THE CHEMICAL-COMPOSITION OF MARTIAN SOIL AND ROCKS RETURNED BY THE MOBILE ALPHA-PROTON X-RAY SPECTROMETER - PRELIMINARY-RESULTS FROM THE X-RAY MODE

The alpha proton x-ray spectrometer (APXS) on board the rover of the M ars Pathfinder mission measured the chemical composition of six soils and five rocks at the Ares Vallis landing site, The soil analyses show similarity to those determined by the Viking missions. The analyzed r ocks were partially covered by dust but otherwise compositionally simi lar to each other. They are unexpectedly high in silica and potassium, but low in magnesium compared to martian soils and martian meteorites , The analyzed rocks are similar in composition to terrestrial andesit es and close to the mean composition of Earth's crust. Addition of a m afic component and reaction products of volcanic gases to the local ro ck material is necessary to explain the soil composition.