Water content of the Martian soil: Laboratory simulations 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 μm absorption features that have long been attributed to hydrated materials on the Martian surface. This interpretation is consistent with a series of chemical weathering models suggesting an abundance of palagonites, clays, and other hydrated mineral phases in the Martian fines. Little work, however, has been done to constrain the actual water content of the Martian surface materials. New laboratory data presented here show that the ISM spectra are consistent with up to 4% water by weight and that the deep hydration features observed in the spacecraft data could be due to less than 0.5% water if the hydrated phases are present in the form of grain coatings. These results are consistent with the somewhat uncertain in situ measurements obtained by the Viking landers which yielded approximately 2 wt % water from samples heated to 500°C. On the basis of this work, 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 Martian soil.