Hydrogeologic evolution of Gale crater and its relevance to the exobiological exploration of Mars

The presence of an Amazonian impact crater lake in the Noachian crater Gale (located in the Aeolis northwest subquadrangle of Mars) is indicated by evidence from young floor deposits, streamlined terraces, layers, and channels observed on the central sedimentary deposit. Evidence for the filling of this lake by two processes is described: (a) the drainage of the aquifer in the Aeolis Mensae region, supported by extended mass-wasting and rim sliding in the crater at the contact with the mensae and (b) the overspilling of the northern rim by an Amazonian south transgression of the Elysium Basin. This last hypothesis is supported by hydrologic features such as channels and channel-like depressions north of the crater and by the crescent-like shape of the central sedimentary deposit. The presence of an impact melt sheet and uplifted central peak may have also generated hydrothermal activity, including an early crater lake, shortly after the formation of the crater in the Noachian period. With time, decreasing heat flux, and changing climates Gale may have experienced transitions in aqueous environments from warm and wet to cold and ice-covered water that could have provided suitable eases for various communities of microorganisms. Preservation of the biological and climatic record may have been favored in this paleolacustrine environment, which probably occured episodically over two billion years. (C) 1999 Academic Press.