P33C-01 INVITED
Coordinated Orbital and Landed Observations for Understanding Martian Soil and Rock Mineralogy and Textures
Coordinated observations between the orbiting Mars Express OMEGA hyperspectral imager (0.4 to 5 micrometers) and the MER rovers Spirit and Opportunity have provided a self-consistent view of surface materials at the rover traverse sites and beyond. Coordinated observations now include the Mars Reconnaissance Orbiter CRISM hyperspectral imager (0.4 to 4 micrometers), the two rovers, and the Phoenix Lander (touched down 5/25/08). Coordinated observations with Phoenix include dozens of near simultaneous and simultaneous measurements of the atmosphere and surface, spaced throughout the northern hemisphere summer period. For Spirit the results show that the Gusev plains are dominated by weakly altered basaltic sands with a variable cover of nanophase iron oxide-rich dust. The hydrated sulfate and opaline silica deposits found by Spirit could not be seen using orbital data because of their small areal extent and subsurface provenance (exposed by rover wheels). Opportunity joint observations show that the Meridiani Plains are covered by aeolian deposits dominated by basaltic sand, hematitic concretions, and outcrops with orbital spectral signatures consistent with weakly hydrated nanophase iron oxides, although jarosite (OH-bearing) and hydrated phases were detected by Moessbauer and, on surfaces excavated by the Rock Abrasion Tool, by Mini-TES. Phoenix joint observations show that water ice frost is retained during the summer in a nearby small (~6 km) bowl-shaped crater and on the north facing slopes of the ~10 km wide Heimdall Crater. The landing site and immediate surroundings are on the differentially eroded ejecta deposits from Heimdall, and the soil exposures are dominated by basaltic sandy silt deposits mixed with nanophase iron oxide-rich dust. No carbonates, sulfates, nitrates, or phyllosilicates are evident in the orbit- based spectra, but ice is present in the subsurface. Monitoring of the Phoenix site during the transition from summer to fall will allow us to track the deposition of water ice and perhaps carbon dioxide ice as the northern seasonal cap forms.
P33C-02 INVITED
Properties of Martian Dust Aerosols From the Combination of MER and MGS/MRO Observations
Remote sensing observations of the Martian atmosphere do not easily lend themselves to the notion of ground truth. While one might consider a high fidelity measurement from a surface platform (such as optical depth from direct solar imaging) to represent some degree of ground truth, connecting a series of orbital spacecraft observations to those of a surface station requires a temporal, as well as a spatial, overlap to account for the dynamical nature of the atmosphere. This concept of an overflight was exploited during the Viking era to provide a connection between lander and orbiter measurements of optical depth. Combined analyses have produced additional physical insights into Martian aerosols. Hunt (1979, JGR, 84, 8301) and Martin (1986, Icarus, 66, 2) examine Infrared Thermal Mapper observations obtained near the site with the context of lander data; Clancy and Lee (1991, Icarus, 193, 35) employ the lander optical depths as a demonstration of the viability of their emission phase function retrieval methodology. However, the overflights of the Viking era remain fundamentally limited by two aspects: the absence of multi-instrument coordinated (i.e., simultaneous, or nearly so) observations, and the lack of similar instrument capabilities on both the surface and the orbital platforms. Overflights of the Mars Exploration Rovers (MER) by the Mars Global Surveyor (MGS) and the Mars Reconnaissance Orbiter (MRO) have been able to overcome the limitations inherent in the earlier efforts. As a result, the MER-MGS and MER-MRO opportunities have been able to provide unique constraints on some basic dust properties (e.g., single scattering albedo, refractive indices). During our presentation, we will highlight the deriviation of infrared refractive indices from the MER-MGS data and the visible-near infrared single scattering albedo (as well as estimates of the refractive indices) from the MER-MRO data. The utility of these results for other applications such as atmospheric correction of retrieved surface reflectances, atmospheric heating rates, and additional atmospheric retrievals will be discussed briefly.
