Huygens is closing in on Titan

Berkeley -- Though scientists hope that Huygens will survive the plunge, it
will be flying blind through hydrocarbon haze and methane clouds to a
surface that could consist of seven-kilometer-high ice mountains and
liquid methane seas.

That's the picture that emerges from a series of articles -
half of them by University of California, Berkeley, researchers -
published in the journal Geophysical Research Letters last month and
detailing what scientists know to date about the surface, atmosphere
and magnetic field of Titan. This view sets the stage for an analysis
of new data soon to arrive from the Cassini spacecraft and Huygens
probe.

"These (journal) papers really give a state-of-the-art picture
of Titan, before Cassini goes into orbit around Saturn and the Huygens
probe goes into Titan's atmosphere," said Imke de Pater, a professor of
astronomy at UC Berkeley who wrote the introductory paper in the series
and co-authored four of the nine papers. The papers came out of a
meeting De Pater hosted last November at UC Berkeley to discuss what
has been gleaned to date about the moon from optical, infrared and
radar telescopes, including the Hubble Space Telescope and the twin
Keck Telescopes in Hawaii.

Scientists expect the current sketchy picture of Titan's
surface, totally obscured by clouds and haze, will much improve when
the Cassini spacecraft, which is carrying the Huygens probe, starts an
intense observation of Titan later this month. While on-board infrared
imaging cameras can pierce the cloud cover, however, they can only
reveal bright and dark spots on the surface, which are difficult to
interpret. What Huygens will encounter at Titan's surface will remain a
mystery until the probe plops into an ocean or parachutes to solid
ground.

"Based upon their spectral characteristics, the bright areas
imaged by various Earth-bound telescopes and the Hubble Space Telescope
could be a mixture of rock and water ice," de Pater said. Such a
mixture appears relatively bright in comparison with substances like
tar and liquid hydrocarbons, which absorb essentially all sunlight at
these wavelengths and hence appear very dark.

"The dark areas could contain liquid hydrocarbons," she said. "But they're all still a mystery."

"The dark areas could contain liquid hydrocarbons"
Some scientists have suggested that one large bright area, Xanadu, is a
mountain of rock and water ice that stands out because runoff
(hydrocarbon rain) has washed off the dark hydrocarbon particles. UC
Berkeley graduate student J. Taylor Perron and de Pater concluded in
one of the papers that such an ice continent, primarily composed of
water ice, could be no higher than 3 to 7 kilometers - that is, at
most, 23,000 feet, about the height of Mt. Aconcagua in Argentina. That
is even more impressive on a globe less than half the diameter of
Earth.

The Huygens probe, which will take from two to two and a half
hours to float to the surface, is aiming for a landing site in a dark
area bordering a bright area near the equator, so it could land instead
in a gasoline-like hydrocarbon brew of methane, propane or butane.
Though the probe is designed to float, its builders expect, at most, 45
minutes of data once it sets down. A few minutes would be cause for
celebration.

The Cassini/Huygens spacecraft was launched from Kennedy Space
Center in 1997, the product of an international collaboration between
three space agencies - the National Aeronautics and Space
Administration, the European Space Agency and the Italian Space agency
- involving contributions from 17 nations. It arrived at Saturn in July
2004, beginning a four-year mission to photograph and collect data on
Saturn, its rings and moons. This Oct. 26, it will get within 1,000
kilometers of Titan - closer than ever before - turning its remote
sensing instruments on that moon's surface and atmosphere. Cassini will
release the Huygens probe on Christmas Day, Dec. 25.

The second largest moon in the solar system and the only one
with a thick, methane-rich, nitrogen atmosphere, Titan intrigues
scientists because of its resemblance to a young Earth. The atmospheres
of both Titan and the early Earth were dominated by nearly the same
amount of nitrogen, and the chemistry discovered on Titan could provide
clues to the origins of life on our planet.

De Pater and chemistry graduate student Mate Adamkovics have
used the adaptive optics on the Keck Telescope in Hawaii to image the
hydrocarbon haze that envelops the moon, taking snapshots at various
altitudes from 150 to 200 kilometers down to the surface. In the movie
they constructed from these snapshots, haze is very evident in the
atmosphere at about 30-50 kilometers over the South Pole. Stratospheric
haze at about 150 kilometers is visible over a large area in the
northern hemisphere but not the southern hemisphere, an asymmetry
observed previously. And at the southern hemisphere's tropopause - the
border between the lower atmosphere and the stratosphere at about 42
kilometers altitude - cirrus haze is visible, analogous to cirrus haze
on Earth.

These observations agree with a theory of haze formation whereby
sunlight creates haze particles at a high altitude - 400 to 600
kilometers above the surface - that are blown to the winter pole, where
the haze accumulates as a polar "hood." The haze particles start to
settle out and are carried by a lower-elevation return flow to the
summer hemisphere.

Laboratory experiments by Melissa Trainer of the University of
Colorado, Boulder, reported in the journal suggest that the haze
particles could be polycyclic aromatic hydrocarbons if the methane
concentration in the atmosphere is high - around 10 percent - though
they would be primarily long-chain hydrocarbons at low concentrations.
The Huygens probe will measure gas concentrations as it plummets
through the atmosphere, hopefully testing this connection between
methane concentration and aerosol composition.

Cassini's observations of Titan over the next four years should
yield much more information about the atmospheric haze and surface
topography, as well as raise new questions. De Pater urges ground-based
astronomers to continue to observe Titan's moon, "so the
Cassini/Huygens data can be tied in with the long-term data base on
Titan's seasons," she wrote.

De Pater herself will be peering at Titan through the Keck
Telescope on Jan. 15 when the Huygens probe disappears into the
atmosphere.

"I'm skeptical that we'll see a meteor trail, as some have
predicted, but our observations will give us a good image of Titan at
the time of probe entry, which could be very relevant to calibrating
Titan at entry time," de Pater said.

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