Floods on Mars Released from Groundwater by Impact. Chi-yuen Wang, Michael Manga and Alex Wong, Department of Earth and Planetary Science, University of California, Berkeley, CA 94720; [email protected] Introduction: On Earth, large earthquakes commonly cause saturated soils to liquefy  and streamflow to increase ; evidence for liquefaction caused by meteoritic impact is also preserved in the sedimentary record . Saturated soils on Mars may experience undrained consolidation, pore-pressure buildup and liquefaction when subjected to ground shaking during meteoritic impact . Here we estimate the extent of liquefaction and the amount of groundwater release on Mars caused by impact, using a relation between the liquefaction limit and crater diameter. The result is consistent with the suggestion that groundwater released by impact on Mars may have produced floods and formed the outflow channels. Liquefaction limit during impact: We derive a relation between the liquefaction limit Rmax (the farthest distance between the earthquake source and the occurrence of liquefaction) and crater diameter D, for all planets with saturated soils , from data for earthquake-induced liquefaction on Earth  and a scaling relation between crater diameter and impact energy . For Earth, the relation predicts Rmax = 200 + 100 km for the Oasis crater in Lybia (D = 11.5 km), Rmax = 5,000 + 2000 km for the Chicxulub crater in Mexico (D ~ 100 km), and the Upheaval Dome crater, with D = 4 km and Rmax = 60 + 30 km - all consistent with observations. For Mars, the relation predicts global occurrence of liquefaction and streamflow for D > 100 km. A possible liquefaction-induced debrisflow deposit occurs ~450 km south of the Lowell crater (D ~ 250 km). This deposit, if correctly explained, is also consistent with prediction. Groundwater released by impact: Estimates of the amount of groundwater released by impact on Mars, estimated from crater-size distribution predicited by Frey’s timeline  and a power law of Hartmann and Neukum  together with reasonable assumption on porosity and permeability of the Martian soils, are consistent with the suggestion that groundwater released by impact on Mars may have produced floods and formed the outflow channels if the regolith in the channels was removed by many separate flood events . One particular surface manifestation of liquefaction on Mars is the chaotic terrain often found at the heads of outflow channels. The checkerboard patterns of gaps between blocks of chaotic terrain suggest some combinations of lateral spreading and collapse. Liquefaction on Earth often leads to lateral spreading and collapse of the surface, so chaotic terrain on Mars may be a manifestation of the same effect, but on a much larger scale. The difference in scale may reflect the different thicknesses of the fractured frozen strata in the two cases. References:  Terzaghi, K., R.B. Peck, and G. Mesri (1996) Soil Mechanics in Engineering Practice, 3rd ed., John Wiley & Sons, New York, 549 pp.  Montgomery, D.R., and M. Manga (2003), Science, 300, 2047-2049.  Underwood, J.R. (1976) Geol. Romana, 15, 337-340; Warme, J.E., and H.C. Kuehner (1998) Intern. Geology Rev., 40, 189-216; Terry, D.O., J.A. Chamberlain, P.W. Stoffer, P. Messina, and P.A. Jannett, (2001) Geology, 29, 10551058.  Clifford, S.M. (1997) Lunar Planet. Sci. Conf., 27, 241.  Wang, C.-Y., A. Wong, D. Dreger, M. Manga (2006) Bull. Seism. Soc. Am., (in press).  Melosh, H.J. (1989) Impact Cratering - A Geologic Process, Oxford University Press, New York, 245 pp.  Wang, C.-Y., M. Manga, and A. Wong (2005) Icarus, 175, 551-555.  Frey, H.V. (2004) Lunar Planet. Sci. Conf. XXXV Abstract, 1382.  Hartmann, W.K., and G. Neukum, 2001, Space Sc. Rev., 96, 165-194.  Williams, R.M., R.J. Phillips, and M.C. Malin (2000) Geophys. Res. Lett., 27, 1073-1076.