THE IDENTIFICATION OF 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. However, neither in-situ nor remote-sensing observations have provided unequivocal evidence of these environments. In this context, targeted, in-situ observations of coarse clasts embedded within, but compositionally distinct from, finer-grained layered deposits may unambiguously distinguish glaciolacustrine settings from volcaniclastic 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 to the Curiosity Rover within Gale Crater. A dropstone, consistent with glaciolacustrine origin, may 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) minimal disruption to bed continuity, particularly for layers above a clast, relative to bomb sags. Although terrestrial dropstones commonly associate with fine-grained marine/lacustrine sediments and bomb sags with coarser, high-energy eruptive sediments, the distinctions may fade at glaciolacustrine volcanic environments. Spectrally coded MER Pancam 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. Our preliminary assessment of the Home Plate clast, together with the proposed near-neutral pH alteration environments at the Comanche and related outcrops, 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. Furthermore, these seemingly contradictory initial morphologic and geochemical results may be consistent with an environment where glaciolacustrine, volcaniclastic, and hydrothermal processes were active simultaneously.