P13D-1549
Global Mineral Maps of Mars: Examination of Compositional Variation Within Solid Solution Series
We have produced updated global mineral maps from Mars Global Surveyor Thermal Emission Spectrometer (TES) infrared data using a spectral library that includes spectra of important phases that were not available in previous mapping efforts [1]. Our refined global mineral maps are broadly similar to those published previously using TES data, but improvements in our spectral library and discernible spatial correlations give us greater confidence that we can examine the distribution of not just mineral groups, but the distribution of phases within solid solution series. For example, variations in the abundances and distribution of several compositions across the Mg-Fe olivine series have been observed and confirmed manually [2]. We have tentatively identified pigeonite for the first time on a global scale, as well as larger fractions of orthopyroxene than were identified in previous TES-based mapping; these results have greater consistency with visible to near infrared (VNIR) data. Furthermore, some low-Ca pyroxene compositions appear to have distributions that correlate with Noachian (and Hesperian) terrains. High Ca (clino-) pyroxenes are dominated by intermediate Mg-Fe compositions (augite), and in particular, by a phase that has relatively high Ti content [3]. We observe spatially correlated distributions of compositions within the sulfate and phyllosilicate groups, although these remain to be confirmed through manual examination. Serpentines are not identified at detectable levels, suggesting that if they are present, their abundances are small (e.g., < ~10%). Noachian terrains are where many phases produced by aqueous alteration have been identified in VNIR data; regional variations in primary phase composition and abundance within these terrains almost certainly play a major role in determining what alteration phases are produced and at what abundances. Global mineral mapping at Mars thus provides critical context for understanding local-scale mineralogical variations, such as those that appear related to aqueous weathering of igneous lithologies in Noachian terrains. [1] Bandfield, J.L., JGR, 107, doi:10.1029/2001JE001510, 2002. [2] Koeppen, W. C. and V. E. Hamilton, JGR, submitted. [3] Hamilton, V.E., JGR, 108, doi:10.1029/2003JE002052, 2003.
P13D-1550
Petrogenesis of Alkaline Magmas in Gusev Crater, Mars
The Spirit rover has encountered a variety of volcanic rocks in Gusev crater, including picritic basalts and different types of alkaline basalts. Phase equilibrium experiments [1] suggested that the picritic basalts (Adirondack- class) represent batch melts of a primitive Marian mantle compositionally similar to the Dreibus-Wänke (DW)- mantle [2]. Here we demonstrate that the Gusev series of alkaline basalts also represent products by melting of the DW-mantle. Three melt compositions obtained by partial melting experiments of the DW-mantle [3] were used as parental melt compositions; their degrees of melting range from ~10% to ~30%. Liquid lines of descent for these melts are calculated by the MELTS program for both fractional and equilibrium crystallization over a wide range of parameters (P = 1 bar to 5 kbar, H2O = 0-1 wt%, fO2 = QFM-3 to QFM). These parameters affect liquidus temperatures and stabilities of minerals, but do not significantly change the compositional trends of liquids when spinel and olivine are the only crystallizing phases (except for Cr). In the spinel+olivine liquidus fields, the MELTS program yields the compositional trends almost identical to the results from the melting experiments [1]. Ten rock samples from three rock classes (Irvine, Backstay and Barnhill) are compared with the MELTS results, because they represent alkaline basalts that do not contain significant amounts of xenocrystic and/or cumulus materials and have experienced relatively low degree of aqueous alteration. Because most of these alkaline basalts are enriched in incompatible elements (e.g. Ti, Na) but have similar MgO contents compared to those of the Adirondack-class, they could not have formed by fractionation of a parental magma similar to the Adirondack- class rocks. Instead, these alkaline basalts can be explained by fractionation of olivine and spinel from the lower- degree melts of the DW-mantle than the Adirondack-class. Thus, all the Gusev basalts (both picritic and alkaline) could have formed by different degrees of melting of the same DW-mantle. Differences in the degree of melting may reflect temporal or spatial thermal structure of adiabatic mantle upwelling beneath Gusev crater. [1] Monders et al. (2007) MPS 42, 131-148. [2] Dreibus and Wänke (1985) Meteoritics 20, 367-382. [3] Bertka and Holloway (1994) CMP 115, 323-338.
P13D-1551
The Martian lithosphere in the Tharsis region: A comparison between MEX gravity data and the MOLA topography model
The first European Mars Mission, Mars Express (MEX), is operating in orbit around Mars since Januar 2004. The Mars Express Radio-Science Experiment (MaRS) is performing gravity measurements above selected target areas during the pericenter passes at an altitude from 250 km to 350 km. MEX has a much higher sensitivity to gravity attractions at small scales than the NASA mission Mars Global Surveyor (MGS) due to this low pericenter altitude. A total of 70 Doppler observations above selected target areas could be recorded at the ESA ground station in New Norcia and at the antennas of the Deep Space Network (DSN). Profiles of the gravitational acceleration could be computed after low-pass filtering. These residual accelerations will be compared with the MOLA topography model from MGS by computing the cross-correlations between these both datasets for all gravity operations belonging to the same target area to make a statement about the compensation status of the particular local and regional Martian lithosphere respectively.
