Stratigraphic and mineralogic investigations of chloride materials; Towards unraveling the timing and conditions of aqueous alteration in the southern highlands of Mars.

We present a regional-scale, comprehensive spectral, morphological and thermophysical mapping effort to elucidate the compositional stratigraphy of chloride deposits ins Terra Sirenum and Noachis Terra. Previous work [1,2] indicates chloride materials are thermophysically distinct and exhibit a relatively featureless red slope in the visible near infrared (VNIR) and blue slopes superimposed on basaltic regolith in the middle infrared (MIR), consistent with a mixture of anhydrous chlorides and silicates [1,2]. A puzzling aspect of martian chloride deposits has been the apparent lack of other weathering or evaporite phases associated with the deposits. On Earth, evaporative settings are often characterized by a multitude of evaporite and phyllosilicate phases including carbonates, sulfates, and nitrates. A global analysis conducted by [3] found that only ~9% of the chloride sites were associated with minerals such as phyllosilicates. Most of these occurrences are in Terra Sirenum where [4] observed that chloride materials lie stratigraphically above Noachian phyllosilicates. Our initial spectroscopic results in Noachis Terra [5] indicate that the only other aqueous phase associated with the chlorides is an Fe/Mg-phyllosilicate-bearing unit. Here, chloride salts formed subsequent to the formation of the hydrated clays similar to observations in Terra Sirenum [4]. Ponding and evaporation of surface runoff and/or groundwater discharge likely led to the formation of chlorides in Noachis Terra, consistent with previous findings globally [1,6]. Here, we will discuss the initial results of our detailed spectroscopic investigations of chloride sites in Terra Sirenum.

[1] Osterloo, M. M. et al. (2010) JGR, 115, E10. [2] Jensen, H. B. and Glotch T. D. (2011) JGR, 116, E12. [3] Ruesch O. et al. (2012) JGR, 117, E00J13. [4] Glotch, T. D. et al. (2010), Geophy. Res. Lett. 37, L16202. [5] Philips B. P et al. (2019) LPSC L #3209. [6] Osterloo M. M. and B.M. Hynek (2015) LPSC XLVI #1054.