P33C-03 INVITED
Phoenix and Mars Reconnaissance Orbiter Coordinated Atmospheric Science
The Phoenix Mars lander (PHX) spacecraft and the Mars Reconnaissance Orbiter (MRO) have collaborated during the course of the Phoenix mission to simultaneously observe the same atmospheric column with a variety of instruments. PHX carries pressure and near-surface air temperature sensors, an upward-looking LIDAR that probes up to 20 km altitude, a wind telltale, a humidity sensor, and a multi-spectral camera that can be used to observe the atmosphere and obtains aerosol and water vapor amounts. MRO carries 4 instruments that we employ in our coordinated campaign. We use the Mars Climate Sounder (MCS) which provides vertical profiles of atmospheric temperature, aerosols, and water vapor and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) which provides column abundances of atmospheric aerosols and water vapor. We also use the Mars Color Imager (MARCI) which provides very wide-angle, context views of the aerosols. Finally, we occasionally use the High Resolution Imaging Science Experiment (HiRISE) camera for examining frosts on the ground in concert with our other measurements. The coordinated campaign between the two spacecraft began shortly after PHX landed safely in the northern plains of Mars (68 N), in the Martian late springtime and continued for more than 3 months, into the PHX extended mission. The campaigns were structured to provide both diurnal and seasonal observations focussed on water vapor, water-ice clouds, and dust to gain insight into major outstanding questions: (a) what are the relative roles of the different reservoirs of water, (b) is there net water transport out of this region, (c) what is responsible for the internnual variability in the atmospheric aerosols, and (d) what is the relationship between the dust, water and CO2 cycles? The use of two spacecraft to examine the same atmospheric column allows cross-calibration of experiments. Additionally, MRO supplements PHX by providing context in the northern polar region. PHX supplements MRO by providing the detailed measurements near the surface and by providing a full set of information at multiple times of day. Full diurnal campaigns were acquired every 5-10 degrees L. This timeframe is of interest since the peak of the water cycle occurs near Ls=110 (early summer) and as summer progresses, water vapor rapidly declines, while dust and water-ice clouds increase. Initial results of these campaigns will be discussed.
P33C-04 INVITED
Mars Aeolian Features and Processes Observed Concurrently From Orbit and the Ground
The last four years have provided the opportunity to study Mars through the concurrent operation of orbiters and rovers to observe processes related to active winds. Data have been obtained on the characteristics of active sand and dust at the MER sites for comparisons with features and active processes seen from orbit. Combined with modeling, results give new insight into surface modification by windblown material. For example, the operation area of Spirit seen from orbit is criss-crossed with dark linear features thought to be tracks left by dust devils (DD). The rover traversed one track and obtained Microscopic Imager data showing that sand grains within the track are relatively clear of dust, while those outside the track are partly mantled with dust. Subsequent observations show that as active DD cross the plains (entraining dust into the atmosphere) they leave behind low-albedo tracks, which are later gradually obscured by settling of dust. The rate of deposition can be calculated from the solar panel output and appears to be continuous, proportional to optical depth τ, minus a constant. Dust removal from the panels is in discrete episodes. Observations of terrain and the atmosphere were made from the ground and from orbit: 1) before the dust devil "season" (τ = 0.578), 2) during active dust devil formation (τ = 0.918), 3) after active dust devil formation, but during active dust clouds (τ = 2.061), and 4) after clearing of dust from the atmosphere (τ = 0.487). Results were compared with temperatures derived from the Thermal Emission Spectrometer for the relevant seasons. Results suggest that spring heating of the surface enhances DD formation, but as atmospheric dust-loading progresses, active DD "shut-off," possibly because atmospheric conditions become thermally stable. In addition to dust activity, movement of coarser grains (i.e., "sand") has also been observed. For example, MOC, HiRISE, THEMIS, and HRSC images from orbit show abundant bedforms, revealed by Spirit to be composed of grains larger than a few hundred microns in diameter. Imaging of Spirit's deck after periods of aeolian activity showed the presence of similar grains, as well as the bounce marks of their passage in the deposited dust suggesting emplacement by saltation; showing that at least some sands currently are active. This interpretation was verified by a sequence of images from Spirit which showed active movement of small ripples across the surface. Despite these results, questions remain regarding current versus relict aeolian features, and the specific pathways of aeolian transport in complex terrains, such as the Columbia Hills in Gusev crater. These questions are being addressed through current research and the acquisition of new data from both the ground and orbit.
