Teams of astronomers at the California Institute of
Technology (Caltech) and at the University of California, Berkeley,
have discovered methane clouds near the south pole of Titan,
resolving a fierce debate about whether clouds exist amid the haze of
the moon’s atmosphere.

The new observations were made using the W. M. Keck II 10-meter and
the Gemini North 8-meter telescopes atop Hawaii’s Mauna Kea volcano
in December 2001. Both telescopes are outfitted with adaptive optics
that provide unprecedented detail of features not seen even by the
Voyager spacecraft during its flyby of Saturn and Titan.

The results are being published by the Caltech team in the December
19 issue of Nature and by the UC Berkeley and NASA Ames team in the
December 20 issue of the Astrophysical Journal.

Titan is Saturn’s largest moon, larger than the planet Mercury, and
is the only moon in our solar system with a thick atmosphere. Like
Earth’s atmosphere, the atmosphere on Titan is mostly nitrogen.
Unlike Earth, Titan is inhospitable to life due to the lack of
atmospheric oxygen and its extremely cold surface temperatures (-183
degrees Celsius, or -297 degrees Fahrenheit). Along with nitrogen,
Titan’s atmosphere contains a significant amount of methane.

Earlier spectroscopic observations hinted at the existence of clouds
on Titan, but gave no clue as to their location. These early data
were hotly debated, since Voyager spacecraft measurements of Titan
appeared to show a calm and cloud-free atmosphere. Furthermore,
previous images of Titan had failed to reveal clouds, finding only
unchanging surface markings and very gradual seasonal changes in the
haziness of the atmosphere.

Improvements in the resolution and sensitivity achievable with
ground-based telescopes led to the present discovery. The
observations used 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 the stars. Using adaptive optics, details as small as 300
kilometers across can be distinguished at the enormous distance of
Titan (1.3 billion kilometers), equivalent of reading an automobile
license plate from 100 kilometers away.

The images presented by the two teams clearly show bright clouds near
Titan’s south pole.

“We see the intensity of the clouds varying over as little as a few
hours,” said post-doctoral fellow Henry Roe, lead author for the UC
Berkeley group. “The clouds are constantly changing, although some
persist for as long as a few days.”

Titan experiences seasons much like Earth, though its year is 30
times longer due to Saturn’s distant orbit from the sun. Titan is
currently in the midst of southern summer, and the south pole has
been in continuous sunlight for over six Earth years. The researchers
believe that this fact may explain the location of the large clouds.

“These clouds appear to be similar to summer thunderstorms on Earth,
but formed of methane rather than water. This is the first time we
have found such a close analogy to the Earth’s atmospheric water
cycle in the solar system,” says Antonin Bouchez, one of the Caltech
researchers.

In addition to the clouds above Titan’s south pole, the Keck images,
like previous data, reveal the bright continent-sized feature that
may be a large icy highland on Titan’s surface, surrounded by linked
dark regions that are possibly ethane seas or tar-covered lowlands.

“These are the most spectacular images of Titan’s surface which we’ve
seen to date,” says Michael Brown, associate professor of planetary
astronomy and lead author of the Caltech paper. “They are so detailed
that we can almost begin to speculate about Titan’s geology, if only
we knew for certain what the bright and dark regions represented.”

In 2004, Titan will be visited by NASA’s Cassini spacecraft, which
will look for clouds on Titan during its multiyear mission around
Saturn. “Changes in the spatial distribution of these clouds over
the next Titan season will help pin down their detailed formation
process,” says Imke de Pater, professor of astronomy at UC Berkeley.
The Cassini mission includes a probe named Huygens that will descend
by parachute into Titan’s atmosphere and land on the surface near the
edge of the bright continent.

The team conducting the Gemini observations consists of Roe and de
Pater from UC Berkeley, Bruce A. Macintosh of Lawrence Livermore
National Laboratory, and Christopher P. McKay of the NASA Ames
Research Center. The team reporting results from the Keck telescope
consists of Brown and Bouchez of Caltech and Caitlin A. Griffith of
the University of Arizona.

The Gemini observatory is operated by the Association of Universities
for Research in Astronomy under a cooperative agreement with the
National Science Foundation, involving NOAO/AURA/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.
This research has been funded in part by grants from NSF and NASA.

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Press Release Figures

Fig. 1:
An image of Titan taken with the W.M. Keck II telescope on
December 3, 2001.
Complex bright and dark features around Titan’s “continent” can
be seen at their highest resolution ever.
Several small bright clouds can also be made out near Titan’s
south pole.
Credit: M.E. Brown, A.H. Bouchez, C.A. Griffith

Fig. 2:
Images of Titan taken with the W.M. Keck II telescope during three
nights in December 2001.
The upper row shows images of just Titan’s troposphere (lower atmosphere),
which contains, in addition to the newly discovered south polar methane
clouds, a “haze” covering the south polar cap.
The limb-brightening is also caused by a tenuous global atmospheric
haze layer.
Images in the lower row show Titan’s surface rotating as well as
the same methane cloud features near the south pole.
Credit: H.G. Roe, I. de Pater, B.A. Macintosh, C.P. McKay

Fig. 3:
Images of changing methane clouds near Titan’s south pole, taken with
the W.M. Keck II telescope in December 2001 and February 2002.
Nearly the same face of the moon is viewed in all three images.
The large storm in the 28 Feb. image is over 1400 km long.
Credit: M.E. Brown, A.H. Bouchez, C.A. Griffith