Using a new climate
predicting model, scientists can predict cloud formation on Titan 30 years into
the future.
That may sound like a long
range forecast, but 30 Earth years correspond to just one year on Titan, Saturn's largest moon.
The new computer model,
however, will help scientists predict where and when large seasonal clouds will appear and disappear across
the moon, enabling them to plan productive future observations of the hazy
moon.
While the Cassini spacecraft has provided stunning images and near-infrared
scans of Titan's clouds, until now scientists couldn't say for sure how the
ethane/methane clouds form. The new general circulation model, however,
combines haze circulation and cloud physics to help solve the riddle.
The model matches past and
present observations, and researchers expect it to accurately forecast where
clouds will pop up in the future.
Unlike previously proposed
models, the new model has the advantage of being teamed up with Cassini images to confirm its accuracy. Taken together,
scientists have determined that Titan's clouds form and move much like those on
Earth and Mars.
"Consistently, our
model produces clouds at places where clouds are actually observed," said
study leader Pascal Rannou of the Service d'Aeronomie, IPSL Universite de
Versailles-St-Quentin, France. "But it also predicts clouds that have not,
or not yet, been observed."
The research is detailed in
the Jan. 13 issue of the journal Science.
Earth-like cloud engine
Titan's atmosphere, like
those at Earth and Mars, is a thermodynamic engine--warm air moves towards cool
air and stirs things up in between. Warm air is generally found near the
equator of a planet, where it rises into the upper atmosphere and drifts
towards the poles, the coolest region of the orb.
As the air reaches higher
altitudes and polar latitudes, the drop in temperature causes any moisture to
condense into clouds.
Clouds form at the poles,
however, simply because the air is cool enough for condensation. Currently,
scientists can only confirm the presence of clouds at Titan's southern pole.
However, they expect to find similar clouds up north when that pole comes into
view, probably in 2010.
On Earth and Mars, clouds
condense in the tropics, around 30 degrees North and South. Titanic clouds form
instead around 40 degrees and stretch to 60 degrees. Scientists aren't quite
sure what drives this difference, but believe Saturn's tidal effects might play
a role.
Season in the Sun
The model forecasts 30
Earth years into the future, but since that time equals one year on Titan, Rannou believes the cloud formation patterns follow
seasonal trends. Clouds appear in Titan's southern hemisphere during the summer
season. But during the southern winter, they follow summer to the northern
hemisphere.
"On Titan, the clouds
appear in the summer time. This is opposite from Earth--when we have summer in
the southern hemisphere, clouds are transferred to the north," Rannou told SPACE.com. "We should spend all our
days in winter on Titan, compared to Earth."
The model predicts that in
the next few years, the mid-latitude clouds around 40 degrees south will move
northwards. The clouds would likely return to the south in 2015.
"If cloud formation is
really due to air circulation, this is what it will do," Rannou said.
Model upgrade
The new model produces
accurate cloud forecasts by taking into account the circulation of the moon's
haze. Haze circulation affects where on the moon the columns of warm air will
rise and form clouds.
Also, the new model makes
predictions in two dimensions, Rannou said, an
upgrade over earlier one-dimensional predictions. However, having only two
dimensions raises questions about what goes on in the vertical layers.
"In the near future we
plan to improve it to be three-dimensional, providing a complete circulation
model," Rannou said.
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