Distinguishing Between Bombsags and Dropstones on Mars with Implications for Gusev and Gale Craters

Glaciolacustrine environments on Mars have been proposed at several locations, including Greg Crater[1], Gorgonum Chaos[2], and Meridiani Planum[3]. However, observations by orbiters and rovers provide only equivocal evidence, including difficulties in distinguishing between these environments and volcaniclastic counterparts. This creates a need to quantify differences between dropstones, typically associated with fine-grained marine or lacustrine sediments, and bombsags, typically associated with coarser, high-energy eruptive sediments. Impact cratering excavation may also provide a mechanism to entrain clasts into sediments, either directly or as a debris source for cold based glaciers and ice cover. However, we focused on the first two mechanisms. We developed quantitative measures to distinguish dropstones from bombsags using in situ and high-resolution remote sensing images, supplemented by chemical and mineralogical information. The development step employed images of paleo samples of both glacial and volcanic clasts on Earth, which represent the meta-stable configuration over geologic time scales. The resulting flow chart method involving distributional parameters can be applied in situ on Mars, advancing early work[4]. Four key indicators yield values that may distinguish between bombsag and dropstone deposits: (1) compositional distinctness from the sedimentary host layer, (2) penetration depth ratio, (3) impact symmetry ratio, and (4) clast population density. Dominance of cold-based mountain glaciers on Mars[5] may reduce debris entrainment and distal sampling, weakening the utility of Indicator 1, but terrestrial examples of significant entrainment and deposition by cold-based glaciers exist[6] We applied our newly developed flow chart to the putative bombsag at Gusev crater[7] as a case study. Any divergence from the consensus of the Home Plate clast as a bombsag would refine current models of pyroclastic activity[8], structural evolution[7], and atmospheric composition[9]. A convergence with the consensus would reinforce each of these models, all relying implicitly on the clast at Home Plate as a pyroclastic artifact in a phraetomagmatic setting of late Noachian Mars. This case study established the utility of our flow chart method to opportunistically identify clast environments that the Curiosity Rover may encounter at Gale Crater. Future work will involve either computational extrapolation for reduced gravity environments, or experimental assessment in a micro-gravity environment. These future analyses will strengthen our newly developed methodology as a tool to distinguish glacial and volcanic environments on Mars. This may prove useful particularly if Curiosity were to observe bed-disrupting clast configurations along its traverse, particularly in Gale Crater. References [1] Kargel J. & Furfaro R. LPSC 43, 2629 (2012) [2] Howard A. & Moore J. GRL 31, L01702 (2004) [3] Michalski J. & Niles P. Geology 40, 419-422 (2012) [4] Thomas G. & Connell. R. JSR 55, 243-249 (1985) [5] Head J. & Marchant D. Geology 31, 641 - 644 (2003) [6] Atkins C. et al. Geology 30, 659 - 662 (2002) [7] Lewis K. et al. JGR 113, E12 (2008) [8] Squyres S. et al. Science 316, 738-742 (2007) [9] Manga M. et al. GRL 39, L01202 (2012)