UC Berkeley astronomers create movie showing hydrocarbon haze that Huygens probe will encounter in Titan’s atmosphere

MOVIE URL is http://astron.berkeley.edu/~madamkov/titan/.

Berkeley – As the Cassini-Huygens spacecraft approaches a July
encounter with Saturn and its moon Titan, a team of University of
California, Berkeley, astronomers has produced a detailed look at the
moon’s cloud cover and what the Huygens probe will see as it dives
through the atmosphere of Titan to land on the surface.

Astronomer Imke de Pater and her UC Berkeley colleagues used adaptive
optics on the Keck Telescope in Hawaii to image the hydrocarbon haze
that envelops the moon, taking snapshots at various altitudes from
150-200 kilometers down to the surface. They assembled the pictures
into a movie that shows what Huygens will encounter when it descends
to the surface in January 2005, six months after the Cassini
spacecraft enters orbit around Saturn.

“Before, we could see each component of the haze but didn’t know
where exactly it was in the stratosphere or the troposphere. These
are the first detailed pictures of the distribution of haze with
altitude,” said atmospheric chemist Mate Adamkovics, a graduate
student in UC Berkeley’s College of Chemistry. “It’s the difference
between an X-ray of the atmosphere and an MRI.”

“This shows what can be done with the new instruments on the Keck
Telescope,” added de Pater, referring to the Near Infrared
Spectrometer (NIRSPEC) mounted with the adaptive optics system. “This
is the first time a movie has been made, which can help us understand
the meteorology on Titan.”

Adamkovics and de Pater note than even after Cassini reaches Saturn
this year, ground-based observations can provide important
information on how Titan’s atmosphere changes with time, and how
circulation couples with the atmospheric chemistry to create aerosols
in Titan’s atmosphere. This will become even easier next year when
OSIRIS (OH-Suppressing Infra-Red Imaging Spectrograph) comes on-line
at the Keck telescopes, de Pater said. OSIRIS is a near-infrared
integral field spectrograph designed for the Keck’s adaptive optics
system that can sample a small rectangular patch of sky, unlike
NIRSPEC, which samples a slit and must scan a patch of sky.

De Pater presented the results and the movie on Thursday, April 15,
at an international conference in The Netherlands on the occasion of
the 375th birthday of the Dutch scientist Christiaan Huygens. Huygens
was the first “scientific director” of the Acad=E9mie Fran=E7aise and the
discoverer of Titan, Saturn’s largest moon, in 1655. The four-day
conference, which started April 13, is taking place at the European
Space & Technology Centre in Noordwijk.

The movie can be viewed at http://astron.berkeley.edu/~madamkov/titan/.

The Cassini-Huygens mission is 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 was launched
from Kennedy Space Center on Oct. 15, 1997. The spacecraft will
arrive at Saturn in July, with the Cassini orbiter expected to send
back data on the planet and its moons for at least four years. The
orbiter also will relay data from the Huygens probe as it plunges
through Titan’s atmosphere and after it lands on the surface next
year.

What makes Titan so interesting is its seeming resemblance to a young
Earth, an age when life presumably arose and before oxygen changed
our planet’s chemistry. The atmospheres of both Titan and the early
Earth were dominated by nearly the same amount of nitrogen.

The atmosphere of Titan has a significant amount of methane gas,
which is chemically altered by ultraviolet light in the upper
atmosphere, or stratosphere, to form long-chain hydrocarbons, which
condense into particulates that create a dense haze. These
hydrocarbons, which could be like oil or gasoline, eventually settle
to the surface. Radar observations indicate flat areas on the moon’s
surface that could be pools or lakes of propane or butane, Adamkovics
said.

Astronomers have been able to pierce the hydrocarbon haze to look at
the surface using ground-based telescopes with adaptive optics or
speckle interferometry, and with the Hubble Space Telescope, always
with filters that allow the telescopes to see through “windows” in
the haze where methane doesn’t absorb.

Imaging the haze itself hasn’t been as easy, primarily because people
have had to observe at different wavelengths to see it at specific
altitudes.

“Until now, what we knew about the distribution of haze came from
separate groups using different techniques, different filters,”
Adamkovics said. “We get all that in one go: the 3-D distribution of
haze on Titan, how much at each place on the planet and how high in
the atmosphere, in one observation.”

The NIRSPEC instrument on the Keck telescope measures the intensity
of a band of near-infrared wavelengths at once as it scans about 10
slices along Titan’s surface. This technique allows reconstruction of
haze versus altitude because specific wavelengths must come from
specific altitudes or they wouldn’t be visible at all because of
absorption.

The movie Adamkovics and de Pater put together shows a haze
distribution similar to what had been observed before, but more
complete and assembled in a more user-friendly way. For example, haze
in the atmosphere over the South Pole is very evident, at an altitude
of between 30 and 50 kilometers. This haze is known to form
seasonally and dissipate during the Titan “year,” which is about 29
1/2 Earth years.

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.

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.

The observations were made on Feb. 19, 20 and 22, 2001, by de Pater
and colleague Henry G. Roe of the California Institute of Technology,
and analyzed by Adamkovics using models made by Caitlin A. Griffith
of the University of Arizona, with co-author S. G. Gibbard of
Lawrence Livermore National Laboratory.

The work was sponsored in part by the National Science Foundation and
the Technology Center for Adaptive Optics.