Coordinated spectroscopy, geochemistry, and mineralogy of mars brine pond analogue sediments from antarctica

Chlorides, phyllosilicates, and sulfates at or near the surface of Mars indicate a complex history of aqueous activity [1-5]. Understanding this history is critical to understanding past and present martian habitability. Fortunately, excellent analogues for the cold and xeric martian climatic conditions, and for the martian regolith, are available in the McMurdo Dry Valleys (MDV) region of Antarctica [6] (Fig. 1). Here, liquid surface water is unstable most of the year [7], and yet, life flourishes [8]. Our study focuses on the mineralogical assemblages that have developed in MDV sediments due to the activity of a transient brine pond, referred to as the VXE-6 pond (Fig. 2). This pond provides an analogue for ancient salt ponds on Mars, and for the observed aqueous mineral assemblages on the surface. Introduction: The climate and geology of the MDV is in many ways analogous to martian settings. Despite an extremely cold (mean annual temperature of -18°C) [6,7] and dry (<10 cm precipitation/year with sublimation >50 cm/year) [6,8] environment, liquid water occurs in the MDV. This water – in lakes, ponds, and streams – is highly saline, thus bearing a close similarity to runoffand groundwater-fed saline water bodies inferred to have existed in the martian past, and importantly, inferred to have produced the observed suite of aqueous minerals on the martian surface [e.g. 5,9,10]. Furthermore, the Ferrar dolerite of the MDV contains abundant pyroxene, feldspars, and phyllosilicates [11,12], providing a close analogue for martian regolith in its weathered form. Study Site: We performed coordinated spectral, geochemical, and mineralogical analyses of sediments collected in 1980 [13] at a dry, shallow-groundwaterfed transient salt pond in the South Fork of Wright Valley (Fig. 1 & 2). Our samples represent sediments collected from six depth intervals (0-1, 1-4, 4-7, 8-10, 12-15, 20-24 cm) in a pit excavated at the pond site, as well as sediments collected at ten one-cm depth intervals from a core taken at the site. Methods: In a previous study [14], we compared various methods of sample preparation and found spectral signatures to be similar regardless of method. Therefore, we here report on spectral analysis of original coarse soil pit and core samples. These samples were analyzed via visible/near-infrared (VNIR) reflectance spectroscopy using an ASD spectrometer at the SETI Institute, as in [15]. Major and trace element geochemical analysis was performed by X-ray Fluorescence (XRF) at the Bureau Veritas in Vancouver and by Instrumental Neutron Activation Analysis (INAA) at the University of Vienna. Mineralogical analysis was performed by X-ray Diffraction (XRD) using a Bruker D8 Advance with a LYNXEYE XE detector at the University of Hawai‘i at Mānoa.