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

This study focuses on topographic characterization of ice-associated north polar region impact craters and several enigmatic polar craterforms using new information from the Mars Orbiter Laser Altimeter (MOLA), an instrument aboard the Mars Global Surveyor (MGS) orbiter. We find that, for ice-associated craters, the topography reveals several surprising results not previously apparent from orbital images alone. First, geometric properties for several impact craters associated with ice and frost deposits suggest that cavity infill is as high as 80% of reconstructed, preerosion levels. Second, craters associated with ice also demonstrate unique cavity geometries relative to their nonpolar counterparts. Finally, in some cases, ice-associated impact features are anomalously deep, on the basis of depths modeled from the best available seating laws. We suggest that burial of these impact features by either episodic advance of the polar cap margin or by continuous deposition at the highest rates previously estimated for the north polar region of Mars has occurred. Subsequent stripping has exhumed the features, leaving behind cavity infill deposits, and a few of these display topographic levels 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 the topography of several enigmatic polar craterforms. In particular, a few craterforms within similar to 150 km of the permanent north polar cap appear to resemble simple, effusive lava shield volcanoes found on Earth. Their geometric properties cannot be reconciled with previous suggestions that they were manifestations of martian hydromagmatic processes (i.e., maar volcanism). MOLA's initial measurements of impact craters and other craterforms in the north polar latitudes of Mars support the concept of a geologically recent surface, with evidence of effusive volcanism and enhanced sedimentation. In addition, the measured impact crater depths for ice-associated craters 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.