On Jan. 14, 2005, the Huygens probe will plow into the orange
atmosphere of Saturn’s moon, Titan, becoming the first spacecraft to
attempt to land on a moon in our solar system since the Soviet Union’s
Luna 24 touched down on Earth’s moon in 1976.
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."
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.
De Pater’s research is supported by the National Science Foundation. The
Nov. 17, 2003, workshop on Titan was sponsored by the Center for
Integrative Planetary Studies at UC Berkeley.
