WATER-CONTENT OF THE MARTIAN SOIL - LABORATORY SILMULATIONS OF REFLECTANCE SPECTRA

Reflectance spectra from the surface of Mars collected by instruments such as the imaging spectrometer (ISM) onboard the 1988 Soviet Phobos 2 spacecraft exhibit strong 3 mu m absorption features that have long been attributed to hydrated materials on the Martian surface. This int erpretation is consistent with a series of chemical weathering models suggesting an abundance of palagonites, clays, and other hydrated mine ral phases in the Martian fines. Little work, however, has been done t o constrain the actual water content of the Martian surface materials. New laboratory data presented here show that the ISM spectra are cons istent with up to 4% water by weight and that the deep hydration featu res observed in the spacecraft data could be due to less than 0.5% wat er if the hydrated phases are present in the form of grain coatings. T hese results are consistent with the somewhat uncertain in situ measur ements obtained by the Viking landers which yielded approximately 2 wt % water from samples heated to 500 degrees C. On the basis of this wo rk, we expect the TEGA instrument on the Mars '98 lander to find less than 4% adsorbed or bound water in the upper few centimeters of the Ma rtian soil.