NASA's Cassini spots monstrous ice cloud on Titan's south polar region

NASA's Cassini spacecraft has detected a monstrous new cloud of frozen compounds on the Saturn moon Titan's south polar region.

The ice cloud in the moon's low- to mid-stratosphere -- a stable atmospheric region above the troposphere, or active weather layer -- suggests that winter comes in like a lion on the largest moon of Saturn, NASA said.

Cassini's camera had already imaged an impressive cloud hovering over Titan's South Pole at an altitude of about 300 km.

However, that cloud, first seen in 2012, turned out to be just the tip of the iceberg. A much more massive ice cloud system has now been found lower in the stratosphere, peaking at an altitude of about 200 km.

The new cloud was detected by Cassini's infrared instrument - the Composite Infrared Spectrometre, or CIRS - which obtains profiles of the atmosphere at invisible thermal wavelengths.

The cloud has a low density, similar to the Earth's fog but likely flat on top, NASA said.

"When we looked at the infrared data, this ice cloud stood out like nothing we have ever seen before," said Carrie Anderson from NASA's Goddard Space Flight Centre in Greenbelt, Maryland.

"It practically smacked us in the face," Anderson said.

The scientists pointed out that the ice clouds at Titan's pole do not form in the same way as the Earth's familiar rain clouds.

The ice particles are made up of a variety of compounds containing hydrogen, carbon and nitrogen.

The size, altitude and composition of the polar ice clouds help scientists understand the nature and severity of Titan's winter.

From the ice cloud seen earlier by Cassini's camera, scientists determined that temperatures at the South Pole must get down to at least -150 degrees Celsius.

The new cloud was found in the lower stratosphere where temperatures are even colder, the scientists said.

For the past few years, Cassini has been catching glimpses of the transition from fall to winter at Titan's South Pole.

As each Titan season lasts about 7-1/2 years on the Earth's calendar, the South Pole will still be enveloped in winter when the Cassini mission ends in 2017, NASA said.

The findings were presented at American Astronomical Society's annual meeting at National Harbor, Maryland, on Wednesday.