P13D-1552
Was there a period of global contraction on Mars?
It has been suggested that a period of global contraction occurred on Mars during the Late Noachian-Early Hesperian in association with basaltic resurfacing and wrinkle ridge formation around Tharsis. To evaluate this hypothesis, we calculated the horizontal strain for all faults, both thrust and normal, using the comprehensive MOLA-derived Knapmeyer database and Kostrov's formula. Strains for the faults were then used to calculate the global radius change, assuming the source of stress was global contraction alone as has been done for Mercury, but also including the globally averaged contribution of normal fault strain. These calculations were done for both the proposed Late Noachian-Early Hesperian pulse of global contraction and the entire span of Martian history. Compressional faults were assumed to be either all thrust faults (with larger offsets and cutting the Martian surface, corresponding to an upper bound on the strain) or wrinkle ridges (with superjacent folding, smaller offsets, and no intersection with the surface, for a lower bound). Four scenarios were evaluated: wrinkle ridges for the entirety of Mars history, wrinkle ridges during the Late Noachian-Early Hesperian, thrust faults for the entirety of Mars history, and thrust faults during the Late Noachian-Early Hesperian. Surface strains are -0.023% for the wrinkle ridges only during the Late Noachian-Early Hesperian, -0.028% for wrinkle ridges only over the history of Mars, -0.145% for thrust faults during the hypothesized global contraction pulse, -0.181% for all thrust faults, 0.045% for normal faults during the Late Noachian-Early Hesperian, and 0.087% for normal faults only over the entire history. The only decreases in radius, ~1.6 km, were obtained for both the Late Noachian-Early Hesperian pulse and over Mars' history with all compressional structures assumed to be thrust faults, in conflict with the observations of Tharsis-related structures being wrinkle ridges rather than deep-seated thrust faults. Our results show that extensional strains associated with normal faults and other sources of stress such as Tharsis may have masked any signature of global contraction.
P13D-1553
Spatial and Compositional Variations of Olivine in Terra Tyrrhena, Mars
Global modeling of thermal and VIS/NIR spectra of the Martian highlands reveal that many materials contain 10 - 20% olivine that spans a relatively large range of Mg/Fe contents (forsterite numbers 100 - 35). We examine the olivine-bearing locales in Terra Tyrrhena using TES-derived mineralogies, spectral variation maps made from THEMIS IR images, THEMIS and MOC visible images, MOLA topographic profiles, and geologic maps. Basaltic materials containing up to 20% forsteritic olivine (>Fo85) are exposed in knobs and craters near the Hellas rim (71E, 24S); the stratigraphic layer is >550 meters thick and may represent very thick, olivine- bearing flows or the base of an exhumed pluton. Mg-rich (Fo70) olivine is ubiquitous throughout Terra Tyrrhena, but locally high abundances occur in crater ejecta and alluvial fans that spill into craters. This suggests that Mg-rich olivine is a common component of the Martian crust that is in situ primarily in the subsurface, and this composition of olivine is commonly brought to the surface by exhumation via craters. Olivine-bearing materials with intermediate compositions (Fo50) are distributed within flat-lying intercrater flows that embay eroded ridges and knobs. The flows appear younger relative to the Noachian terrain, though they have also been subsequently eroded. Intermediate compositions predominantly occur closer to Syrtis Major, but also appear in small locations near the Hellas rim. Fe-rich olivine (<Fo40) is uncommon in Terra Tyrrhena and is confined to flat-floored craters that are filled with lava or sediments. Fe-rich olivine also commonly occurs in conjunction with more Mg-rich olivine compositions and may represent the spectral modeling of intermediate compositions, zoning within olivine, or Fe-rich groundmass in the basalts of Terra Tyrrhena. Our results suggest that many Mg-rich compositions of olivine occurring in Terra Tyrrhena today may have formed very early in the Noachian and survived weathering during early wet periods by residing in the subsurface. Intermediate and Fe-rich compositions are in place, younger (late Noachian or later), and may have never seen water at the Martian surface.
