Séminaire à Guyancourt le jeudi 6 mai à 11h. Steve Clifford (Senior Staff Scientist, Lunar and Planetary Institute, Houston, Texas): "The Evolution and Fate of Water on Mars". Various lines of evidence suggest that Mars may possess an inventory of water equal to a global equivalent layer as much as 1-km deep. Currently, only a few percent of this amount is found in visible reservoirs such as the Martian atmosphere and polar caps. The remainder is believed to reside in the subsurface, as both ground ice and groundwater. Four billion years ago, the geologic evidence suggests that the distribution and state of water were quite different, including the presence of a northern ocean that may have covered up to a third of the planet and supported a dynamic hydrologic cycle -- including the episodic occurrences of rain. This seminar will discuss how the climatic and geothermal evolution of Mars is believed to have affected the distribution and state of water, the possible origin and survival of life, and the potential use of geophysical methods, such as ground penetrating radar, to test these hypotheses.