Evaporites in Martian Paleolakes: Observations and Implications

Details Evaporites in Martian Paleolakes: Observations and Implications Evaporites in Martian Paleolakes: Observations and Implications

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p41b..06w&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P41B-06

Biology

[5200] Planetary Sciences: Astrobiology, [5410] Planetary Sciences: Solid Surface Planets / Composition, [5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes, [6225] Planetary Sciences: Solar System Objects / Mars

Scientific paper

Ancient lakes on Mars have long been inferred from morphologic evidence [e.g., 1], and are considered high-priority targets in the search for Martian biomarkers. Minerals precipitated from lake water reflect water chemistry and temperature, as well as the composition of the contemporaneous atmosphere, providing constraints on habitability. However, proposed paleolakes have until recently shown little evidence for evaporite minerals such as carbonates and sulfates. We previously reported CRISM detections of sulfates and phyllosilicates in finely bedded deposits within impact craters in Terra Sirenum [2]. Subsequent mapping reveals that Al-phyllosilicates are found not only within these ~10 craters, but also on the intercrater plains. Sulfates, however, are found only within the craters Columbus (29S, 166W) and Cross (30S, 158W). Cross contains the acid sulfate alunite [3], while Columbus has predominantly polyhydrated Ca and possibly Mg sulfates in a “bathtub ring” around its walls. Thermal infrared data are consistent with ~40% clay and ~16% sulfate abundances in the Columbus ring, suggesting strong alteration, possibly in a lacustrine setting. Since most craters in the region lack major inlet valleys, they may have been filled by groundwater. Indeed, global hydrologic models [4] predict enhanced Noachian/Hesperian groundwater upwelling in this region, and a new regional model predicts the greatest thicknesses of evaporites in Columbus and Cross craters specifically. Therefore, groundwater may have caused regional alteration, before ponding and evaporating in the largest craters to form sulfates. A new CRISM image reveals sulfate in another deep lacustrine setting. A depression within Shalbatana Vallis (3N, 43.3W) has been described as a Hesperian-aged paleolake based on topography and morphology, including inlet channels that feed six fan-shaped deposits interpreted as deltas, the largest of which preserves features inferred to be shorelines [5]. The valley wall adjacent to this fan contains Fe/Mg-phyllosilicate, and near the bottom of the depression, over 300 meters below the shoreline elevation, polyhydrated sulfate is detected in finely bedded deposits. These sulfate deposits contrast with carbonate-bearing sediments seen in Jezero crater, another inferred paleolake site [6,7]. This difference in dominant anion could reflect local differences in water chemistry, groundwater vs. meteoric inputs, or possibly a change in atmospheric composition over time. Finding more examples of lake evaporites on Mars should improve our understanding of their paleo-environmental and astrobiological implications. [1] Cabrol N A and Grin E A (1999) Icarus 142, 160. [2] Wray J J et al. (2009) LPSC 40, #1896. [3] Swayze G A et al. (2008) AGU Fall Meeting, #P44A-04. [4] Andrews-Hanna J C et al. (2007) Nature 446, 163. [5] Di Achille G et al. (2009) GRL 36, L14201. [6] Ehlmann B L et al. (2008) Nature Geosci. 1, 355. [7] Ehlmann B L et al. (2008) Science 322, 1828.

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