BOULDER HALOS ON MARS SHOW SYSTEMATIC DIFFERENCES IN REGOLITH BOULDER SIZE DISTRIBUTIONS FROM THE CRATER LENGTH SCALES TO HEMISPHERIC LENGTH SCALES

Impacts on Mars form craters that excavates subsurface materials (regolith, bedrock, ice, etc.) ranging in size from fine sediments to boulders. Lunar studies have shown that the grain size distribution in fresh ejecta deposits follows a power law relation with the average grain size decreasing away from the crater center, and that impact fragmentation of bedrock results in larger boulder clasts surrounding larger impact craters. When impacts occur on Mars into ice-rich latitude dependent mantle surfaces, rock rings called “boulder halos” can form when boulders persist at the surface long after impact craters have been filled in and rim material has been eroded away. Boulder halos are ubiquitous across the middle to high latitudes of Mars, but are vastly more common in the northern hemisphere than the southern hemisphere. However, the size distribution of debris in Martian boulder halos is not well known. While it is possible that climatic conditions or surface processes drive differences in boulder halo abundance between the northern and southern hemisphere of Mars, here we explore the possibility that boulder composition (southern highlands sedimentary units vs. northern plains lava flow material) and differential erosion rates may drive differences in boulder halo abundance. We show that across 20 sites in each hemisphere, the lack of correlation between boulder halo size and average boulder size suggests that there is a thick layer of boulder-rich regolith underlying latitude-dependent mantle ice deposits, implying that impacts merely excavate this regolith layer instead of creating new boulders by fragmenting the underlying bedrock. Furthermore, boulder sizes are on average larger in the northern hemisphere than in the southern. This suggests that the southern hemisphere boulders and therefore underlying bedrock may be composed of friable material that is more prone to comminution than that of the northern hemisphere bedrock. Accordingly, boulder halos may serve as a probe not just into the presence or absence of near-surface ice on Mars, but also as a probe into the physical properties of underlying regolith and/or bedrock.