Implications for the Abundance of Water on Early Mars as Evidenced by the Presence of Secondary Minerals in Martian Meteorite

Evidence for Water on Early Mars: Although numerous lines of evidence suggest that Mars must have been a "wet" planet at some time in its past, there is no universal agreement on the volume of hydrous activity, its style, or the characteristics of the corresponding climates through time. Morphological evidence for the presence of surface water on Mars is abundant [1-3] as is evidence for the existence of ground ice [4-9]. Evidence for glacial activity on Mars supports a colder, though still hydrous planet [10,11]. Other features suggest that some fluid and ground ice activity on Mars has not been limited to the ancient past [6,8,9,12]. Mineralogical and geochemical evidence for a wet Mars arise from hematite deposits found on the surface [13,14] and from igneous minerals in the martian meteorites [15-17]. The question, therefore, is not whether water existed on Mars, but when it existed, in what state, its abundance, the locations of reservoirs (both spatially and in terms of depth), and if/when this water became less active. Interpretations of the martian hydrologic history range from a warm and wet Mars with oceans and an active hydrologic cycle [18] to a drier planet, where short-term hydrospheres were activated by impact events [19], to a frozen Mars where a water is present as a frozen subsurface layer [20]. Estimates for the abundance of water on Mars range from an initial 3.6 m to a current 0.2 m global thickness layer [21] to an initial depth of >100 km ac-creted and lost early [22] to a current subsurface 1400 m [20]. A global layer of frozen, saturated ground ice has been proposed by [18]. Little of this hydrous inventory is available for direct inspection. Only in the last few years have we been able to quantitatively measure the abundance of water in the outermost surface layers by remote means [23,24] though remotely sensed images have provided us with evidence for decades. Ground Truth: The Martian Meteorites: The ~30 martian meteorites serve as " ground truth " for the examination of the historical record of Mars, including the abundance of water present in the crust over time. The record of alteration within these rocks should be an indication of the pervasiveness of water in the crust. The fact that the amount of aqueous alteration present in the martian meteorites is very low suggests that either the availability of liquid water at the …