Saturn's Satellite Titan
Surface View
From Lori Stiles, (602) 621-1877 University of Arizona, News Services -- Sent
Nov. 8, 1994 -- UA SCIENTIST & TEAM DISCOVER SURFACE FEATURES COVER TITAN
(Science contacts: Peter H. Smith, (602) 621-2725; Mark Lemmon, (602) 621-1485;
UA Lunar and Planetary Laboratory) Scientists for the first time have made
images of the surface of Saturn's giant, haze-shrouded moon, Titan. They mapped
light and dark features over the surface of the satellite during nearly a
complete 16-day rotation. One prominent bright area they discovered is a surface
feature 2,500 miles across, about the size of the continent of Australia. Titan,
larger than Mercury and slightly smaller than Mars, is the only body in the
solar system, other than Earth, that may have oceans and rainfall on its
surface, albeit oceans and rain of ethane-methane rather than water. Scientists
suspect that Titan's present environment -- although colder than minus 289
degrees Fahrenheit, so cold that water ice would be as hard as granite -- might
be similar to that on Earth billions of years ago, before life began pumping
oxygen into the atmosphere. Peter H. Smith of the University of Arizona Lunar
and Planetary Laboratory and his team took the images with the Hubble Space
Telescope during 14 observing runs between Oct. 4 - 18. Smith announced the
team's first results last week at the 26th annual meeting of the American
Astronomical Society Division for Planetary Sciences in Bethesda, Md.
Co-investigators on the team are Mark Lemmon, a doctoral candidate with the UA
Lunar and Planetary Laboratory; John Caldwell of York University, Canada; Larry
Sromovsky of the University of Wisconsin; and Michael Allison of the Goddard
Institute for Space Studies, New York City.
Titan's atmosphere, about four times
as dense as Earth's atmosphere, is primarily nitrogen laced with such poisonous
substances as methane and ethane. This thick, orange, hydrocarbon haze was
impenetrable to cameras aboard the Pioneer and Voyager spacecraft that flew by
the Saturn system in the late 1970s and early 1980s. The haze is formed as
methane in the atmosphere is destroyed by sunlight. The hydrocarbons produced by
this methane destruction form a smog similar to that found over large cities,
but is much thicker. Smith's group used the Hubble Space Telescope's
WideField/Planetary Camera 2 at near-infrared wavelengths (between .85 and 1.05
microns). Titan's haze is transparent enough in this wavelength range to allow
mapping of surface features according to their reflectivity. Only Titan's polar
regions could not be mapped this way, due to the telescope's viewing angle of
the poles and the thick haze near the edge of the disk. Their image- resolution
(that is, the smallest distance seen in detail) with the WFPC2 at the
near-infrared wavelength is 360 miles. The 14 images processed and compiled into
the Titan surface map were as "noise" free, or as free of signal interference,
as the space telescope allows, Smith said. Titan makes one complete orbit around
Saturn in 16 days, roughly the duration of the imaging project. Scientists have
suspected that Titan's rotation also takes 16 days, so that the same hemisphere
of Titan always faces Saturn, just as the same hemisphere of the Earth's moon
always faces the Earth. Recent observations by Lemmon and colleagues at the
University of Arizona confirm this is true. It's too soon to conclude much about
what the dark and bright areas in the Hubble Space Telescope images are --
continents, oceans, impact craters or other features, Smith said. Scientists
have long suspected that Titan's surface was covered with a global
ethane-methane ocean. The new images show that there is at least some solid
surface. Smith's team made a total 50 images of Titan last month in their
program, a project to search for small scale features in Titan's lower
atmosphere and surface. They have yet to analyze images for information about
Titan's clouds and winds. That analysis could help explain if the bright areas
are major impact craters in the frozen water ice-and-rock or higher-altitude
features. The images are important information for the Cassini mission, which is
to launch a robotic spacecraft on a 7-year journey to Saturn in October 1997.
About three weeks before Cassini's first flyby of Titan, the spacecraft is to
release the European Space Agency's Huygens Probe to parachute to Titan's
surface. Images like Smith's team has taken of Titan can be used to identify
choice landing spots - - and help engineers and scientists understand how
Titan's winds will blow the parachute through the satellite's atmosphere. UA
scientists play major roles in the Cassini mission: Carolyn C. Porco, an
associate professor at the Lunar and Planetary Laboratory, leads the 14-member
Cassini Imaging Team. Jonathan I. Lunine, also an associate professor at the
lab, is the only American selected by the European Space Agency to be on the
three-member Huygens Probe interdisciplinary science team. Smith is a member of
research professor Martin G. Tomasko's international team of scientists who will
image the surface of Titan in visible light and in color with the Descent
Imager/Spectral Radiometer, one of five instruments in the Huygens Probe's
French, German, Italian and U.S. experiment payload. Senior research associate
Lyn R. Doose is also on Tomasko's team. Lunine and LPL professor Donald M.
Hunten are members of the science team for another U.S. instrument on that
payload, the gas chromatograph mass spectrometer. Hunten was on the original
Cassini mission science definition team back in 1983.
Four global projections of the HST Titan data, separated in longitude by 90
degrees. Upper left: hemisphere facing Saturn. Upper right: leading hemisphere
(brightest region). Lower left: the hemisphere which never faces Saturn. Lower
right: trailing hemisphere. Not that these assignments assume that the rotation
is synchronous. The imaging team says its data strongly support this assumption
-- a longer time baseline is needed for proof. The surface near the poles is
never visible to an observer in Titan's equatorial plane because of the large
optical path.
Last Update: 2005-Nov-29
This is the Web Edition of "A Trip Into Space", a Coimbra-based electronic book on space science. Both the texts and the photos are by courtesy of National Aeronautics and Space Administration.
Saturn 