P13D-1554
Identification and Mapping of Olivine-Rich Basalt Bedrock Outcrops in Ganges and Eos Chasma on Mars
An extensive olivine-rich basalt unit has been identified and mapped in Ganges and Eos Chasma on Mars using Thermal Emission Imaging System (THEMIS) and Thermal Emission Spectrometer (TES) data. A 100 m/pixel THEMIS decorrelation stretch mosaic was created using bands 8, 7, and 5 (11.79 μm, 11.04 μm, and 9.35 μm respectively) as the olivine-rich unit has a deep absorption in THEMIS band 7 data. These outcrops were mapped spatially using this mosaic and were found to extend for more than 1,100 km in length. In addition to the THEMIS data, TES data were used to constrain the composition and abundance of the olivine. By using spectral ratios of the unit and of the surrounding canyon floor material, it is possible to determine that the olivine abundance of this unit is ~10-15% and its composition is estimated to have a Mg/(Mg+Fe) ratio of ~68. In addition to compositional data, this unit has been characterized using elevation data from the Mars Orbiter Laser Altimeter (MOLA). By using MOLA elevation data in conjunction with the compositional data, constraints on the continuity, extent, dip, and orientation of this layer can be made. These data indicate that this unit lies on a nearly uniform surface, dipping slightly (~0.013 degrees) to the northeast. They also indicate that it persists through topographically high areas and outcrops only in predicted locations. Estimates of the thickness of this unit were made utilizing MOLA data, with thickness ranging from ~40 m to ~110 m, illustrating that this layer is thin in comparison to the ~4.5 km of overlying material. High-resolution imagery from the Mars Orbiter Camera and the High Resolution Imaging Science Experiment provide insights into the small-scale morphologies associated with these outcrops, which appear rough, pitted, and largely unmantled with a slightly higher albedo than the surrounding materials. These observations are consistent with THEMIS thermal inertia for an in place rocky unit, with values of >~300-600 JK-1m-2s-1/2. One of the largest in situ, stratigraphic layers identified on Mars has been mapped and characterized using a variety datasets. Several possible emplacement mechanisms for this unit include a basaltic sill, a rifting event, or more likely large flood lavas or a volcaniclastic deposit.
P13D-1555
Possible Volcanic Province in Western Promethei Terra, Mars
The western Promethei Terra region (36-50°S, 90-106°E) studied is roughly ~700 km across. It occupies a unique area on the smoothened eastern Hellas basin rim, and consists of two parts: a gentler (~0.07°, eastward of ~97°E) and a steeper (~0.88°, W of ~97°E) regional slope. The Noachian cratered terrain surrounds the region in the NE, E, and S. The large canyons of Harmakhis, Reull and Teviot Valles cut through the central area and the smooth Hesperian plains [1-10] of the western and central areas display a set of features that does not occur elsewhere on the eastern side of the Hellas basin. The plains have multi-layered interiors as seen on the walls of the canyons that cut them. Similar stacks of sub- horizontal layers are seen in other Martian regions with exposed lava plain interiors, for example, in Lunae Planum and Syrtis Major. These are classic volcanic provinces the layered structure of which was formed by successive emplacement of sheet lava flows that followed the general topographic trend. The average visible thickness of the Promethei layers is ~70-80 m and the typical measured canyon wall slope is ~25-30o. This gives an estimate of the thickness of the layers, which is ~35-45 m. The full layer stack thickness, estimated from observations, is ~1.3 km. Consistent with the observed layering, there are narrow wrinkle ridges (WR) that deform the surface of the plains. WR mostly occur in the eastern portion of the area near Reull and Teviot Valles but some of them are seen near Harmakhis Vallis in the west. Additional long straight narrow ridges (widths < km, heights 10s m, lengths 10s km), which occur in mostly NE-SW-oriented groups, are seen on the surface of the plains. The regional topography does not appear to control the distribution of the ridges. Their morphologic characteristics, areal distribution, and close association with the lava plains are consistent with and suggest that the straight ridges may represent exhumed dikes [11], which have served as feeders for the lava plains. The identified features and layered structure suggest that the regional basement material is of volcanic origin. The lava sheets have experienced post-emplacement compression and dike injections. The approximate volume of the layered material in this region is estimated to be ~0.3 x 106 km3 and the time of emplacement of the material may correspond to the Late Noachian-Early Hesperian epochs. References: [1] Potter (1976) USGS I-941. [2] King (1978) USGS I-1073. [3] Scott & Carr (1978) USGS I-1083. [4] Scott &. Tanaka (1986) USGS I-1802-A. [5] Greeley & Guest (1987) USGS I-1802-B. [6] Crown et al. (1992) Icarus 100, 1-25. [7] Price (1998) USGS I-2557. [8] Mest & Crown (2002) USGS I-2730. [9] Mest & Crown (2002) USGS I- 2763. [10] Leonard & Tanaka (2001) USGS I-2694. [11] Head et al. (2006) Geology 34, 285-288.
