Recognizing Dropstone Occurrences and their Potential for Insight into Glaciolacustrine Environments on Mars

Glaciolacustrine environments, such as temporally persistent ice-covered lakes, have been proposed for several locations on Mars, including Gusev Crater[1], Greg Crater[2], and Gorgonum Chaos[3]. Morphologic features consistent with the Hesperian-age lava flows dammed by hundred-meter thick ice-rich deposits within Gusev Crater have also been described[4]. However, neither in-situ nor remote-sensing observations have provided unequivocal evidence for the existence of glaciolacustrine environments. In this context, targeted observations by in-situ missions (i.e., MER and MSL rovers) of coarse clasts embedded within, but compositionally distinct from, finer-grained layered deposits holds the potential for unambiguously distinguishing glaciolacustrine settings from volcaniclastic[5] alternatives. We use the single clast found at the Home Plate deposit in the Columbia Hills of Gusev Crater as a test case to develop a comprehensive strategy for identifying dropstones, also applicable with upcoming data from the Curiosity Rover within Gale Crater. A dropstone, consistent with glaciolacustrine origin, can be identified from 3 key indicators: (1) compositional distinctness from the surrounding sedimentary host; (2) smaller clast populations than would be expected for volcaniclastic bomb sags; (3) little, if any, disruption in bed continuity, particularly in the layers above a clast, relative to bomb sag occurrences. Although terrestrial dropstones commonly associate with fine-grained marine sediments and bomb sags with coarser sediments indicative of high-energy eruptive settings, the distinctions may fade at glaciolacustrine volcanic environments[6]. Spectrally coded MER Panoramic camera images show a common bluish tinge for the Home Plate clast and neighboring sediment layers, suggesting compositional similarity. Alternatively, the apparent similar mineralogy may be from surface alteration, while the interiors may be compositionally distinct; desert varnish analogs have been identified elsewhere on Mars[7]. Our preliminary assessment of the Home Plate clast, together with the proposed near-neutral pH alteration environments at the Comanche and related outcrops[8], provides tentative evidence for the existence of an ancient ice-covered lake within Gusev Crater, potentially reaching depths comparable to the ~100 m height of Columbia Hills. Greater depths are possible, given evidence for an eroded summit above the Tennessee Valley area[9]. Furthermore, these seemingly contradictory initial morphologic and geochemical results may be consistent with an environment where glaciolacustrine, volcaniclastic, and hydrothermal processes were active simultaneously. References [1] Grin, E. Icarus 130, 461-474 (1997) [2] Kargel, J. S. & Furfaro, R. A LPSC 43, 2629 (2012) [3] Howard, A. D. & Moore, J. M. Geophys. Res. Lett. 31, L01702 (2004) [4] Gregg, T. K. P. et al. Icarus 192, 348-360 (2007) [5] Lewis, K. W. et al. J. Geophys. Res. 113 E12S36 (2008) [6] Cousins, C. R. & Crawford, I. A. Astrobiology 11, 695-710 (2011) [7] Knoll, A. H. et al. J. Geophys. Res. 113, E06S16 (2008) [8] Ruff, S. W. LPSC 43, 2898 (2012) [9] Cole, S. B. et al. LPSC 43, 1134 (2012)