Discovery and Distribution of Chloride-Bearing Deposits in the Ancient Cratered Terrain of Mars From THEMIS

Details Discovery and Distribution of Chloride-Bearing Deposits in the Ancient Cratered Terrain of Mars From THEMIS Discovery and Distribution of Chloride-Bearing Deposits in the Ancient Cratered Terrain of Mars From THEMIS

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p13d1563o&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P13D-1563

Mathematics

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5400 Planetary Sciences: Solid Surface Planets, 5464 Remote Sensing, 5470 Surface Materials And Properties

Scientific paper

We have identified and mapped a number of spectrally distinct deposits (~250) in mid-infrared data acquired by the 2001 Odyssey Thermal Emission Imaging System (THEMIS). These deposits are interpreted to contain a chloride salt component based on their spectral signatures in THEMIS and Mars Global Surveyor Thermal Emission Spectrometer (TES) data. Thermal inertia derived from THEMIS nighttime observations, indicate that the chloride-bearing materials are possibly cemented or indurated. Individually, most chloride-bearing deposits are small in area (<~25 km2), but they are globally widespread, as we have identified them throughout low albedo regions of the southern highlands of Mars. These regions correspond to mid-to-late Noachian terrains, as well as early Hesperian ridged plains units. The chloride-bearing deposits commonly occur in topographic lows relative to the surrounding terrain, and some appear to follow channel-like outlines. Less typically we observe them in small crater floors. Images acquired by the Mars Orbiter Camera (MOC) and the High Resolution Imaging Science Experiment (HiRISE) indicate geomorphology consistent with formation in an evaporitic environment. HiRISE imagery (at 25.3 cm/pixel) over a large exposure in Terra Sirenum shows the chloride-bearing materials to be light-toned and highly fractured. The fracturing is sub-polygonal and is similar to desiccation cracks in evaporitic environments. Cross cutting relationships indicate that the chloride-bearing materials are older than the surrounding basaltic materials, and commonly appear to have been exposed by erosion. In the HiRISE image we observe additional occurrences of chloride materials within the regional terrain, indicating that the chloride deposits are likely more extensive than what is discernable at THEMIS IR scales. The origin and diagenesis of each chloride deposit is likely complex, however it is probable that water played a role in each instance, either via direct precipitation of ground water or standing water, or via efflorescence from evaporative pumping, volcanic out gassing, or atmospheric-surface interactions. Many chloride salts are extremely hygroscopic and can be further modified by fluctuations in humidity. Identification of a hygroscopic chloride could indicate brine activity subsequent to initial deposition. The identification of another water-related material in the ancient cratered terrain of Mars is further evidence that Mars once had a hydrologic cycle (albeit possibly short in duration) that was more active than at present.

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