P13D-1556
Nitrates on Mars: Evidence from the 15/14N isotopic ratio
I. The 14/15N isotopic ratio of nitrogen in the bulk atmosphere of Mars is 170 +/- 15, while primitive nitrogen in SNCs is about 278, a fractionation factor of 1.62. Fractionating effects imply the 14/15N ratio of loss to space is between about 265 and 296 [1,2], consistent with primitive N. Estimates of N escape, 3--8E5 cm-2 s-1, imply the current time-scale for its removal is between 500 and 1300 Myr. These findings imply a source of primitive nitrogen, and a steady state fractionation, as suggested by Wallis [1]. However, our modeling of juvenile outgassing predicts an outgassing rate an order of magnitude lower than modeled escape. Here, we suggest that nitrate decomposition by impacts is the source. II. Judging from estimates of the water inventory [3], and comparison to Earth, Mars probably had between 250 and 600 mbars of N2 [1]. With the approximate 20:1 C:N ratio, this suggests between about 5 and 10 bars of CO2. During the period of intense bombardment, atmospheric nitrogen is subjected to shock heating which allows equilibrium reactions between the dissociated atoms [4,5]. Simulation of impact processes under the evolving impact flux [6] suggests that about a quarter of the initial, atmospheric nitrogen is fixed, forming nitrates in the soil -- about 60-150 mbars. III. During Mars' history, fractionating loss of N to space increases the bulk atmospheric fractionation. However, in analogy to carbonates [7], nitrates may be decomposed by the shock/heating of impacts, a process that moderates 14/15N. Nitrates are decomposed to a radius about twice that of the impactor, providing a slow recycling of N. IV. However, the distribution of nitrates in the soil affects the quantity needed to explain the current 62% fractionation. This is because the distribution of impactors is heavily weighted towards the low-mass end [8]. A concentrated surface deposit minimizes the amount of nitrates needed, while deep nitrates are out of reach of the numerous, small impactors. But nitrates are highly soluble, and are probably leached from the surface, as chlorides are at Endurance or nitrates at Atacama. If nitrates become entrained in the hydrosphere, the formation of the cryosphere would tend to produce regions of concentrated brines, perhaps near the surface where nitrate concentrations would be high. Much of the nitrates may have been washed into the northern lowlands. V. If 100 mbars of nitrates exist in the uper crust, a model consisting of a layer 100 m deep with 10% mass fraction of NaNO3 is plausible. The corresponding mass fraction of N would be 1.6% in the layer. References: [1] {Wallis}, M.~K., 1989. E&PSL, 93, 321--324, [2] Fox, J.~L., 1993. J Geophys Res, 98, 3297-3310, [3]Carr , M.~H., 1986. Icarus, 68, 187--216, [4] {Mancinelli}, R.~L. and {McKay}, C.~P., 1988. Origins of Life, 18, 311-325, [5] {Navarro-Gonz{á}lez}, R., et al. \nat, 412, 61-64, [6] Manning, C.~V., et al.,(2006). Icarus, 180, 38--59, [7] {Carr}, M.~H. 1989. Icarus, 79, 311--327, [8] Brown, P. et al., 2002. Nature 420, 294-296.
P13D-1557
Geologic context and characteristics of bedrock exposures in Mare Serpentis: Implications for martian surface evolution
Examining the spatial and stratigraphic relationships, geologic context, and composition of martian surface units contributes to our understanding of martian surface processes, as well as to the reconstruction of geologic history for a given region. Here we report observations on exposures of bedrock and/or blocky materials (hereafter, "high-TI" units) that have been identified in the low-albedo region of Mare Serpentis, Mars. These units occur in intercrater plains as well as crater floors, and exhibit THEMIS-derived thermal inertia values ranging from ~550 to >1200 J m-2K-1s-1/2. They are compositionally distinct from surrounding low TI plains and crater ejecta, with higher pyroxene abundance, and lower plagioclase abundance, than the surrounding plains. Additionally, a few of the high-TI exposures do not appear spectrally-uniform; rather, they exhibit isolated areas of less mafic, lighter-toned material; these unique areas are smaller than can be resolved in TES or OMEGA data. The high-TI exposures commonly superpose the ejecta from large diameter (>40 km) craters, indicating that they are younger than the earliest martian crust in this region. Stratigraphic relationships observed in high- resolution imagery are not clear. In some areas, it appears that plains materials have been transported across the margins of the high-TI units, whereas in others, erosion has removed some of the plains sediment leaving the margin of the high-TI unit apparently overlying the plains. Possible geologic scenarios for the formation of these compositionally distinct materials, and for their relationship to the surrounding plains, are currently being investigated. For example, the low inertia plains materials might be related to the high-TI units through a chemical or physical weathering process. Alternatively, the high-TI units may be unrelated to the surrounding plains, potentially implying a volcanic origin. Evidence for and against each scenario will be presented. Determining the true relationship between these materials may have implications for surface evolution processes here and elsewhere on Mars.
P13D-1558
Phyllosilicate identification in Mawrth Vallis: an analysis of CRISM multispectral data and targeted images FRT4ECA and HRS307A
Large exposures of Al- and Fe/Mg-phyllosilicates have been identified in the Mawrth Vallis region using data from OMEGA (Poulet, F., Bibring, J.- P., Mustard, J. F., Gendrin, A., Mangold, N., Langevin, Y., Arvidson, R. E., Gondet, B. & Gomez, C., 2005, Nature, 438, 632-627; Noe Dobrea and Michalski, 2006, AGU Fall Meeting 2006, #P23D-009; Loizeau et al 2007, JGR, 112 (E8), 10.1029/2006JE002877) and CRISM. Regional mapping with CRISM multispectral data shows an excellent correlation with OMEGA results. The higher-resolution CRISM multispectral mapping data improve upon the OMEGA detections, finding additional, smaller deposits. We have identified several Al- and Fe/Mg-phyllosilicate minerals in three 5x5 degree multispectral map tiles projected at 256 pixels per degree and high-resolution hyperspectral images HRS307A and FRT4ECA. HRS307A lies within the southern part of the main channel and contains units exposed by channel creation, allowing us to study its depositional history. FRT4ECA is northeast of HRS307A, just south of the northern part of the main channel and contains knob-type features not seen in abundance in other Mawrth images. In FRT4ECA, Fe/Mg-phyllosilicates such as nontronite have been identified using the characteristic absorptions at 2.41um, 2.28um, 1.91um, and 1.42um and Al-phyllosilicates such as montmorillonite by the characteristic absorptions at 2.21 um, 1.91 um, and 1.41um. These two minerals occur in two distinct layers and are consistent with mineral species identified in other regions of Mawrth, implying wide-spread deposition in an aqueous environment rather than local hydrothermal alteration. Smaller exposures of other phyllosilicates such as kaolinite and ferrosaponite have also been observed indicating a variety of aqueous processes in this region.
