November 11, 2015
New observations made near the south pole of Titan by NASA’s Cassini spacecraft add to the evidence that winter comes in like a lion on this moon of Saturn.
Scientists have detected a monstrous new cloud of frozen compounds in the moon’s low- to mid-stratosphere – a stable atmospheric region above the troposphere, or active weather layer.
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).
The new cloud was detected by Cassini’s infrared instrument – the Composite Infrared Spectrometer, or CIRS – which obtains profiles of the atmosphere at invisible thermal wavelengths. The cloud has a low density, similar to Earth’s fog but likely flat on top.
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. Because each Titan season lasts about 7-1/2 years on Earth’s calendar, the south pole 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 is presenting the findings at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society at National Harbor, Maryland, on Nov. 11.
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.
image credit: NASA JPL-CalTech, SSI