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.