P13D-1559
Layering of Al- and Fe/Mg-phyllosilicates in Western Mawrth Vallis, Mars, and Implications for Aqueous Processes During the Noachian Period
Phyllosilicates are abundant in the Mawrth Vallis outflow channel region and occur in layers, knobs and craters. This project builds on previous studies of the Mawrth Vallis region using OMEGA spectra [1,2,3], where a combination of hydrated material, Al-rich phyllosilicates, and Fe-rich phyllosilicates were observed. For this study we are focusing on the region west of the major outflow channel where phyllosilicates occur in discrete layers associated with the stratigraphy. Mars Express OMEGA and Mars Reconnaissance Orbiter CRISM and CTX images are investigated in parallel in order to identify specific phyllosilicate minerals and map these within the Noachian terrain. CRISM multispectral data sampled at 256 pixels per degree and map projected into 5-degree tiles, plus high resolution hyperspectral images HRL43EC and FRT672C are available to date and have been analyzed here. We observe three distinct types of phyllosilicate-bearing layers. One is an Al-phyllosilicate with a band near 2.20-2.21 um that is consistent with montmorillonite and kaolinite, and another is an Fe/Mg- phyllosilicate exhibiting bands at 2.30 and 2.39 um, which are consistent with ferrosaponite. A third layer is characterized by a broad Fe2+ slope, possibly due to ferrous mica, and a broad band near 2.2-2.27 um that could be due to hydrated silica or glass or poorly crystalline AlSi-bearing phases. Although montmorillonite is the spatially dominant Al-phyllosilicate over most of the Mawrth Vallis region, kaolinite is also identified in ~100 m x 100 m patches in some areas. The presence of kaolinite south and west of the mouth of the valley, but not further south along the outflow channel [4], possibly indicates longer periods of aqueous activity and more highly altered material in the western region. The presence of ferrous mica would also imply the presence of more water than other clay minerals commonly require. In many locations the Fe2+-bearing layer often occurs in between the Al-phyllosilicate and Fe/Mg-phyllosilicate layers. The stratigraphic sequence consists of, from bottom to top: (i) a dark and relatively smooth Fe/Mg-rich phyllosilicate layer that is partially covered by (ii) a brighter and rougher layer containing a ferrous material plus a hydrated phase, then (iii) a bright layer with a rough surface containing Al-phyllosilicate, and finally (iv) a dark, rough-textured mantling unit on top. One of the simpler explanations for the occurrence of these three phyllosilicate-bearing layers is changes in the aqueous chemistry and/or depositional environment over time during the Noachian period such that different phyllosilicate minerals formed and were deposited in layers. [1] Poulet et al. (2005) Nature, 438, 632-627. [2] Loizeau et al. (2007) JGR, 112, E08S08, doi:10.1029/2006JE002877. [3] Noe Dobrea and Michalski (2006) AGU Fall Mtg, abs.#P23D-0091. [4] Wray et al. (2007) 7th Mars Conf., abs.#3119.
P13D-1560
The extent of phyllosilicates in the northern highlands around Mawrth Vallis: CRISM observations of Western Arabia Terra
The largest spatial exposure of phyllosilicates on the surface of Mars occurs on the highland plains around Mawrth Vallis. It extends for about 300 kilometers southward from the edge of the dichotomy boundary, covering an area of over 200 x 300 km (e.g., Poulet et al 2005; Noe Dobrea and Michalski 2006; Loizeau et al 2007) over an elevation range of ~2000 m. At least two different types of phyllosilicates (Fe/Mg smectites, and Al smectites), presenting complex spatial and stratigraphic relationships, have been identified in OMEGA data. Stratigraphically, the phyllosilicate-bearing units appear exposed from under a darker, indurated, and more heavily cratered unit, suggesting that it has recently been exhumed (Michalski and Noe Dobrea, 2007) from under an overlying unit. It is unclear, however, whether the clay-bearing units extend into the southern highlands and into the northern lowlands. In this investigation, we use both OMEGA spectral cubes (~1 km/pixel) and CRISM multispectral data sampled at 256 pixels per degree and map projected into 5-degree tiles (~230 m/pixel) to further constrain the extent of phyllosilicates in the Mawrth Vallis region. We find a good correlation between the spatial distribution of phyllosilicates as detected by CRISM and OMEGA, where coverage overlaps. We identify additional small, localized phyllosilicate exposures in CRISM tiles in places up to 100 km farther than the boundaries previously identified with OMEGA. These exposures are found around craters to the south, in interior deposits of a crater to the east, and in association with hummocks in the lowlands to the north. Additional phyllosilicate signatures are observed in darker materials at the base of the escarpment that constitutes the dichotomy boundary in this area. The exposures show spatial variability of the 2.2- and 2.3-μ m absorptions, suggesting complex spatial and stratigraphic relationships similar to those previously observed in the largest exposures. The identification of phyllosilicate exposures in the southern highlands may suggest that the clay-bearing unit observed around Mawrth Vallis is in fact an exposure of a much more extensive clay unit that may underlie at least part of the southern highlands. Additionally, the identification of phyllosilicates in some of the hummocks of the northern lowlands suggests that these hummocks may be erosional remnants of a formerly more extensive unit, leading to the possibility that the dichotomy boundary has erosionally regressed.
