JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. E6, 5042, 10.1029/2001JE001510, 2002 Global mineral distributions on Mars Joshua L. Bandfield NASA Goddard Space Flight Center, Greenbelt, Maryland, USA Received 27 April 2001; revised 29 January 2002; accepted 26 March 2002; published 27 June 2002. [ 1 ] Determining the mineralogy of Mars is an essential part of revealing the conditions of the surface and subsurface. A deconvolution method was used to remove atmospheric components and determine surface mineralogy from Thermal Emission Spectrometer data at 1 pixel per degree (ppd). Minerals are grouped into categories on the basis of compositional and spectral similarity, and global concentration maps are produced. All binned pixels are fit well with RMS 0.005 errors in ofemissivit y. Higher RMS errors are attributed to short wavelength particle size effects on dust-covered surfaces. Significant concentrations (>0.10) of plagioclase, high-Ca pyroxene, sheet silicates/high-Si glass, and hematite are detected and display distributions consistent with previous studies. Elevated concentrations of plagioclase and high-Ca pyroxene are consistent with basaltic surfaces and are located in low-albedo highlands regions 45 °north S. Significant of concentrations of plagioclase and sheet silicates/high-Si glass and low concentrations of high-Ca pyroxenes are consistent with andesitic surfaces and are concentrated in both southern and northern high-latitude, low-albedo regions. Andesitic surfaces in the southern hemisphere have a lower spectral contrast than northern surfaces. An isolated surface located in Solis Planum is spectrally distinct but compositionally similar to other surfaces interpreted to be andesitic in composition. Concentrations of olivine below the detection limit correctly identify its presence in two of three locations. Potassium feldspar, low-Ca pyroxene, basaltic glass, olivine, sulfate, carbonate, quartz, and amphibole are not detected with confidence at 1 ppd. The results presented here indicate a predominance of volcanic compositions within Martian dust-free INDEXsurfaces. T ERM S: 5410 Planetology: Solid Surface Planets: Composition; 5464 Planetology: Solid Surface Planets: Remote sensing; 5470 Planetology: Solid Surface Planets: Surface materials and properties; 5494 Planetology: Solid Surface Planets: Instruments and techniques;KEYWORD S: Mars, infrared spectroscopy, surface mineralogy, remote sensing 1. Introduction maps presented here allo w for recove ry of compositional di st ribut ions. This is di st inct from previ ous efforts that [ 2] An essential part of revealing the presentdetermined and past surface unit distributions, which had spectral conditions of the surface and the subsurface ofsign Marsatu re is s defined before distributio ns were determined determining its mineralogy. Igneous compositions can[ Bandfield pro- et , al.2000a]. The compositions and distributions vide insight into mechanisms such as crustal formatiof on,ma theat er ia ls o n t he su r f ac e hig hli gh t u niq ue as w el l as loss processes, and magma differenti ation. Knowledgefamiliar ofcond itions and processes that have occurred during t hes e com pos it ions ca n al so be use Mars’d t historo co y.nstr It aiis n hopedoth erthat future studies will use the data parameters such as bulk composition and source setmat displayed erials here to help determine this history. for the Marti an soil and dust. Evaporit e, clay, and other sedimentary compositions can provide confirmation 1.1.of pastPrevious Spectroscopi c Mineralogical Work liquid wate r environments on or near the surface. These 1.1.1. Low-albedo regions material s may provide an idea of the temporal and spati al [ 4 ] Low-albedo regions on Mars (lambert albedo of less ex te nt of t he Ma r ti an wea t her ing reg i me. Hyd rot he rm al than 0.20 based on albedo histogramsMellon of et al. alteratio n can al so produce mineral signatures that indicate [2000]) have been known to contain distinct spectral ab- a past water-rich environment. Metamorp hic compo sitions sorptions near 1 andm m 2as well as the 10m m and 20 can be used to constrain the extent to which burial and wavelength regions [Adams, 1968, 1974; Adams and Mc- deformation al mechanis ms have occurred in the Martian Cord, 1969; Singer, 1980; Singer et al., 1979; Christensen, past. 1982; Singer and McSween, 1993; Mustard et al., 1993, [ 3] The purpose of this study is to provide global distri- 1997; Christensen, 1998]. Laboratory and telescopic and b ut i on s o f M ar t i an su r f a ce ma t e r i al s. T he mi ne r al o gi ca l Phobos-2 Infrared Spectrometer for Mars (ISM) data have This paper is not subject to U.S. copyright. been used to determine the presence of Ca-rich pyroxenes Published in 2002 by the American Geophysical Union. and to conclude that olivine is not a dominant mineral phase Paper number 2001JE001510 [Singer, 1980; Singer et al., 1979; McCord et al., 1982; 9 - 1 9 - 2 BANDFIELD: GLOBAL MINERAL DISTRIBUTIONS ON MARS Mustard et al., 1993]. Mustard et al. [1997] investigated the et al., 1973; Aronson and Emslie, 1975; Toon et al., 1977; 1 mm and the 2.2 mm absorptions in ISM data and Clancy et al., 1995]. While the results have demonstrated that argued for the coexistence of variable relative abundances the Martian dust may be composed of materials similar to of both low-Ca and high-Ca pyroxenes within several montmorillonite and palagonite, there are clear discrepancies equatorial dark regions. The presence of these pyroxenes between the Martian spectra and those approximated using is consistent with the compositions of a variety of igneous the optical constants of these materials. compositions, including the olivine-poor SNC meteorites, basalts, andesites, and komatiites. 