New observations of Saturn’s moon Titan by NASA’s Cassini spacecraft have detected a monstrous new cloud of frozen compounds at the moon’s south pole, in the low- to mid-stratosphere – a stable atmospheric region above the troposphere, or active weather layer. The cloud has a low density, similar to Earth’s fog but likely flat on top. Researchers presented the findings at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society on November 11, 2015.

Each Titan season lasts about 7-1/2 years on Earth’s calendar. For the past few years, Cassini has been catching glimpses of the transition from fall to winter at Titan’s south pole – the first time any spacecraft has seen the onset of a Titan winter. As winter has been setting in, a cloud system called the south polar vortex has been forming.

Cassini’s camera had already imaged an impressive cloud hovering over Titan’s south pole at an altitude of about 186 miles (300 kilometers). 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 124 miles (200 kilometers).

Carrie Anderson of NASA’s Goddard Space Flight Center said:

When we looked at the infrared data, this ice cloud stood out like nothing we’ve ever seen before. It practically smacked us in the face.

The ice clouds at Titan’s pole don’t form in the same way as Earth’s familiar rain clouds, say scientists.

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.

Cassini arrived at Saturn in 2004 – mid-winter at Titan’s north pole. As the north pole has been transitioning into springtime, the ice clouds there have been disappearing. Meanwhile, new clouds have been forming at the south pole. The build-up of these southern clouds indicates that the direction of Titan’s global circulation is changing.

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 -238 degrees Fahrenheit (-150 degrees Celsius).

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Bottom line: New observations of Saturn’s moon Titan by NASA’s Cassini spacecraft have detected a monstrous new cloud of frozen compounds at the moon’s south pole. Researchers presented the findings at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society on November 11, 2015.

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