Abstract
Titan is the only satellite in our Solar System with a dense atmosphere.
The surface pressure is 1.5 bar ( ref. 1) and, similar to the Earth,
N-2 is the main component of the atmosphere. Methane is the second
most important component(2), but it is photodissociated on a timescale
of 10(7) years ( ref. 3). This short timescale has led to the suggestion
that Titan may possess a surface or subsurface reservoir of hydrocarbons(4,5)
to replenish the atmosphere. Here we report near-infrared images
of Titan obtained on 26 October 2004 by the Cassini spacecraft. The
images show that a widespread methane ocean does not exist; subtle
albedo variations instead suggest topographical variations, as would
be expected for a more solid ( perhaps icy) surface. We also find
a circular structure similar to 30 km in diameter that does not resemble
any features seen on other icy satellites. We propose that the structure
is a dome formed by upwelling icy plumes that release methane into
Titan's atmosphere.
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