When the
Cassini-Huygens probe landed
on the surface of Saturn's largest moon, Titan,
last year, it found no evidence of the ethane
ocean long thought to cover the satellite's surface.
Instead,
scientists observed dune structures that could be dust-like combinations of
ethane and smog particles, according to a new study in the current issue of Nature.
Titan's
dense atmosphere is composed mostly of nitrogen with a small amount of methane.
This methane is broken up by the Sun's ultraviolet light to produce a dense orange-brown
smog that hides the satellite's surface.
Scientists
thought that ethane, one of the products of these reactions with the Sun, was
abundant enough to have condensed and rained down to form a kilometer-deep
ocean across the entire surface of the satellite.
But
observations of the surface suggest that it is instead covered by dunes, which
Donald Hunten of the University of Arizona thinks could be made of a
combination of ethane and smog particles.
Titan's
ethane can't condense into liquid rain because "the smog particles grab the
ethane before it has a chance to form drops," Hunten said.
The
resulting particles deposit on the moon's surface and pile up to form dunes
that might be as deep as several kilometers, Hunten says. The particles would
be more like dust than sand though, so Hunten has dubbed them "smust" (a
combination of "smog" and "dust").
Hunten
based his proposal for this mechanism on the observed behavior of ethane in Jupiter's atmosphere, where at certain
levels, it condenses onto smog particles.
"Basically,
I used Jupiter as the laboratory to show that the ethane is sticking to the
particles," Hunten told Space.com.
"I think
there's a very deep deposit of them on the ground [of Titan], but we can't
confirm that with observations," he added.
To confirm
Hunten's theory, laboratory experiments would have to be conducted to show that
ethane does indeed condense onto the smog particles.
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