North polar region craterforms on Mars: Geometric characteristics from theMars Orbiter Laser Altimeter

This study focuses on topographic characterization of ice-associated north polar region impact craters and several enigmatic polar craterforms using n ew information from the Mars Orbiter Laser Altimeter (MOLA), an instrument aboard the Mars Global Surveyor (MGS) orbiter. We find that, for ice-associ ated craters, the topography reveals several surprising results not previou sly apparent from orbital images alone. First, geometric properties for sev eral impact craters associated with ice and frost deposits suggest that cav ity infill is as high as 80% of reconstructed, preerosion levels. Second, c raters associated with ice also demonstrate unique cavity geometries relati ve to their nonpolar counterparts. Finally, in some cases, ice-associated i mpact features are anomalously deep, on the basis of depths modeled from th e best available seating laws. We suggest that burial of these impact featu res by either episodic advance of the polar cap margin or by continuous dep osition at the highest rates previously estimated for the north polar regio n of Mars has occurred. Subsequent stripping has exhumed the features, leav ing behind cavity infill deposits, and a few of these display topographic l evels above the surrounding preimpact surface. In at least one case, cavity interior deposits show layering with a typical thickness of similar to 10 m, suggesting episodic deposition and ablation of materials as in the polar layered terrain. In addition to ice-associated craters, we investigated th e topography of several enigmatic polar craterforms. In particular, a few c raterforms within similar to 150 km of the permanent north polar cap appear to resemble simple, effusive lava shield volcanoes found on Earth. Their g eometric properties cannot be reconciled with previous suggestions that the y were manifestations of martian hydromagmatic processes (i.e., maar volcan ism). MOLA's initial measurements of impact craters and other craterforms i n the north polar latitudes of Mars support the concept of a geologically r ecent surface, with evidence of effusive volcanism and enhanced sedimentati on. In addition, the measured impact crater depths for ice-associated crate rs suggest at least a few cases of enhanced excavation and thus a possibly weaker target relative to the typical northern hemisphere plains. (C) 2000 Academic Press.