The 30-Year Forecast: Predicting Titan's Clouds
These false-colour images of Titan were obtained by the Cassini-Huygens Visual Infrared Mapping Spectrometer during the 26 October/13 December Titan fly-bys In contrast to the reddish surface, bright clouds residing above most of the atmospheric absorption appear whitish in these representations.
Credit: NASA/JPL/University of Arizona

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 "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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