P13D-1561
Comparisons between phyllosilicate-bearing strata at Mawrth Vallis and Terra Meridiani
Recent observations of ancient terrains with phyllosilicate minerals and somewhat younger terrains with layered sulfate-rich rocks have led to the suggestion of reordering the nomenclature of Martian geologic epochs so that they are associated with the time periods when these materials were formed. The earlier "Phyllosian" era would be associated with the time that the phyllosilicate minerals were formed. This would correspond to the current early Noachian. This would be followed by the "Theiikian" era which is when the sulfate minerals were formed and corresponds to the late Noachian/early Hesperian. Large exposures of phyllosilicate-rich rocks have been observed at a number of areas including the Mawrth Vallis region. Sulfate-rich rocks have been directly sampled at Meridiani Planum by the Opportunity rover and mapped from orbit. Several distinct spectral/stratigraphic units have been mapped at Mawrth Vallis based on the presence of H2O and metal-OH absorption features. These include materials with a 2.2 micrometer Al-OH feature, nominally attributed to montmorillonite, with a 2.3 micrometer Mg or Fe-OH feature, nominally attributed to nontronite, and materials with a 1.9 micrometer H2O but no discernable metal-OH band. Sulfate minerals have not been identified at Mawrth Vallis. These spectral units, based on short-wave infrared features, also tend to have distinct visible and near infrared color properties which are mappable, at higher spatial resolutions, by color HRSC data. Examination of these color units with high resolution HiRISE images indicates that they have distinct textural properties as well. The recent discovery of exposures of phyllosilicate-bearing rocks in eastern Terra Meridiani invites comparisons with the phyllosilicate-bearing rocks in Mawrth Vallis. Initial examination of HiRISE images of the eastern Meridiani phyllosilicate region indicates different surface textures for the Meridiani phyllosilicate region versus the Mawrth Vallis region. The latter has smooth to fractured surfaces while the former has a rougher, more knobby texture. Multispectral, hyperspectral, and thermal emissivity characteristics of the two regions are also being examined using color HRSC, hyperspectral OMEGA, CRISM, and TES and multispectral THEMIS data.
P13D-1562
Sedimentary Fan Deposits in Jezero Crater Lake, in the Nili Fossae Region, Mars: Meter- scale Layering and Phyllosilicate-Bearing Sediments
Sedimentary deposits that are clearly related to valley networks on Mars are relatively unusual and have been recognized in only a few locations on Mars. However, such sediments have the potential to preserve an excellent record of the surface environment at the time of their emplacement, including data on the potential habitability of the surface, which makes them important targets of study for both orbital and landed missions. An exceptional example of valley network-derived sedimentary deposits is located in Jezero crater, a ~40-km diameter impact crater, northwest of the Isidis Basin near the Nili Fossae (centered at 77.6 E and 18.4 N). Two valleys debouch into this crater from the west and north, and each valley appears to have deposited a sedimentary fan. On the eastern end of the crater, an outlet valley is observed, and based on the elevation of the outlet valley, Jezero crater must have contained a sizeable lake at one time, with a volume of at least 250 km3, of comparable volume to terrestrial Lake Tahoe (V ~ 160 km3). The fan deposits have been interpreted as deltaic sediments, deposited into this lake. New data from MRO and other recent missions has motivated the reexamination of Jezero crater and its sedimentary deposits. Meter-scale layering is commonly observed on fan deposits in HiRISE data. Layers appear to dip gently (<10-15°) and outcrop in sinuous patterns. Many unconformities are discernable between packets of the layered material. We interpret these sediments as having been deposited via lateral accretion during meander migration, with observed unconformities resulting from numerous channel switching events on the fan. CRISM visible/near-infrared observations of the fan materials indicate that where these light- toned, finely-layered packets of sedimentary material are exposed, they commonly have spectra consistent with iron-magnesium phyllosilicate. Phyllosilicates are also found on the Jezero crater interior off the margins of the primary deposits. Given both the unique geological setting and mineralogy of the Jezero crater fan materials, it is a prime target for future landed missions, such as MSL.
