Scientists using the international Cassini
spacecraft have studied the rapid change in seasons on Saturn’s moon Titan,
following equinox in August 2009, which saw the formation of a swirling vortex
and a build up of exotic gases at unexpectedly high altitudes.
Titan is the only other body in the Solar
System with a thick nitrogen-rich atmosphere like Earth’s. Titan’s atmosphere
also contains methane and hydrogen, with trace amounts of other gases including
hydrocarbons that form at high altitudes as a result of reactions with
sunlight. These complex molecules filter down into the lower atmosphere and
eventually combine to produce orange smog.
A separate layer of haze is found at a much
higher altitude of 400–500 km and can be seen at the limb of the moon,
apparently detached from the rest of the atmosphere. This haze was thought to
represent the ceiling of Titan’s ‘middle atmosphere’ circulation which extends
from pole to pole in one giant cell, but new results from Cassini suggest
otherwise.
Titan’s changing seasons
When Cassini arrived in the Saturn system in
2004, Titan sported a vortex with a ‘hood’ of enriched gas and dense haze high
above its north, winter pole. After equinox in August 2009, spring arrived in
the moon’s northern hemisphere while the southern hemisphere headed towards
autumn.
The change in solar heating was reflected by
a rapid reversal in circulation direction in Titan’s single pole-to-pole
atmospheric cell, with an upwelling of gases in the summer hemisphere and down
welling in the winter hemisphere.
“Even though the amount of sunlight reaching
the south pole was decreasing, the first thing we saw there during the six months
after equinox was actually an increase in temperature at altitudes of 400–500
km, as atmospheric gases that had been lofted to these heights were compressed
as they subsequently sank into a newly forming southern vortex,” says Dr Nick
Teanby from the University of Bristol, UK, and lead author of the study
reported in the journal Nature.
“This heating effect is the same one that
causes compressed air in a bicycle pump to heat up, and provided the smoking
gun that the change in seasons was underway.” In the months that followed, up
to a hundred-fold increase in atmospheric gas concentration was measured over
the South Pole at the same high altitudes. Cassini’s instruments found that
these gas molecules were sinking through the atmosphere at a rate of 1–2
millimetres per second.
Dr Teanby’s team conclude that for the
enrichment and motion to be seen throughout these altitudes, the actual source
of the complex gas molecules must be higher still, and that the detached haze
layer cannot signal the top of the atmospheric circulation cell. The new
observations instead suggest that these complex haze molecules are produced
higher up, but that when they drop down to the 400–500 km level, a change in
the character of the haze takes place, perhaps as individual particles clump
together.
“It’s impressive to see such dramatic
solar-driven seasonal changes on a world where the sunlight is nearly a hundred
times weaker than it is on Earth,” adds Dr Teanby. “Since a year on Titan is
nearly 30 Earth years long, for the atmosphere to change over a period of just
six months is extremely rapid.” “Models have predicted this change in Titan’s
atmospheric circulation for nearly 20 years, but Cassini has provided the first
direct observations of it actually happening,” says Nicolas Altobelli, ESA’s
Cassini project scientist.
For further information visit: http://www.esa.int/esaCP/SEMLA6EQZ9H_index_2.html
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