More Stormy Weather on Titan

PASADENA, Calif.– 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 the California Institute of Technology, discovered methane
clouds near the south pole of Saturn’s largest moon, resolving a
debate about whether such clouds exist amid the haze of its
atmosphere.

Now Roe and his colleagues have found similar atmospheric
disturbances at Titan’s temperate mid-latitudes, 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 will appear in the January 1, 2005, issue of the Astrophysical
Journal.

“We were fortunate to catch these new mid-latitude clouds when they
first appeared in late 2003 and early 2004,” says Roe, who is a
National Science Foundation Astronomy and Astrophysics 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 the Earth’s atmosphere. These
distortions are what cause the well-known twinkling of the stars.
Using adaptive optics, details as small as 300 kilometers across can
be distinguished despite the enormous distance of Titan (1.3 billion
kilometers). That’s equivalent to reading an automobile license plate
from 100 kilometers away.

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 these recently discovered atmospheric features.

Like Earth, Titan’s atmosphere is mostly nitrogen. Unlike Earth,
Titan is inhospitable to life due to the lack of atmospheric oxygen
and to its extremely cold surface temperatures (-297 degrees
Fahrenheit). Along with nitrogen, Titan’s atmosphere also contains a
significant amount of methane, which may be the cause of the
mid-latitude 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 mid-latitude 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
mid-latitude 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–volcanic activity that spews an icy mix of
chemicals–is another mechanism that could cause disturbances. Hints
about what is happening on this frigid world could be obtained as the
Huygens probe, which will be released from Cassini on Christmas day,
drops through Titan’s atmosphere in January 2005.

If the clouds are being caused by these geological conditions, says
Roe, they should stay at the observed 40-degree latitude and
repeatedly occur above the same surface feature or features.
Meanwhile, if a seasonal shift in the winds is forming the clouds
then their locations should move northward as Titan’s season
progresses into southern summer. “Continued observations with the
Gemini and Keck telescopes will easily distinguish between these two
scenarios,” says Roe.

The Gemini observatory is operated by the Association of Universities
for Research in Astronomy under a cooperative agreement with the
National Science Foundation, involving the National Optical Astronomy
Observatory, AURA, and the NSF as the U.S. partner. The W.M. Keck
Observatory is operated by the California Association for Research in
Astronomy, a scientific partnership between the California Institute
of Technology, the University of California, and the National
Aeronautics and Space Administration.