NASA/JPL/Space Science Institute

A mosaic of nine processed images recently acquired during Cassini’s first very close flyby of Saturn’s moon, Titan, on October 26. The view is centered on 15 degrees South latitude, and 156 degrees West longitude. Brightness variations across the surface and bright clouds near the south pole are visible.

Stormy weather on Saturn's moon

Titan, it turns out, may be a very stormy place. In 2001, a group of astronomers led by Henry Roe, now a postdoctoral scholar at Caltech, discovered methane clouds near the south pole of Saturn’s largest moon, resolving a debate about whether such clouds exist amid the haze of Titan’s atmosphere.

Now Roe and his colleagues have found similar atmospheric disturbances at Titan’s temperate midlatitudes, about halfway between the equator and the poles. In a bit of ironic timing, the team made its discovery using two ground-based observatories, the Gemini North and Keck 2 telescopes on Mauna Kea, in Hawaii, in the months before the Cassini spacecraft arrived at Saturn and Titan. The work appeared in the January 1 issue of the Astrophysical Journal.

“We were fortunate to catch these new midlatitude clouds when they first appeared in late 2003 and early 2004,” says Roe, who is an O. K. Earl Postdoctoral Scholar at Caltech. Much of the credit goes to the resolution and sensitivity of the two ground-based telescopes and their use of adaptive optics, in which a flexible mirror rapidly compensates for the distortions caused by turbulence in Earth’s atmosphere. These distortions are what cause the well-known twinkling of stars. Using adaptive optics, details as small as 300 kilometers across can be distinguished despite the distance to Titan (1.3 billion kilometers).

Still to be determined, though, is the cause of the clouds. According to Chad Trujillo, a former Caltech postdoctoral scholar and now a scientist at the Gemini Observatory, Titan’s weather patterns can be stable for many months, with only occasional bursts of unusual activity like those recently discovered atmospheric features.

Like Earth, Titan has an atmosphere that is mostly nitrogen. Unlike Earth, Titan is inhospitable to life due to the lack of atmospheric oxygen and to its extremely cold surface temperatures, as cold as -297 degrees Fahrenheit. Along with nitrogen, Titan’s atmosphere also contains a significant amount of methane, which may be the cause of the midlatitude clouds.

Conditions on Earth allow water to exist in liquid, solid, or vapor states, depending on localized temperatures and pressures. The phase changes of water between these states are an important factor in the formation of weather in our atmosphere. But on Titan, methane rules. The moon’s atmosphere is so cold that any water is frozen solid, but methane can move between liquid, solid, and gaseous states. This leads to a methane meteorological cycle on Titan that is similar to the water-based weather cycle on Earth.

While the previously discovered south polar clouds are thought to be a result of solar surface heating, the new midlatitude clouds cannot be formed by the same mechanism. One possible explanation for the new clouds is a seasonal shift in the global winds. More likely, says Roe, surface activity might be disturbing the atmosphere at the midlatitude location. Geysers of methane slush may be brewing up from below, or a warm spot on Titan’s surface may be heating the atmosphere. Cryovolcanism is another mechanism that could cause disturbances. Hints about what is happening on this frigid world could be obtained as the Huygens probe, which was released from Cassini in late December, drops through Titan’s atmosphere this month.

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