Sulfate and hematite stratigraphy in Capri Chasma, Valles Marineris

Introduction: Analysis of OMEGA and CRISM spectral data for sulfate-rich Interior Layered Deposits (ILDs) in Valles Marineris has revealed complex assemblages of iron oxides and multiple sulfates [1-7]. We have been developing stratigraphic assemblages to assess the similarities and differences among chasma [3, 5, 7]. Here we present sulfate and iron oxide stratigraphy within the central ILD of Capri Chasma. Background: Ferric minerals have been previously identified in basinal chasmata throughout Valles Marineris with OMEGA data, but could not be definitively resolved as ferric oxide + sulfate or ferric sulfate [1]. TES discovered crystalline gray hematite throughout Valles Marineris and Meridiani Planum [12], and the MER rover Opportunity confirmed the detection in Meridiani [13]. Many of the OMEGA detections of “ferric oxide” can now be resolved as red hematite, often with kieserite, in CRISM data. The co-occurrence of red hematite with gray hematite from TES indicates a weathering process [3]. While VNIR instruments cannot resolve gray hematite, it could be present with the red hematite and darkening the spectra. The presence of sulfate and hematite together suggests an incomplete maturation process [14]. Datasets: OMEGA is a visible-near infrared hyperspectral imager on the ESA/Mars Express mission [9]. It has a 300m-4.8km spatial sampling, and a 7 to 20 nm spectral resolution in 352 spectral bands over 0.35-5.1 μm. CRISM, a visible-near infrared hyperspectral imager on Mars Reconnaissance Orbiter (MRO), has similar spectral characteristics and takes targeted observations at up to 18m/pixel [10]. The MRO HiRISE camera is capable of acquiring co-aligned imagery with CRISM and can resolve details down to ~30 cm/pixel [11]. Stratigraphic relationships: Hematite, kieserite, and PHS are found in consistent relationships across the Capri Chasma central ILD. Fig 1 shows well-exposed contacts along the northern edge of the ILD. Kieserite and PHS are clearly separated layers with different textures – bright fluted kieserite and dusty, more friable PHS. In the south of Fig 1, contacts between kieserite and PHS are complicated by a ~25 km diameter impact crater. PHS is found above kieserite in the thickest parts of the ILD in a subhorizontal layer ~400m thick (dip is <2° to the SW). Thinner parts of the ILD do not show PHS above the kieserite, suggesting erosion removed the topmost material. Hematite is found with both sulfate types, but is more commonly found with kieserite. This is similar to the hematite/sulfate relationships in West Candor Chasma [3] and Ophir Chasma. Fig 1b illustrates the common association of hematite with kieserite (purple color); kieserite without hematite appears blue. PHS with hematite is yellowish, while PHS without hematite is green. Hematite is Figure 1. Northern edge of central ILD in Capri Chasma with CRISM targeted data over CTX. a) CRISM color RGB: 2.5, 1.5, 1.08 μm. b) CRISM parameter RGB: .86 μm band depth, 1.9 μm band depth, 2.1 μm band depth, in this case indicating hematite, polyhydrated sulfate, and kieserite, respectively.