Titan is mapped

For the first time, images have been made of the surface of Saturn's giant haze-shrouded moon, Titan. Light and dark features were mapped over the surface of the satellite during most of a complete 16-day rotation. One prominent bright area discovered is a surface feature 2,500 miles across, about the size of the continent of Australia.

Peter Smith (of the University of Arizona) and his team used the Hubble Space Telescope's WideField/Planetary Camera 2 at near-infrared wavelengths (between 0.85 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 with the WFPC2 at the near-infrared wavelength is 360 miles.

Smith's team made a total of 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.

Titan itself is a moon of Saturn larger than Mercury and slightly smaller than Mars. It 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. Titan's present environment -- although 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.

Titan's atmosphere, about four times as dense as Earth's atmosphere, is primarily nitrogen laced with 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.

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 scientists at the University of Arizona confirm this is true.

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.

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, but the new images show that there is at least some solid surface.

Compiled by Warren Brown