P33C-05
Surface Properties and Characteristics of Mars Landing Sites from Remote Sensing Data and Ground Truth
Surface characteristics at the six sites where spacecraft have successfully landed on Mars can be related favorably to their signatures in remotely sensed data from orbit and from the Earth. Comparisons of the rock abundance, types and coverage of soils (and their physical properties), thermal inertia, albedo, and topographic slope all agree with orbital remote sensing estimates and show that the materials at the landing sites can be used as ground truth for the materials that make up most of the equatorial and mid- to moderately high-latitude regions of Mars. The six landing sites sample two of the three dominant global thermal inertia and albedo units that cover ~80% of the surface of Mars. The Viking, Spirit, Mars Pathfinder, and Phoenix landing sites are representative of the moderate to high thermal inertia and intermediate to high albedo unit that is dominated by crusty, cloddy, blocky or frozen soils (duricrust that may be layered) with various abundances of rocks and bright dust. The Opportunity landing site is representative of the moderate to high thermal inertia and low albedo surface unit that is relatively dust free and composed of dark eolian sand and/or increased abundance of rocks. Rock abundance derived from orbital thermal differencing techniques in the equatorial regions agrees with that determined from rock counts at the surface and varies from ~3-20% at the landing sites. The size-frequency distributions of rocks >1.5 m diameter fully resolvable in HiRISE images of the landing sites follow exponential models developed from lander measurements of smaller rocks and are continuous with these rock distributions indicating both are part of the same population. Interpretation of radar data confirms the presence of load bearing, relatively dense surfaces controlled by the soil type at the landing sites, regional rock populations from diffuse scattering similar to those observed directly at the sites, and root-mean-squared slopes that compare favorably with 100 m scale topographic slopes extrapolated from altimetry profiles and meter scale slopes from high-resolution stereo images. The third global unit has very low thermal inertia and very high albedo, indicating it is dominated by deposits of bright red atmospheric dust that may be neither load bearing nor trafficable. The landers have thus sampled the majority of likely safe and trafficable surfaces that cover most of Mars and show that remote sensing data can be used to infer the surface characteristics, slopes, and surface materials present at other locations.
P33C-06
Pancam Spectral Variations Across Home Plate: Bonestell Panorama, Gusev Crater, Mars
Visible/near-infrared color variations across the surface of the Home Plate (HP) structure were first observed by the Spirit Pancam multispectral camera using images acquired from the top of Husband Hill on sol 595, ~700m away from HP. Orbital imaging by the HiRISE camera on sol 1325 showed consistent color trends with Pancam in which the western edge of HP was "redder" than the "bluer" eastern portion. This suggested the eastern rim materials of HP are not as contaminated by airfall dust and/or are less oxidized. Pancam spectra of brushed rock targets indicate that western dust-free rock surfaces have higher 535nm band depths (consistent with higher Fe3+/Fe measured by the Mossbauer spectrometer), potentially caused by finely crystalline red hematite. The western rocks also exhibited less negative 601nm band depths than in the east, which could result from lower pyroxene/olivine ratios or the presence of goethite. The spectral variations across HP combined with in situ geochemical data around the rim suggest that the volcanic and/or hydrothermal nature of the HP system resulted in localized, high temperature events on the eastern side, compared to lower temperature alteration on the western side that produced greater amounts of nanophase ferric oxides. This hypothesis is being investigated using 13 band scenes acquired from Spirit's winter location on the northern rim of HP. Pancam began imaging on sol 1477 as part of an extensive mosaic (the "Bonestell Panorama"). Preliminary analyses confirm higher red/blue ratios along the western rim, but also redder regions on the eastern rim not as obvious in Sol 595 images. HiRISE acquired a color image of HP on Sol 1591 that shows less color variability on HP than the sol 1325 image. Dust fallout from the 2007 dust storm (sols 1240 to 1330) may be the cause of these temporal color variations. Additional analysis is required to determine whether surficial dust deposits are the dominant cause of the original color dichotomy on HP.
P33C-07
Evidence for High and Low Temperature Alteration across Home Plate, Gusev Crater
Over the last ~2 years in Gusev Crater, the Mars Exploration Rover Spirit has observed coherent variations in mineralogy and geochemistry along an almost circular traverse of Home Plate, an 80 m-diameter outcrop of layered, basaltic tephra. Observations of Home Plate from orbit by the High Resolution Imaging Science Experiment (HiRISE) camera (0.3 m per pixel) and from the summit of Husband Hill (0.7 km to the north) by the Panoramic Camera (Pancam) onboard Spirit show clear longitudinal differences in visible/near- infrared (VNIR) colors, where its eastern region is more blue and western region is more red. Up close, Pancam spectra of rock targets brushed by the Rock Abrasion Tool (RAT) revealed similar variations and confirm that color contrasts observed at greater distances reflect meaningful differences in outcrop mineralogy. Mineralogical observations by the Spirit Mössbauer Spectrometer and Miniature Thermal Emission Spectrometer (Mini-TES) are consistent with the VNIR data, indicating that pyroxene and magnetite dominate the Fe-bearing assemblage at the east side, while olivine, nanophase ferric oxide (npOx), and glass are more abundant at the west. Alpha Particle X-Ray Spectrometer (APXS) observations indicate that eastern Home Plate has higher concentrations of Si, Al, Zn, Ni, and K, while Cl and Br are higher in the west. Compositional similarities in major elements between the two sides of Home Plate, as well as geologic observations indicate that upper, cross-bedded materials that span Home Plate belong to the same stratigraphic unit. However the compositions of more fluid-soluble elements and Fe-bearing minerals in the upper unit vary independently of stratigraphy. We propose that these variations are the result of two distinct alteration regimes: one that produced npOx at the west and another that recrystallized olivine to form pyroxene by Si addition at the east. Abundant npOx at the west is the likely product of breakdown and oxidation of glass or other igneous phases by either low temperature hydrothermal alteration or chemical weathering. Some mass transport during the recrystallization event is implied by small but systematic changes in composition across Home Plate (e.g., decreasing SiO2 and Zn from east to west). Under hydrothermal conditions, SiO2 solubility is increased and Zn and Ni can form temperature-dependent complexes with Cl. The higher concentrations of SiO2, Zn, and Ni in eastern Home Plate rocks indicate that higher temperatures were likely attained there (likely ~300° C to subsolidus temperatures). The localized nature of the high temperature alteration indicates perhaps that the event was relatively short-lived, temperature gradients were steep, and lateral advection was minor across Home Plate.
