The veils of Saturn's most mysterious moon have begun to lift in
Cassini's eagerly awaited first glimpse of the surface of Titan, a world
where scientists believe organic matter rains from hazy skies and seas of
liquid hydrocarbons dot a frigid surface.
Surface features previously observed only from Earth-based telescopes
are now visible in images of Titan taken in mid-April by Cassini through
one of the narrow angle camera's spectral filters specifically designed
to penetrate the thick atmosphere. The image scale is 230 kilometers (143
miles) per pixel, and it rivals the best Earth-based images.
The two images displayed here show Titan from a vantage point 17
degrees below its equator, yielding a view from 50 degrees north latitude
all the way to its south pole. The image on the left was taken four days
after the image on the right. Titan rotated 90 degrees in that time. The
two images combined cover a region extending halfway around the moon. The
observed brightness variations suggest a diverse surface, with variations
in average reflectivity on scales of a couple hundred kilometers.
The images were taken through a narrow filter centered at 938
nanometers, a spectral region in which the only obstacle to light is
the carbon-based, organic haze. Despite the rather long 38-second
exposure times, there is no noticeable smear due to spacecraft motion.
The images have been magnified 10 times and enhanced in contrast to bring
out details. No further processing to remove the effects of the overlying
atmosphere has been performed.
The superimposed grid over the images illustrates the orientation of
Titan -- north is up and rotated 25 degrees to the left -- as well as the
geographical regions of the satellite that are illuminated and visible.
The yellow curve marks the position of the boundary between day and night
on Titan. The enhanced image contrast makes the region within 20 degrees
of this day and night division darker than usual. The Sun illuminates
Titan from the right at a phase angle of 66 degrees. Because the Sun is
in the southern hemisphere as seen from Titan, the north pole is canted
relative to the boundary between day and night by 25 degrees.
Also shown here is a map of relative surface brightness variations on
Titan as measured in images taken in the 1080-nanometer spectral region
in 1997 and 1998 by the Near Infrared Camera and Multi-Object Spectrometer
on NASA's Hubble Space Telescope. These images have scales of 300
kilometers (186 miles) per pixel. The map colors indicate different
surface reflectivities. From darkest to brightest, the color progression
is: deep blue (darkest), light blue, green, yellow, red and deep red
(brightest). The large, continent-sized, red feature extending from 60
degrees to 150 degrees west longitude is called Xanadu. It is unclear
whether Xanadu is a mountain range, giant basin, smooth plain, or a
combination of all three. It may be dotted with hydrocarbon lakes but
that is also unknown. All that is presently known is that in Earth-based
images, it is the brightest region on Titan.
A comparison between the Cassini images and the Hubble map indicates
that Xanadu is visible as a bright region in the Cassini image on the
right. The dark blue northwest-southeast trending feature from 210
degrees to 250 degrees west longitude, and the bright yellow/green region
to the east (right) and southeast of it at minus 50 degrees latitude and
180 to 230 degrees west longitude on the Hubble map, can both be seen in
the image on the left.
It is noteworthy that the surface is visible to Cassini from its
present approach viewing geometry, which is not the most favourable for
surface viewing. These early Cassini observations are promising for
upcoming imaging sequences of Titan in which the resolution improves
by a factor of five over the next two months. These results are
encouraging for future, in-orbit observations of Titan that will be
acquired from lower, more favorable phase angles.
The first opportunity to view small-scale features (2 kilometers or
1.2 miles) on the surface comes during a 350,000 kilometer (217,500 mile)
flyby over Titan's south pole on July 2, 2004, only 30 hours after
Cassini's insertion into orbit around the ringed planet.
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, manages the Cassini-Huygens mission for NASA's Office of
Space Science, Washington, D.C. The Cassini orbiter and its two onboard
cameras, were designed, developed and assembled at JPL. The imaging team
is based at the Space Science Institute, Boulder, Colo.
For more information, about the Cassini-Huygens mission visit,
http://saturn.jpl.nasa.gov and the Cassini imaging team home page,
http://ciclops.org.