P13D-1563
Discovery and Distribution of Chloride-Bearing Deposits in the Ancient Cratered Terrain of Mars From THEMIS
We have identified and mapped a number of spectrally distinct deposits (~250) in mid-infrared data acquired by the 2001 Odyssey Thermal Emission Imaging System (THEMIS). These deposits are interpreted to contain a chloride salt component based on their spectral signatures in THEMIS and Mars Global Surveyor Thermal Emission Spectrometer (TES) data. Thermal inertia derived from THEMIS nighttime observations, indicate that the chloride-bearing materials are possibly cemented or indurated. Individually, most chloride-bearing deposits are small in area (<~25 km2), but they are globally widespread, as we have identified them throughout low albedo regions of the southern highlands of Mars. These regions correspond to mid-to-late Noachian terrains, as well as early Hesperian ridged plains units. The chloride-bearing deposits commonly occur in topographic lows relative to the surrounding terrain, and some appear to follow channel-like outlines. Less typically we observe them in small crater floors. Images acquired by the Mars Orbiter Camera (MOC) and the High Resolution Imaging Science Experiment (HiRISE) indicate geomorphology consistent with formation in an evaporitic environment. HiRISE imagery (at 25.3 cm/pixel) over a large exposure in Terra Sirenum shows the chloride-bearing materials to be light-toned and highly fractured. The fracturing is sub-polygonal and is similar to desiccation cracks in evaporitic environments. Cross cutting relationships indicate that the chloride-bearing materials are older than the surrounding basaltic materials, and commonly appear to have been exposed by erosion. In the HiRISE image we observe additional occurrences of chloride materials within the regional terrain, indicating that the chloride deposits are likely more extensive than what is discernable at THEMIS IR scales. The origin and diagenesis of each chloride deposit is likely complex, however it is probable that water played a role in each instance, either via direct precipitation of ground water or standing water, or via efflorescence from evaporative pumping, volcanic out gassing, or atmospheric-surface interactions. Many chloride salts are extremely hygroscopic and can be further modified by fluctuations in humidity. Identification of a hygroscopic chloride could indicate brine activity subsequent to initial deposition. The identification of another water-related material in the ancient cratered terrain of Mars is further evidence that Mars once had a hydrologic cycle (albeit possibly short in duration) that was more active than at present.
P13D-1564
The Martian Soil Formation Experiments by Hydrothermal Alteration of Basaltic Rock
Soil formation processes on terrestrial planets have an important role on evolution of environments of the planets. In this study, we carried out hydrothermal alteration experiments to elucidate the soil formation processes on the Martian surface. Major rock of the Martian crust is iron-rich basaltic rock, and subject to sulfuric acid-bearing hydrothermal alteration. The experiments are carried out on the starting material of the simulated Martian basalt with H2SO4 solution in hydrothermal conditions. Experimental temperatures and pressures are 100 ~ 300°C, water vapor pressure, and 400 ~ 500°C, 100 MPa. Acidity of the solution and fluid-rock ratio are pH1.5 or 3.0, and 50:1 or 10:1 (100 ~ 150°C), 10:1 or 2:1 (200 ~ 500°C). Run durations are 1 or 4 weeks. Composition of the starting material is referred to the average Martian surface composition analyzed by Mars Pathfinder probe. The starting material is prepared from a basaltic rock, iron rich dunite and peridotite. In the run products, olivine grains reacted with low pH fluid at low temperature condition including 100 ~ 150°C. Characteristic phases in the run products are hematite and clay minerals. Hematite occurs in products at 100 ~ 150°C and 400 ~ 500°C. More hematite grains occur in products with pH1.5 and larger fluid-rock ratio. Morphology of the hematite is granular to spheroidal with diameters of 0.5 to 3 micron meters in products at 100 and 150°C. Hematite in products at 400 ~ 500° is euhedral crystals with the diameter of 5 to 20 micron meters. Clay minerals with Fe-rich rim substituted olivine crystals were formed under low pH and over 300°C. Hydrothermal alteration of mafic minerals including olivine with sulfuric acid-bearing fluid occurs efficiently at lower pH and higher fluid-rock ratio. The characteristic products of this alteration are hematite and clay minerals. Acidic hydrothermal alteration may have essential role to form the Martian soil rich in iron oxide. Especially, morphology of hematite is strongly depends on temperatures of the hydrothermal alteration. Direct observation of the Martian soil may provide us information on the conditions of hydrothermal alteration which occurred on the Martian surface.
