Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6240.pdf

EVIDENCE FOR HESPERIAN ACIDIC ALTERATION IN IUS CHASMA. C. M. Weitz1, J. L. Bishop2, J. Flahaut3, C. Gross4, A.M. Saranathan5, Y. Itoh5, and M. Parente5, 1Planetary Science Institute (1700 E Fort Lowell, Suite 106, Tucson, AZ 85719, [email protected]), 2SETI Institute (Carl Sagan Center, 189 Bernardo Ave., Mountain View, CA 94043), 3CRPG (15 rue Notre Dame des Pauvres, BP 20 54500 Vandœuvre les Nancy, France), 4Free University of Berlin (Berlin, Germany), 5 University of Massachusetts Amherst (Dept. Electrical & Computer Engi- neering, University of Massachusetts, 151 Holdsworth Way, Amherst, MA 01003).

Introduction: Recent studies of Ius Chasma, Me- CRISM results: CRISM spectra taken from sev- las Chasma, Noctis Labyrinthus, and Mawrth Vallis eral images across the doublet unit show there are vari- have revealed unique surface materials that could be ations in the strength and location of the 2.2-2.3 µm related to acidic alteration [1-7]. These mystery phases absorptions. As shown in Figure 2, some doublet spec- exhibit a “doublet” absorption in CRISM spectra be- tra have equal strength absorptions at 2.22 and 2.27 tween 2.2 and 2.3 µm that occurs in a variety of pat- µm (b), whereas others have a much stronger 2.22 µm terns, but is distinct from the commonly observed min- absorption relative to the 2.27 µm (c) or a stronger eral bands attributed to Al- and Fe-rich phyllosilicates. 2.27 µm relative to the 2.22 µm (d). These results indi- This “doublet” unit is typically characterized by a pair cate that a simple mixture cannot explain the observed of bands at 2.21-2.23 and 2.25-2.28 µm (attributed to shifts and intensity of spectral features, and at least two OH combination vibrations). This “doublet” deposit types of doublet materials are present. The lab spec- also exhibits an absorption near 1.9 µm due to the H2O trum that most closely matches the doublet material in stretch plus bend combination mode. The band centers spectrum b is an acid leached Fe-smectite (Fig. 2e) [8]. and relative intensity of the doublet features vary The lab spectrum that most closely matches the dou- greatly, suggesting a process such as acid weathering blet material in spectrum c is a soil with mixtures of could be acting on OH-bearing minerals to produce jarosite, Ca-sulfate, and montmorillonite (j) [9]. The altered phases that differ depending on the type of sub- spectrum from d must have more jarosite to explain the strate, water/rock ratio, solution chemistry, and dura- stronger 2.27 µm band. Similar doublet materials at tion of aqueous processes. Mawrth Vallis showed a wider variety of doublet In this study, we focused on the “doublet” deposit properties with bands near 2.21-2.23 and 2.25-2.28 µm observed within central Ius Chasma, specifically along [4,6,10]. Geryon Montes (Fig. 1), which is a central horst brack- eted to the north and south by graben. We examined CRISM, HiRISE, CTX, and HRSC images to deter- mine what morphology and materials correlate to the “doublet” deposit. We also utilized HRSC Digital Ter- rain Models (DTMs) to explore the stratigraphy and elevation associated with the doublet type materials.

Figure 2. CRISM spectra (a-d) from image FRTA202. Spectrum ‘a’ is a hydrated sulfate+smectite. Spectrum ‘b’ has equal strength absorptions at 2.22 and 2.28 µm. Spectrum ‘c’ has a stronger absorption at 2.22 relative Figure 1. CTX mosaic of central Geryon Montes. to 2.28 µm. Spectrum ‘d’ has a stronger 2.28 µm ab- CRISM spectral parameter images are overlain in color sorption relative to the 2.22 µm. Spectra e-j are labora- (R=BD1900R2, G=D2200, B=D2300) and show the tory spectra, where e=acid leached smectite, doublet material (yellow-green) and hydrated sul- f=magnesium polyhydrated sulfate, g=jarosite-gypsum fate+smectite material (magenta) correlate to light- mixture, h=jarosite, i=butlerite, and j=soil with jaro- toned deposits along the Geryon Montes wallrock. site, Ca-sulfate, and montmorillonite. Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6240.pdf

