SELECTION OF THE MARS PATHFINDER LANDING SITE

The Mars Pathfinder spacecraft will land on a depositional fan near th e mouth of the catastrophic outflow channel, Ares Vallis (19.5 degrees N, 32.8 degrees W). This site offers the prospect of analyzing a vari ety of rock types from the ancient cratered highlands, intermediate-ag e ridged plains, and reworked channel deposits. Analyses of these rock s by Pathfinder instruments will enable first-order scientific questio ns to be addressed, such as differentiation of the crust, the developm ent of weathering products, and the nature of the early environment, a s well as their subsequent evolution on Mars. Constraints imposed by ( 1) spacecraft and rover designs (which are robust), (2) entry, descent , and landing, (3) scientific potential at various sites, and (4) safe ty were important considerations in site selection. Engineering constr aints require a 70 km by 200 km smooth, flat (low slopes) area located between 10 degrees and 20 degrees N that is below 0 km elevation, wit h average radar reflectivity, little dust, and moderate rock abundance . Three regions on Mars are between 10 degrees and 20 degrees N and be low 0 km elevation: Chryse, Amazonis, and Isidis-Elysium. Science cons iderations favor sites at the mouths of outflow channels (grab bag sit es offer an assay of rock types on Mars), highland sites (early crusta l differentiation and climate), and sites covered with dark (unoxidize d) material. Sites are considered safe if they are clearly below 0 km elevation, appear acceptably free of hazards in high-resolution (< 50 m/pixel) Viking orbiter images and have acceptable reflectivity and ro ughness at radar wavelengths, thermal inertia, rock abundance, red to violet ratio, and albedo. Recent 3.5-cm wavelength radar observations were used to verify elevation, reflectivity, and roughness within the landing ellipses. Three sites meet all of these criteria: Ares Vallis, Tritonis Lacus, and Isidis. Although Isidis appears to be safer than Tritonis and Ares, the greater scientific potential at Ares Vallis res ulted in its Selection. Comparisons of the Grand Coulee (channel) and the depositional Ephrata Fan of the Channeled Scabland in eastern Wash ington, with Ares Vallis and its depositional fan also suggest the Are s Vallis landing site is safe and scientifically interesting.