P33C-08
Martian Phyllosilicates: Characteristics, Enigmas, and New Results from Orbital, Surface, and Laboratory Observations
For decades, evidence of phyllosilicate minerals on Mars was absent or inconclusive. With the advent of two visible/near infrared (VNIR) imaging spectrometers, the Mars Express OMEGA and Mars Reconnaissance Orbiter CRISM, spectral evidence has emerged that indicates hundreds to thousands of isolated occurrences of phyllosilicates around the planet. These locations appear to be limited to the oldest (Noachian) parts of the Martian crust. Dioctahedral and trioctahedral clay minerals, mica, and chlorite have been identified. In contrast to these discoveries made from orbit, no definitive identification of phyllosilicates has been made on the ground. The two Mars Exploration Rovers carry two instruments capable of such identifications: a Mössbauer spectrometer (MB) for the identification of Fe-bearing minerals and a thermal infrared spectrometer (Mini-TES) for the identification of a wide range of primary and secondary minerals. The bedrock at the Meridiani Planum landing site shows some evidence for nontronite (an Fe-smectite) based on Mini-TES results, but this was not corroborated by MB data. At the Gusev Crater landing site, highly altered rocks were encountered in the Columbia Hills, some of which show strong indications of phyllosilicates based upon chemical data from the Alpha Particle X-ray Spectrometer (APXS). However, neither Mini-TES nor MB could confirm their presence even though data from these instruments show clear evidence for other hydrated secondary phases in several locations. Laboratory thermal infrared (TIR) spectroscopy clearly shows sensitivity to even thin (<10 microns) coatings of phyllosilicates. Two features in the low wavenumber range covered by TES and Mini-TES (near 530 and 465 cm-1) are especially diagnostic of the presence of phyllosilicates and primary and secondary amorphous silicates. A feature near 465 cm-1 has been identified in Mini-TES spectra of rocks in the Columbia Hills that MB and APXS spectra indicate are highly altered. A similar feature has been mapped globally using TES spectra and may serve as a proxy for some of the rocks encountered in the Columbia Hills. I have now discovered that many of the locations identified by OMEGA and CRISM as phyllosilicate- bearing display this feature, including parts of Mawrth Vallis, Nili Fossae, and NE Tyrrhena Terra. Full TES spectra from one location adjacent to Nili Fossae look remarkably similar to the Assemblee-type rocks in the Columbia Hills that have the chemical signature of montmorillonite. Additionally, I have found that one of the locations in the Nili Fossae region identified as phyllosilicate-bearing by CRISM shows the TES 530 cm-1 feature along with the 465 cm-1 feature. Together, these are indicative of a dioctahedral smectite. The full spectra strongly resemble that of weathered Columbia River Basalt. Despite these examples of apparent agreement between TIR and VNIR observations, there are other examples in which phyllosilicates mapped by OMEGA and/or CRISM display no diagnostic features in TES spectra or conversely, the TES 465 cm-1 feature has no OMEGA/CRISM counterpart. The latter case may represent locations where unaltered volcanic glass or impact melt is present. The former case may indicate something about the mineralogy, abundance, or physical character of the identified phyllosilicates. In either case, the combination of TIR and VNIR observations provides a potential link between ground observations from the rovers and orbital observations around the globe. A more complete picture of Martian phyllosilicates is likely to emerge with these combined observations, which will become increasingly important in the landing site selection process for the upcoming Mars Science Laboratory rover.