Based on the first direct measurements ever obtained of Martian rocks and terrain, scientists on NASA's Mars Pathfinder mission report in this week's Science magazine that the red planet may have once been much more like Earth, with liquid water streaming through channels and nourishing a much thicker atmosphere.

Among the more significant discoveries of the Mars Pathfinder mission was the identification of possible conglomerate rocks, which suggests the presence of running water to smooth and round the pebbles and cobbles, and deposit them in a sand or clay matrix, says Dr. Matthew Golombek, Mars Pathfinder project scientist at NASA's Jet Propulsion Laboratory, Pasadena, CA. This scenario supports the theory that Mars was once warmer and wetter.

"If you consider all of the evidence we have at Ares Vallis -- the rounded pebbles and cobbles and the possible conglomerate, the abundant sand and dust-sized particles and models for their origins, in addition to the high silica rocks," Golombek says, "it suggests a water-rich planet that may have been more Earth-like than previously recognized, with a warmer and wetter past in which liquid water was stable and the atmosphere was thicker."

A panoramic view of Pathfinder's Ares Vallis landing site, featured on the cover of the Dec. 5 issue of Science, reveals traces of this warmer, wetter past, showing a floodplain covered with a variety of rock types, boulders, rounded and semi-rounded cobbles and pebbles. These rocks and pebbles are thought to have been swept down and deposited by floods which occurred early in Mars' evolution in the Ares and Tiu regions near the Pathfinder landing site.

The cover image, which is a 75-frame, color-enhanced mosaic taken by the Imager for Mars Pathfinder, looks to the southwest toward the Rock Garden, a cluster of large, angular rocks tilted in a downstream direction from the floods. The image shows the Pathfinder rover, Sojourner, snuggled against a rock nicknamed Moe. The south peak of two hills, known as Twin Peaks, can be seen on the horizon, about 1 kilometer (6/10ths of a mile) from the lander. The rocky surface is comprised of materials washed down from the highlands and deposited in this ancient outflow channel by a catastrophic flood.

"Before the Pathfinder mission, knowledge of the kinds of rocks present on Mars was based mostly on the Martian meteorites found on Earth, which are all igneous rocks rich in magnesium and iron and relatively low in silica," Golombek and a team of Pathfinder scientists report in a paper entitled, "Overview of the Mars Pathfinder Mission and Assessment of Landing Site Predictions." The paper summarizes the scientific results of the mission, which are also detailed in six other papers in this issue. The scientists report that chemical analyses of more than 16 rocks and studies of different regions of soil -- along with spectral imaging of rock colors, textures and structures -- have confirmed that these rocks have compositions distinct from those of the Martian meteorites found on Earth.

"The rocks that were analyzed by the rover's alpha proton X-ray spectrometer were basaltic or volcanic rocks, with granite-like origins, known as andesitic rocks," Golombek reports. "The high silica or quartz content of some rocks suggests that they were formed as the crust of Mars was being recycled, or cooled and heated up, by the underlying mantle. Analyses of rocks with lower silica content appear to be rich in sulfur, implying that they are covered with dust or weathered. Rover images show that some rocks appear to have small air sacks or cavities, which would indicate that they may be volcanic. In addition, the soils are chemically distinct from the rocks measured at the landing site."

The remarkably successful Mars Pathfinder spacecraft, part of NASA's Discovery program of fast track, low-cost missions with highly focused science objectives, was the first spacecraft to explore Mars in more than 20 years. In all, during its three months of operations, the mission returned about 2.6 gigabits of data, which included more than 16,000 images of the Martian landscape from the lander camera, 550 images from the rover and about 8.5 million temperature, pressure and wind measurements.

The rover traveled a total of about 100 meters (328 feet) in 230 commanded maneuvers, performed more than 16 chemical analyses of rocks and soil, carried out soil mechanics and technology experiments, and explored about 250 square meters (820 square feet) of the Martian surface. The flight team lost communication with the lander on Sept. 27, after 83 days of daily commanding and data return. In all, the lander operated nearly three times its design lifetime of 30 days, and the small, 10.5 kilogram (23- pound) rover operated 12 times its design lifetime of seven days.

Now known as the Sagan Memorial Station, the Mars Pathfinder mission was designed primarily to demonstrate a low-cost way of delivering a set of science instruments and a free-ranging rover to the surface of the red planet. Landers and rovers of the future will share the heritage of spacecraft designs and technologies first tested in this "pathfinding."

Photojournal note: Sojourner spent 83 days of a planned seven-day mission exploring the Martian terrain, acquiring images, and taking chemical, atmospheric and other measurements. The final data transmission received from Pathfinder was at 10:23 UTC on September 27, 1997. Although mission managers tried to restore full communications during the following five months, the successful mission was terminated on March 10, 1998.