An ancient flood plain on Mars has been chosen as the landing site for NASA's December, 1996 Mars Pathfinder mission, one of the first in a new generation of small, low-cost spacecraft.

Billions of years ago, when water flowed on Mars, great floods inundated the landing site, located on a rocky plain in the area called Ares Vallis (19.5 degrees N, 32.8 degrees W). The site is 850 kilometers (527 miles) southeast of the spot where the first spacecraft to land on Mars, Viking Lander 1, touched down in 1976. Pathfinder will be the first to land on Mars since the twin Viking landers arrived almost 20 years ago.The spacecraft, which will arrive at Mars on July 4, 1997 after a seven-month voyage, will parachute down to Ares Vallis at the mouth of an ancient outflow channel chosen for the variety of rock and soil samples it may present.

The purpose of the new Pathfinder mission is to demonstrate an inexpensive system for cruise, entry, descent, and landing on Mars, said Project Manager Anthony Spear and Project Scientist Matthew Golombek of the Jet Propulsion Laboratory.

The lander, carrying the microrover, will aerobrake in the upper martian atmosphere using an aeroshell and a parachute. Just before impact, airbags will inflate to cushion the landing. The microrover will then roll out to examine the rocks and soil nearby.

Both lander and rover will carry scientific instruments and cameras. The lander will make atmospheric and meteorological observations during descent and function as a weather station on the surface, as well as a radio relay station for the rover.

The constraints on landing site selection had to do with engineering considerations, Spear said. Since the spacecraft are powered by nonsteerable solar arrays, the best site is one with maximum sunshine and in July, 1997, the sun will be directly over the 15 N latitude region of the planet.

The elevation must be as low as possible, Spear added, so the descent parachute has sufficient time to open and slow the lander to the correct terminal velocity. The landing will be within a 100- by 200-kilometer (60- by 120-mile) ellipse around the targeted site that will accomodate uncertainties in navigation and atmospheric entry. Ares Vallis, which meets the engineering constraints, was chosen after a workshop earlier this year attended by 60 scientists who study Mars.

The Ares Vallis site is also a "grab bag" location, according to Golombek, set at the mouth of a large outflow channel in which a wide variety of rocks washed down from highland areas by the ancient floods are potentially within reach of the rover. Even though the exact origins of the samples would not be known, he said, the chance of sampling a variety of rocks in a small area could reveal a lot about Mars. Several potential sites were considered where ancient flood channels emptied into Chryse Planitia, having cut through crustal units and ridged plains where the water would have picked up material and deposited it on the plain.

Other sites that were considered included Oxia Palus, a highlands region that contains highland crust and dark wind-blown deposits; Maja Valles Fan, a delta fan which drained an outflow channel; and the Maja Highlands, just south of Maja Valles. All sites were studied using Viking orbiter data.

Both the Pathfinder lander and rover have stereo imaging systems. The rover also carries an alpha proton x-ray spectrometer to examine the composition of rocks. The imaging system will reveal the mineralogy of surface materials as well as the geologic processes and atmosphere interactions that created and modified the surface. The instrument package will also enable scientists to determine dust particle size and water vapor abundance in the atmosphere.