GROUNDWATER-SURFACE INTERACTION ON MARS: MEGA-LAHARS AT ELYSIUM AND A DRY LYOT CRATER

Under current Martian climatic conditions a cryosphere extends from the surface to a latitudinally-dependent depth, below which liquid groundwater (GW) may exist (Clifford 1993 JGR 98 10973). The confining ability of the cryosphere, along with a megaregolith that is highly permeable on a global scale, leads to the potential for hydrostatic pressure to build in regions of relatively low topography. Disruption of this cryosphere could release GW to the surface. We use MOLA topography to investigate areas with evidence of GW-surface interaction and areas where such activity may be predicted.

Flow deposits, incised fluvial channels, and degraded channeled terrains extending from Elyiusm Mons to Utopia result from debris-rich lahars and water-rich flows emanating from radial fossae on the flanks of Elysium. Fossae were initiated by lateral propagation of dikes from Elysium. Elevation relationships among units and structures reveal that fossae with min. elevations < ~ -3500 m may be sources of lahars and channels while higher fossae are sources only of lava flows. The elevational control of water sources may reflect the min. elevation of the top of a subsurface saturated zone. Disruption of the cryosphere by dike propagation would allow this water access to the surface. Resulting mega-lahars fluidized primarily by groundwater constitute a class of lahar different from those on Earth. The eruption of large volumes of pressurized GW is enabled by conditions of the Martian climate, cryosphere, hydrosphere, and subsurface.

Lyot crater is in the N. Lowlands, an area of min. planetary elevation and hence max. hydrostatic pressure, and is about as deep as the local predicted cryosphere is thick. However, a lack of fluvial features in the crater suggests no GW effused to the surface as a result of major impact. This implies the cryosphere in the N. Lowlands may be thicker than predicted, GW at the time of impact was absent or too deep, or local subsurface conditions (geothermal gradient, effective permeability) differed at Elysium and Lyot and strongly govern the ability of GW to flow to the surface.

Comparison of these and other areas where volcanism, tectonism, or impact may have resulted in GW-surface interaction provides constraints on the global and local variables controlling the state and evolution of the cryosphere and hydrosphere.