1.2. Previous Thermal Emission Spectrometer Results [5] Distinct absorptions in the 9 and 20 mmspectral [9] The Thermal Emission Spectrometer (TES) on board regions were observed in Viking Infrared Thermal Mapper the MGS spacecraft has returned data in the 200–1650 cmÀ1 (IRTM) data [Christensen, 1982, 1998]. The shape and (6–50 mm) portion of the spectrum (see a brief instrument depth of the absorptions correlate well with dark regions description below as well as thorough descriptions of the and are consistent with basalt-like sand surfaces. However, TES instrument and operations by Christensen et al. [1992] the coarse spectral resolution makes unique compositional and Christensen et al. [2001a]). A number of studies have determination impossible and atmospheric removal difficult been used to determine the spectral properties and compo- [Christensen, 1998]. sition of the surface of Mars. 1.1.2. High-albedo regions [10] TES aerobraking data from the Sinus Meridiani [6] A great deal of past spectroscopic work has focused region of Mars contain prominent absorptions at 300, on the composition of Martian high-albedo regions (lambert 450, and >525 cmÀ1, which closely match the absorptions albedo of greater than 0.20 based on albedo histograms of coarse-grained, gray hematite [Christensen et al., 2000a]. of Mellon et al. [2000]). Investigations in the visible and Subsequent investigation of the global data set discovered near-infrared (VNIR) portions of the spectrum have been additional concentrations of hematite within Aram Chaos extensive with respect to Martian high-albedo, low thermal and within the Ophir/Candor regions of Valles Marineris inertia (and presumably dusty) surfaces (see review by Bell [Christensen et al., 2001b]. Spectroscopic evidence indi- [1996]) [Bell et al., 2000; Morris et al., 2000]. While the cates that these materials were most likely formed from details of this work are considerable, these surfaces may be precipitation from aqueous fluids, and morphologic features partially characterized by poorly crystalline iron oxides. In such as layered, friable units of these localities suggest a addition, minor absorptions in the spectral shape strongly sedimentary environment [Christensen et al., 2000a, suggest a small amount of crystalline iron oxide (2–4 2001b]. The hematite is distinct from the nanophase and wt%) [Bell et al., 1990; Morris et al., 1997]. red hematite detected in high-albedo regions that form [7] The high-albedo regions of Mars display subtle under weathering and alteration processes. spectral differences as seen by Viking and Phobos-2 orbiter, [11] Inspection of atmospherically corrected [Bandfield Pathfinder lander, and telescopic observations [Soderblom et al., 2000b; Smith et al., 2000] TES data reveals distinct et al., 1978; McCord et al., 1982; Murchie et al., 1993; 400 and 1000 cmÀ1 absorptions in low-albedo regions, Geissler et al., 1993; Merenyi et al., 1996; Bell and Morris, consistent with earlier studies [Christensen, 1982]. An 1999; Bell et al., 2000; Morris et al., 2000]. These results initial study of atmospherically corrected spectra acquired indicate spatial variations in the abundance and, possibly, of Cimmeria Terra revealed a surface composed primarily of the composition of well-crystallized iron oxide materials. plagioclase feldspar and high-Ca pyroxene that closely Spectral detection of sulfate and other evaporite materials in matches the spectral signature of typical terrestrial basaltic the Martian soil and dust remains elusive despite abundant sands [Christensen et al., 2000b]. Global mapping revealed chemical evidence for oxidized sulfur [Toulmin et al., 1977; two major distinct spectral units within Martian low-albedo Clark et al., 1982; Morris et al., 2000] as well as evidence regions [Bandfield et al., 2000a]. Average spectra of these for duricrust material at the Viking and Pathfinder landing two spectral units were isolated and, using both direct sites and from Mars Global Surveyor (MGS) Mars Orbiter comparison with terrestrial samples and deconvolution Camera (MOC) images [Malin et al., 1998]. techniques, were found to match basaltic and andesitic 1.1.3. Atmospheric dust compositions. The basaltic mineralogy was found to be [8] A number of studies have investigated the Martian similar to that of Christensen et al.