P13D-1565
CRISM Analyses of Noachian Stratigraphy in Argyre Basin
The Argyre basin is a >1500 km, well preserved impact basin in the southern highlands of Mars and the geologic units associated with the basin are mostly Noachian in age (Scott and Tanaka, 1986). Thus, Argyre is an ideal location to characterize the stratigraphy of ancient highland rocks. We analyzed 72-channel CRISM multispectral data that had been map projected to 256 ppd into 40 5-degree map tiles. The data were corrected for illumination by dividing by the cosine of the solar incidence angle. A multiplicative correction for atmospheric gas absorption was applied (Bibring et al., 2005). Spectrally distinct regions were identified by calculating summary parameters (Pelkey et al., 2007); spectra of key areas were examined in detail. A scarp is roughly associated with a contact between the Hpl3 and Npld units in NW Argyre. At the top of the scarp (unit Hpl3) CRISM has detected an olivine signature associated with a depression. Phyllosilicates have been identified extending laterally along the scarp, related to unit Npld (dissected impact breccias). Below the phyllosilicates are found deposits of high-Ca pyroxene hugging scarps in unit Npld. High-Ca pyroxene is also identified in unit Npl1 (undissected impact breccias), at the same stratigraphic level of the Npld pyroxenes. A high-standing knob in Npl1 has a phyllosilicate signature. CRISM investigations also support previous studies by OMEGA, which identified olivine and pyroxene in north Argyre (Gondet et al., 2007). A correlation appears between the low-Ca pyroxene/olivine exposures and the hills of unit Nplh, the oldest geologic unit in Argyre interpreted as uplifted by tectonism during the formation of the impact basin (Scott and Tanaka, 1986). We suggest that as a group these deposits reveal the stratigraphy of the Noachian crust in this region. At the bottom of the stratigraphic column are olivine and low-Ca pyroxene associated with uplifted ancient rocks (unit Nplh). Above these deposits are high-Ca pyroxenes associated with lavas and impact breccia (unit Npl1 and Npld). Within these same units but stratigraphically above the pyroxene are phyllosilicates, preserved in freestanding knobs and along scarp walls. At the top of the stratigraphic column in NW Argyre is olivine associated with the early Hesperian Hpl3 unit.
P13D-1566
Structural, Stratigraphic, and Mineralogic Mapping of Noachian and Hesperian Aged Crust in Xanthe Terra, Mars
The Xanthe region of Mars (10N-17S, 300-345E) contains both Noachian-aged cratered highlands and Hesperian-aged plains and chasmata. In particular, the region is transected by the Capri Chasma running southwest to northeast. The total elevation relief is ~10 km due to the juxtaposition of both highlands and chasmata in this area. The variation in terrain type and elevation makes this region ideal for reconstructing the three-dimensional structure, stratigraphy, and lithology of the crust. This region also contains Aram Chaos, a well-studied site where kieserite and polyhydrated sulfates were identified from OMEGA data (Gendrin et al., 2005) and gray crystalline hematite was identified from TES data (Glotch and Christensen, 2005). Analysis of CRISM multispectral mapping strips (72 bands, 256 pixels/degree, covering ~30% of the study area) confirms the presence of both hydrated sulfate and hematite exposures in Aram Chaos. Phyllosilicate signatures were identified in the CRISM data by the presence of 1.9 and 2.3 μm absorption bands in the upper sections of the walls of Capri Chasma and Ganges Chasma where these chasmata cut across what are mapped as both Noachian Cratered Plains and Hesperian Ridged Plains (based on the geologic map by Scott and Tanaka, 1986). The multispectral data also show 2.3 μm absorptions that are tentatively assigned to a phyllosilicate signature in the chasma walls north of Hydraotes Chaos (1.5N, 325.5E) in a region that cuts across Noachian cratered terrain. Hydrated sulfates were identified in the walls of Hesperian-aged interior layered deposits (ILDs) superimposed on the floor of Capri Chasma (13S, 312E). Results show that within the study area the hydrated sulfates are younger than the phyllosilicates. Future plans include mapping pyroxene and olivine on the Noachian and Hesperian units, tracing the mineralogy to chasmata walls, and placing the phyllosilicate- and sulfate-bearing units in stratigraphic and structural context to help decipher the geologic evolution of the study site.
P13D-1567
True Colors of Mars as Revealed by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board the Mars Reconnaissance Orbiter (MRO)
CRISM acquires images of the Martian surface in 544 channels ranging from 362 to 3920 nm with 6.55 nm spectral sampling. Spatial resolution varies from 18 meters per pixel in full resolution targeted mode to 200 meters per pixel in a multispectral survey observing mode. CRISM's spectral resolution and range, fully covering the visible portion of the spectrum on a single detector with high signal-to-noise ratio, make it ideally suited to accurately represent the appearance of Mars to an observer in orbit. With 50% surface coverage to date in multispectral survey mode and nearly 2000 targeted observations it is possible to produce natural color data products for large portions of the Martian surface at spatial resolutions matching the CRISM data products. To accurately depict surface colors tristimulus values are calculated by discrete summation of calibrated surface radiance and color matching functions followed by color space transformation and correction for output devices (following the work of Bell and Savransky 2006). The results of this work will be a capability to produce natural color depictions of CRISM targeted and multispectral data products. Ongoing efforts to account for and remove atmospheric effects in multispectral survey (McGuire et al. 2007), and targeted hyperspectral data will lead to a complementary simulation of the appearance of the Martian surface as it would be perceived by the human eye in the absence of atmospheric attenuation and scattering. References: Bell III, J.F., D. Savransky, and M.J. Wolff, Chromaticity of the martian sky as observed by the Mars Exploration Rover Pancam instruments, J. Geophys. Res., 111, E12S05, doi:10.1029/2006JE002687, 2006. McGuire, P. et al., this conference