Cassini's Visual and Infrared Mapping Spectrometer (VIMS) has detected what appears to be a massive ethane cloud surrounding Titan's north pole. The cloud might be snowing ethane snowflakes into methane lakes below.
The cloud may be the clue needed in solving a puzzle that has confounded scientists who so far have seen little evidence of a veil of ethane clouds and surface liquids originally thought extensive enough to cover the entire surface of Titan with a 300-meter-deep ocean.
Before the Cassini-Huygens mission began visiting Titan in 2004, "We expected to see lots of ethane -- vast ethane clouds at all latitudes and extensive seas on the surface of Saturn's giant moon Titan," University of Arizona planetary scientist Caitlin Griffith said.
That's because solar ultraviolet light irreversibly breaks down methane in Titan's mostly nitrogen atmosphere. Ethane is by far the most plentiful byproduct when methane breaks down. If methane has been a constituent of the atmosphere throughout Titan's 4.5-billion-year lifetime -- and there was no reason to suspect it had not -- the large moon would be awash with seas of ethane, scientists theorized.
NASA's Cassini spacecraft radar found lakes in Titan's north arctic latitudes on a flyby last July 22. However, "We now know that Titan's surface is largely devoid of lakes and oceans," Griffith said. She is a member of the UA-based Cassini VIMS team, headed by Professor Robert Brown of UA's Lunar and Planetary Lab.
The missing ethane is all the more mysterious because Cassini images suggest that other less abundant solid precipitates from the photochemical reactions in Titan's atmosphere have formed dunes and covered craters on its surface, Griffith said.
VIMS made the first detection of Titan's vast polar ethane cloud when it probed Titan's high northern latitudes on Cassini flybys in December 2004, August 2005, and September 2005.