Composition and Stratigraphy of Acidic or Salty Components at Mawrth Vallis, Mars J. L. Bishop (1), B. Horgan (2), J. J. Wray (3) D. Loizeau (4), C. Gross (5), P. C. McGuire (5), M. Parente (6), F. Seelos (7), C. Viviano-Beck (7), and S. L. Murchie (7) (1) SETI Institute & NASA Ames, Mountain View, CA, USA (jbishop@seti.org), (2) Purdue University, Lafayette, IN, USA, (3) Georgia Institute of Technology, Atlanta, GA, USA, (4) University of Lyon, Villeurbanne Cedex, France, (5) Freie Universität Berlin, Germany, (6) University of Massachusetts, Amherst, MA, USA, (7) JHU-APL, Laurel, MD, USA. Abstract Small outcrops of materials characteristic of acidic or salty environments have been found at Mawrth Vallis. They are characterized by a spectral doublet (2.22- 2.28 µm) and are generally associated with the upper Al/Si-rich phyllosilicate unit and may include acid- ified clays [1], bassanite [2], jarosite [3] or other mat- erials. Here we are investigating these doublet units in more detail to constrain their likely formation conditions and determine how they are related to the phyllosilicate-rich units at Mawrth Vallis. 1. Introduction The phyllosilicate-rich outcrops at Mawrth Vallis exhibit multiple spectral signatures in CRISM images due to a variety of phyllosilicates and other hydrated materials [e.g. 1]. Defining the composition of these hydrated outcrops provides information on the aqueous geochemistry of the region. In this study we are focusing on the components likely to have formed in acidic or salty environments [1-4] that are often found in portions of the upper Al/Si-rich units. These are identified by the presence of a doublet feature between 2.22-2.28 µm that is distinct from the bands characteristic of Al/Si-rich (~2.2 µm) and Fe/Mg-rich (~2.3 µm) phyllosilicates [e.g. 5]. 2. Methods CRISM TRR3 [6] and MTRDR [7] images have been evaluated for this study. In addition to the initial parameters generated to identify minerals of interest in CRISM images [6], newly developed parameters were used in this study that enable improved discrimination among the types of phyllosilicates and other hydrated and OH-bearing phases [8]. 3. Results The acidic type materials at Mawrth Vallis are found in some cases at the juncture of the lower Fe/Mg-rich smectite and the upper Al/Si-rich phyllosilicates [4]. This is observed in Figure 1 for a region north of Oyama crater and west of the main channel. Figure 1: 3D view of MTR3 version of CRISM image HRL000043EC (R is D2300, G is BD2250, B is BD2190). The image is ~8 km across and S is up. Fe/Mg-smectite is shown in red and Al/Si-rich phyllosilicates in blue. The green regions indicate the unit associated with acidic or salty processes. CRISM spectra from several locations in this image are shown in Figure 2 including several examples of the acidic materials. This acidic type unit is identified by a weak doublet or broad band near 2.22-2.28 µm. This can be asymmetric with a deeper band near 2.22-2.23 µm or near 2.26-2.28 µm, indicating this could be due to two components. It could be as simple as acid alteration of the Fe/Mg-smectite. Experiments with SWa-1 Fe(Al)-smectite in HCl observed a change in the 2.29 µm band in spectra of SWa-1 smectite to a broad doublet with centers near 2.23 and 2.27 µm as the sample was acidified [9, Figure 2). The observed features could also be EPSC Abstracts Vol. 9, EPSC2014-694, 2014 European Planetary Science Congress 2014 c  Author(s) 2014 E P S C European Planetary Science Congress