Icarus 211 (2011) 157–171 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus The role of aqueous alteration in the formation of martian soils ⇑ Joshua L. Bandfield a, , A. Deanne Rogers b, Christopher S. Edwards c a Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195-1310, United States b Department of Geosciences, Stony Brook University, 255 Earth and Space Sciences, Stony Brook, NY 11794-2100, United States c School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-6305, United States article info abstract Article history: Martian equatorial dark regions are dominated by unweathered materials and it has often been assumed Received 31 December 2009 that they have not been significantly altered from their source lithology. The suite of minerals present is Revised 25 August 2010 consistent with a basaltic composition and there has been no need to invoke additional processes to Accepted 27 August 2010 explain the origin of these compositions. We have begun to question this result based on detailed obser- Available online 15 September 2010 vations using a variety of datasets. Locally derived dark soils have a mineralogy distinct from that of adja- cent rocky surfaces; most notably a lower olivine content. This pattern is common for many surfaces Keywords: across the planet. Previous work using detailed measurements acquired within the Gusev Plains has Mars, Surface shown that olivine dissolution via acidic weathering may explain chemical trends observed between rock Spectroscopy Geological processes rinds and interiors. Mineralogical trends obtained from rocks and soils within the Gusev Plains are more Regoliths prominent than the elemental trends and support previous results that indicate that dissolution of olivine Infrared observations has occurred. However, clear differences are also present in elemental abundances that indicate a variety of inputs and processes are likely responsible for the formation of martian dark soils. Despite the poten- tial complexity of source materials and processes, it appears that most martian dark regions have likely experienced aqueous alteration and chemical weathering appears to be closely linked to the mechanical breakdown of materials. Regardless of the responsible mechanism, there appears to be a general, though not perfect, correlation between elevated olivine abundance and high-thermal inertia surfaces on Mars. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction that would concentrate elements that are typically found in more soluble species (e.g. Clark, 1981; Keller et al., 2006; Gellert et al., A major goal of martian exploration is to understand how much 2006). Other elemental and mineralogical trends are also indicative and under what conditions liquid water has been present through- of chemical alteration under specific conditions (e.g. Hurowitz out martian history. This record of aqueous processes can be re- et al., 2006) and both orbital and in situ observations have pro- vealed from characteristics such as elemental composition and vided an increasingly detailed picture of properties such as the mineralogy, landscape morphology, stratigraphic relationships, temperature, pH, and abundance of water present. and surface textures. Over the past decades, spacecraft have re- Despite this evidence for aqueous processes at many locations, turned data that have enabled construction of at least a rudimen- martian equatorial dark regions (largely coincident with Surface tary global understanding of aqueous processes on Mars using Type 1 of Bandfield et al. (2000)) have been interpreted as these characteristics. unweathered materials that have not been significantly altered Elemental composition and mineralogy can give a positive indi- from the source material (e.g. Bandfield et al., 2000; Mustard cation of the presence of liquid water at the time of formation. et al., 2005). Spectroscopic observations from visible through ther- Although not without ambiguity, elemental and spectroscopic mal infrared (TIR) wavelengths indicate the presence of plagio- observations of Mars have provided a wealth of evidence for aque- clase, pyroxene, and minor amounts of olivine (e.g. Rogers and ous processes. These observations have provided positive evidence Christensen, 2007) throughout low latitude low albedo martian for chemical sedimentary compositions such as sulfates (e.g. surfaces. This suite of minerals is consistent with a basaltic compo- Gendrin et al., 2005) or weathering products such as certain types sition and, given this information, there was no apparent need to of phyllosilicates (e.g. Poulet et al., 2005). Concentrations of ele- invoke additional processes to explain the presence of these mate- ments such as sulfur or chlorine also indicate the presence of water rials on the martian surface. We and others have begun to question this assumption based on detailed observations from a variety of datasets (McSween ⇑ Corresponding author. et al., 2006; Bandfield and Rogers, 2008; Amundson et al., 2008; E-mail address: [email protected] (J.L. Bandfield). Rogers et al., 2009). The work that we present here integrates a 0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2010.08.028 158 J.L. Bandfield et al. / Icarus 211 (2011) 157–171 variety of local and global observations that indicate a link be- gorized as one of three surface types; (1) steep surfaces of crater tween source rocks and dark soils (the term ‘‘soil” as it is used here and canyon walls; (2) crater floors; and (3) inter-crater plains sur- refers to the finer fraction of the martian regolith and does not im- faces. Rocky surfaces were found by Edwards et al. (2009) to be ply organic content) on Mars. Relatively low thermal inertia dark uncommon on Mars, covering much less than 1% of the surface. soils have a distinctive mineralogy from that of local and regional We use the crater floor and inter-crater plains high-thermal high-thermal inertia rocks and rocky surfaces. Investigation of ele- inertia surfaces for comparison with global compositional data. mental and mineralogical relationships between rocks and soils Although steep surfaces are the most common occurrence of high appears to indicate that aqueous alteration is an important process inertia surfaces, their high slope angles, relatively small exposures, in the formation of martian dark soils. and often spur and gully morphology make compositional investi- On Earth, rocky surfaces are typically broken down into finer gations extremely difficult. For this reason, we have not included sediments through a combination of chemical and physical weath- these steep surfaces in the work presented here. ering and soils are typically composed of both primary and second- In addition to global high inertia identifications, thermal inertia ary compositions (e.g. Jenny, 1941). Although similar processes was derived on a per-pixel basis from individual THEMIS nighttime may be present on Mars, soil formation mechanisms remain poorly temperature images. We used a methodology similar to that of understood. There are a number of basic questions that remain lar- Fergason et al. (2006a) and uncertainties are described in detail gely unanswered when considering soil formation mechanisms on in that work. Although parameters such as surface slope and albe- Mars: What are the relative roles of chemical and mechanical do were not incorporated in the thermal modeling at the spatial weathering in martian soil formation? How do processes such as resolution of the temperature measurements, the pre-dawn tem- meteor impacts or aeolian erosion of friable materials contribute peratures have a relatively low sensitivity to these parameters. to soil formation? Most martian low albedo regions have proper- The methodology used here is well suited for the purpose of ties consistent with silty and sandy soils (e.g. Kieffer et al., 1977) distinguishing dusty, sandy/silty, and rocky surfaces (Fergason and there are basic and fundamental outstanding questions about et al., 2006a). The high resolution thermal inertia data was used the formation and development of these surfaces that cover a sub- for comparison with spatially coincident compositional informa- stantial portion of Mars. tion (described below). 2.3. THEMIS compositions 2. Data and methods Daytime multispectral THEMIS data were used to assess the 2.1. Themis composition of the surface. Although the limited spectral informa- tion prevents retrieval of detailed mineralogical information, spec- For this work, we used 2001 Mars Odyssey Thermal Emission tral units can be linked to lower spatial resolution but higher Imaging System (THEMIS) nighttime temperature data for the der- spectral resolution Thermal Emission Spectrometer (TES) data. This ivation of surface thermophysical properties and daytime multi- type of analysis leverages the capabilities of both datasets and has spectral data was used to derive surface compositional been effectively used in previous work (e.g. Bandfield et al., 2004a; information. THEMIS has a thermal imaging subsystem that con- Rogers et al., 2005; Schnieder and Hamilton, 2006; Rogers and sists of a 320 by 240 pixel uncooled microbolometer array. Multi- Bandfield, 2009). spectral images are acquired in a pushbroom fashion using filters Olivine-rich surfaces are readily identified in decorrelation in nine spectral bandpasses between 7 and 15 lm. Spatial sam- stretch (DCS; Gillespie et al.,