NASA’s Cassini spacecraft has detected a monstrous new cloud of frozen compounds in the low- to mid-stratosphere of Saturn’s moon Titan.
The atmospheric region lies above the troposphere, the active weather layer.
Cassini’s camera had already imaged an impressive cloud (above) but 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 kilometres.
The transition from autumn to winter at Titan’s south pole has been going on for several years. Each Titan season lasts about 7.5 years on Earth’s calendar.
The south pole of the moon will still be enveloped in winter when the Cassini mission ends in 2017.
“When we looked at the infrared data, this ice cloud stood out like nothing we’ve ever seen before,” said Carrie Anderson of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It practically smacked us in the face.”
Anderson presented the findings at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society at National Harbor, Maryland.
The ice clouds at Titan’s pole don’t form in the same way as Earth’s familiar rain clouds.
For rain clouds, water evaporates from the surface and encounters cooler temperatures as it rises through the troposphere. Clouds form when the water vapor reaches an altitude where the combination of temperature and air pressure is right for condensation. The methane clouds in Titan’s troposphere form in a similar way.
However, Titan’s polar clouds form higher in the atmosphere by a different process. Circulation in the atmosphere transports gases from the pole in the warm hemisphere to the pole in the cold hemisphere. At the cold pole, the warm air sinks, almost like water draining out of a bathtub, in a process known as subsidence.
The sinking gases – a mixture of smog-like hydrocarbons and nitrogen-bearing chemicals called nitriles – encounter colder and colder temperatures on the way down. Different gases will condense at different temperatures, resulting in a layering of clouds over a range of altitudes.
Bill Condie is a science journalist based in Adelaide, Australia.
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