Samples collected in Rio Tinto, Spain exhibit a doublet Marineris, which is why the deposit appears patchy in type spectrum that is attributed to a mixture of Al- distribution. phyllosilicates and jarosite that also matches the Ius Chasma doublet [7]. We also show a material with hydrated sulfate and Fe/Mg-smectite signatures corresponding to different and older materials than the doublet (Fig. 2a). Morphology: HiRISE and CTX images of the doublet material are complex and reveal much variabil- ity within the deposit. Figure 3a shows a clean and bright doublet exposure surrounded by slightly higher and darker doublet material, producing a brecciated appearance in the outcrop. Given that the doublet de- Figure 4. HRSC image 931 DTM perspective view posit occurs along steep slopes, it is plausible that (4X vertical exaggeration) of doublet material (white downslope movement of the deposit due to gravity has arrows) draping the wallrock of Geryon Montes in Ius created the observed brecciated appearance, with the Chasma. Colors are the same as in Figure 1. cleaner, lower exposure representing the intact deposit and the darker upper materials representing the dis- Conclusions: Acidic surface water or groundwater placed doublet deposit that has moved downslope. was proposed [1] to explain the formation of the Ius Rounded and curvy ridges associated with the upper, doublet deposit as either altered Fe/Mg-smectites or darker doublet materials could represent flow struc- precipitated poorly crystalline Fe-SiO2 phases. How- tures produced during this downslope movement. ever, the draping nature of the doublet material and its Another doublet exposure lower in elevation (Fig. occurrence at high elevations along the Geryon Montes 3b) has fine scale fracturing, producing a blockier ap- wallrock, which lacks evidence for fluvial flow, is not pearance than observed in Fig. 3a. These morphologic consistent with either a groundwater or surface source differences in the doublet deposit are consistent with for the water that caused the acidic alteration. the CRISM spectra that show spectral variations within A similar doublet material draping the southern the doublet deposit. The hydrated sulfate+smecite ma- wallrock of western Melas Chasma was proposed to be terial (Fig. 3c) exhibits lineations and color variations airfall material, either atmospheric dust or volcanic that are not observed in the doublet materials. ash, that was altered by ice/snow accumulation along the wallrock slopes [3]. We hypothesize that ice/snow accumulation along the Geryon Montes wallrock slopes influenced and controlled sedimentary deposi- tion by trapping and subsequently altering the ash/dust into the hydrated doublet materials. Local differences in the content of sulfur and/or iron in the deposit or Figure 3. Blowups of HiRISE enhanced color images. changes in pH and redox conditions of the water would (a) Clean and brighter exposure surrounded by slightly result in different amounts of each mineral assemblage, darker and higher-standing materials. (b) Doublet ma- which in turn would explain the spectral and morpho- terial with fine scale fracturing and blocky appearance. logic variations observed in the doublet deposit. White arrows show where a darker floor unit overlies The age of the doublet deposit must be middle to the doublet materials. (c) Lineations and color varia- late Hesperian because it overlies Geryon Montes, tions within the hydrated sulfate+smectite deposit. which formed as part of the Valles Marineris system during the Late Noachian to Hesperian [11,12]. Stratigraphy: We used a HRSC DTM to deter- References: [1] Roach, L.H. et al. (2010) Icarus 206, 253- mine the stratigraphy of the doublet material. Figure 4 268. [2] Weitz, C.M. et al. (2011) Geology 39, 899-902. [3] shows where the doublet material drapes along the Weitz, C.M. et al. (2015) Icarus 251, 291-314. [4] Bishop, slope of Geryon Montes wallrock and extends across J.L. et al. (2016) LPSC 47, #1332. [5] Flahaut, J. et al. (2014) ~1.3 km in elevation. These observations indicate the 8th Mars, #1411. [6] Bishop, J.L. et al. (2019) 9th Mars, this doublet material formed after Geryon Montes and it is volume. [7] Kaplan et al. (2016) Icarus 275, 45-64; [8] likely an airfall deposit. Younger eolian debris and Madejova, J. et al. (2009) Vib Spec. 49, 211-218. [9] Perrin mass wasting material from the wallrock have covered et al. (2019) LPSC 50, #1903; [10] Danielsen et al. (2019) much of the doublet deposit throughout western Valles LPSC 50, #3017; [11] Tanaka, K.L. (1986) JGR 91, E139- 158. [12] Tanaka, K.L. et al. (2014) USGS SIM 3292.