacecraft believe they now have very strong evidence for methane
lakes on Saturn's haze-shrouded moon Titan.

When Cassini was launched, planetary scientists expected it would
find Titan covered by seas--if not oceans--of methane, ethane, and
nitrogen liquefied by the moon's relatively balmy 90° K (-183 °C)
temperatures. But those hydrocarbon seas were nowhere to be seen on
Cassini's arrival, and Cassini's Huygens lander later touched down on
methane-damp water ice rather than splashing into a frigid pool. Yet
methane clouds, high methane humidity, and obvious cutting of river
valleys in the icy surface spoke of methane cycling on Titan much the
way water cycles from lakes and oceans to clouds and rain on Earth.

On its latest pass by Titan, Cassini's surface-scanning radar
detected places on the surface that fail to reflect any detectable
signal back to Cassini, says team member Jonathan Lunine of the
University of Arizona, Tucson. That is just the way radar striking a
liquid surface would behave. And these "radar-black" areas ranging
from 1 or 2 kilometers to 30 kilometers in size meet many other
expectations of lakes, says Lunine. The putative shorelines are sharp
boundaries between radar-black and radar-gray, and some possible
tributaries are radar-black as well. In addition, thermal emissions
from radar-black areas suggest they are warmer than their
surroundings, as liquids would be. And the lake-like features reside
poleward of 70°N, where modeling of Titan weather predicts that rain
should be available to fill them; the same sort of topography
equatorward of 70°N, where it shouldn't be raining, has no lake-like
features.

See: http://sciencenow.sciencemag.org/cgi/content/full/2006/725/1

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