A COMPARISON OF PROPOSED MARTIAN SHORE MORPHOLOGY WITH COASTAL LANDFORMS OF THE BONNEVILLE BASIN, UTAH

Mars Global Surveyor has now completed its nominal mission. Among its many science objectives,the question of whether or not lakes and/or oceans existed on Mars is arguably one of the most provocative. Has MGS either verified or refuted published "predictions" of ancient lakes or oceans on Mars? We look specifically at results from the camera and laser altimeter instruments acquired over proposed ocean shorelines. Terrestrial bodies of water create equipotential surfaces that form shorelines at a fixed elevation modulated by storm, solar and lunar tides (though Mars would have lower solar and no lunar tides). An abandoned shoreline is seldom level, but it defines a planar surface subject to tilting, faulting and warping due to structural changes such as isostatic rebound or loading. All these effects have been identified in Pleistocene Lake Bonneville, Utah. Ancient shorelines on Mars would not currently be perfectly level for similar reasons, though it should be possible to recognize systematic variations in elevation along the shoreline due to such effects. The camera and laser altimiters on MGS reveal numerous potential shoreline features that are comparable in scale and preservation state to coastal landforms in Lake Bonneville. Not surprisingly, the best expressions of this morphology are in regions of relatively moderate to high thermal inertia and low albedo - indicating they are probably not buried by eolian fines. These regions include Cydonia and Deuteronilus Mensae. In Cydonia, terraced massifs similar in scale and morphology to Pavant Butte in southern Utah (a volcano surrounded by a broad cut-and-fill terrace at the Bonneville shoreline), are common (the infamous "Face" is an example). In Deuteronilus, terraced escarpments, analogous to mountain front beach terraces in Bonneville, are more common. Finally, smooth curvilinear ridges, similar to barrier beach ridges, have been imaged in both regions by the MGS camera.