LIMNOLOGIC ANALYSIS OF GUSEV CRATER PALEOLAKE, MARS

The survey of the hydrogeologic system formed by Gusev crater and Ma'a dim Vallis (Aeolis subquadrangle of Mars) points out evidence for the existence of an ice-covered lake in Gusev crater. A first lake was for med by the drainage of the aquifer in the region surrounding Gusev bef ore the entry of Ma'adim Vallis in the crater. The existence of a form er lake in Gusev is deduced from the morphology of the Ma'adim delta. Its comparison with terrestrial Antarctic analogs argues for the prese nce of an ice-covered lake in Gusev at the time that the southern part of the crater's rampart was breached by Ma'adim first release, and fo r a subice-lacustrine construction of the valley's delta. Our survey s hows that Ma'adim Vallis may have entered Gusev crater as late as Late Hesperian/Early Amazonian as part of a second lake episode. The relat ionship between the variation of the Gusev lake water-level, the volum e of the lake, and the surface of the lake bed is established by our b athymetric model. The elevation of the former lake is deduced from the elevation of the mesa-like structures in the delta of Ma'adim Vallis. Furthermore, the correlation of the crater frequency of Gusev rampart with Mars' stratigraphic age shows that lakes may have occupied Gusev crater over a period of time covering 2 Gyrs., from the formation of the crater to the last episode of water release from Ma'adim Vallis. T hough it is most likely that the lake was episodical, the recurrence o f abundant water in Gusev crater makes this site a high priority for m issions, either for martian resource exploration, or for the search of life. (C) 1997 Academic Press.