After water, now scientists have discovered fog at the south pole of Saturn’s largest moon - Titan.

According to scientists at California Institute of Technology, Titan’s south pole is spotted “more or less everywhere” with puddles of methane that give rise to sporadic layers of fog.

Titan, seems to be the only place in the solar system - aside from our home planet earth - with copious quantities of liquid (largely, liquid methane and ethane) sitting on its surface, lead author Mike Brown said.

“It seems that earth and Saturn’s moon share yet another feature, which is inextricably linked with that surface liquid: common fog,” he said.

Speaking at the American Geophysical Union’s 2009 Fall Meeting in San Francisco, Brown and his team said, “The presence of fog provides the first direct evidence for the exchange of material between the surface and the atmosphere, and thus of an active hydrological cycle, which previously had only been known to exist on earth“.

Mr. Brown and his colleagues also describe their findings, which was recently published in The Astrophysical Journal Letters.

The researchers made their discovery using data from the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft, which has been observing Saturn’s system for the past five years.

The VIMS instrument provides “hyperspectral” imaging, covering a large swath of the visible and infrared spectrum.

The team searched public online archives to find all Cassini data collected over the moon’s south pole from October 2006 through March 2007 and filtered the data to separate out features occurring at different depths in the atmosphere, ranging from 20 kilometers (12.4 miles) to .25 kilometers (820 feet) above the surface.

They concluded that only possible way to make Titanian fog, then, is to add humidity to the air. And the only way to do that, Mr. Brown says, is by evaporating liquid in this case, methane, the most common hydrocarbon on the moon, which exists in solid, liquid, and gaseous forms.

Mr. Brown notes that evaporating methane on Titan “means it must have rained, and rain means streams and pools and erosion and geology. The presence of fog on Titan proves, for the first time, that the moon has a currently active methane hydrological cycle.”

The presence of fog also proves that the moon must be dotted with methane pools, Mr. Brown said, adding that’s because any ground-level air, after becoming 100 per cent humid and turning into fog, would instantly rise up into the atmosphere like a giant cumulus cloud.

“The only way to make the fog stick around on the ground is to both add humidity and cool the air just a little. The way to cool the air just a little is to have it in contact with something cold, like a pool of evaporating liquid methane.”

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