P53A-1427
MarsVac: Pneumatic Sampling System for Planetary Exploration
We are proposing a Mars Sample Return scheme whereby a sample of regolith is acquired directly into a Mars Ascent Vehicle using a pneumatic system. Unlike prior developments that used suction to collect fines, the proposed system uses positive pressure to move the regolith. We envisage 3 pneumatic tubes to be embedded inside the 3 legs of the lander. Upon landing, the legs will burry themselves into the regolith and the tubes will fill up with regolith. With one puff of gas, the regolith can be lifted into a sampling chamber onboard of the Mars Ascent Vehicle. An additional chamber can be opened to acquire atmospheric gas and dust. The entire MSR will require 1) an actuator to open/close sampling chamber and 2) a valve to open gas cylinder. In the most recent study related to lunar excavation and funded under the NASA SBIR program we have shown that it is possible lift over 3000 grams of soil with only 1 gram of gas at 1atm. Tests conducted under Mars atmospheric pressure conditions (5 torr). In September of 2008, we will be performing tests at 1/6thg (Moon) and 1/3g (Mars) to determine mass lifting efficiencies in reduced gravities.
P53A-1428
A Concept for the in-situ Measurement of Electrical Properties of Planetary Bodies, Comets and Moons in the Solar System
The in-situ investigation of subsurface and atmospheric properties on planetary and cometary bodies or moons is a field of growing interest. We present a concept to measure the electrical properties using electric sensors at the feet of a planetary lander-system. Because of the expected high contact impedances, we suggest capacitive coupling for the injection of current into the regolith. This requires an alternating current, ideally in the frequency range from 100 Hz to 100 kHz, which at the same time provides a good resolution of both electrical resistivity and permittivity. We present a comprehensive theory covering all aspects such as the contact impedance of the electrodes, spurious currents in the lander, and the resolution of subsurface parameters depending on the geometry of the 4-point electrode configuration. Feeding sufficient current into the ground imposes special conditions on the design of the sensor- electrodes and the electronic components. Over resistive ground and at high frequencies the contact impedance will depend on the electrical properties of the subsurface and determines how much current can be injected into the ground. We calculate the contact impedance based on a spherical disc model and show that placing the electrode directly on the ground is always superior to the use of an insulating layer. Another design criteria is that the spurious current flow in the lander must be kept under a certain level. The interaction of the capacitive electrodes with the lander system and the ground is examined by an electric circuit which represents the properties of the subsoil and the lander system. The ratio between the spurious current in the lander and the current in the ground critically depends on the construction of the electrodes. We suggest a PEEK-vacuum solution which minimizes capacitive coupling to the lander while keeping the weight small. For an optimum resolution of the subsurface parameters, the geometry and especially the distance of the electrode to the ground are decisive. Near –surface information on the scale of the electrode size may be obtained directly from the contact impedances of the electrodes, while deeper information in the range of electrode distance may be obtained through a classical 4-point configuration. Within the limits of a space mission, not every position of the electrodes is possible for the geometry of the 4 electrodes. Two solutions for the placement of sensors and their influence on the expected results are discussed. The results show that ideally, the electrodes are as close to the ground as possible. However, a design where the electrodes are placed with a certain distance under the lander-body will also provide an acceptable resolution. Our suggested experiment might be realized within the Russian Phobos Grunt mission.
P53A-1429
Laser-induced Breakdown Spectroscopy of Phyllosilicates for ChemCam Calibration
The ChemCam instrument on the Mars Science Laboratory will use laser-induced breakdown spectroscopy (LIBS) to quickly and accurately determine elemental compositions of surface materials up to 9 meters away from the rover. Some key mineral groups may occur on Mars in sample sizes large enough that single phases can be probed by the LIBS instrument with a laser spot size of ~0.5mm. Of these, phyllosilicates are significant because spectral features attributed to vibrations of OH groups bound to Al, Fe, and Mg in phyllosilicates have recently been identified on Mars by OMEGA and the MRO CRISM instrument. Furthermore, to maximize science return from the ChemCam LIBS data, it is necessary to understand chemical matrix effects, which arise when chemical and physical interactions affect spectral intensities in unpredictable ways. This work explores matrix effects and calibration issues for a group of phyllosilicate minerals. Because they share the same crystal structure, some aspects of matrix effects should be mitigated. LIBS spectra of 17 phyllosilicate samples were recorded at the Los Alamos National Laboratory in ChemCam- and Mars-analogue conditions at a stand-off distance of 9 meters in the spectral range of 224-927 nm. Three analysis techniques are employed to determine elemental concentrations: univariate analysis, which correlates peak intensity with elemental concentration; peak area stepwise regression analysis (PASRA), which correlates multiple peak areas with elemental concentration; and partial least squares analysis (PLS), which correlates the entire spectrum with elemental concentration. The relative effectiveness of each technique's ability to predict known concentrations was assessed and compared. Results show that all three methods give reasonable results for most elements. The conventional univariate analysis is the least effective in predicting chemical compositions accurately. Both PLS and PASRA appear to compensate for or exploit the chemical matrix effects, though PLS is superior in predicting compositions of unknowns in this data set because it utilizes all the channels in a spectrum. The PASRA technique is far less computationally intensive, but may be more instructive with regard to the physics contributing to the chemical matrix effects because it identifies only a few key peaks that are diagnostic of specific elements. However, PASRA will require a larger data to set achieve accuracy comparable to PLS.
P53A-1430
Rotation and internal dynamics of Mars from the LaRa experiment in ExoMars
The LaRa instrument is a coherent transponder using one uplink and one downlink in X-band and proposed to be part of the Humboldt Payload (on the fixed platform lander) of the ExoMars ESA mission. LaRa will measure the variation of Mars' rotation rate (related to the length-of-day) and the orientation of Mars' rotation axis in space (precession and nutations), by measuring Doppler shifts resulting from the motion of Mars relative to the Earth, through monitoring a radio signal between the ExoMars lander and the tracking stations from ESA (ESTRACK stations) and NASA (the Deep Space Network, DSN) on Earth. The primary objective of LaRa is a precise measurement of precession, nutations, and length-of-day. Comparing the data with theoretical modeling allows inferring knowledge on Mars' interior and on the global circulation in its atmosphere. Precession and nutations are induced by the well-known gravitational forcing of the Sun on Mars. Nutations depend on the internal properties of Mars, in particular on the state and dimension of the core. Length-of-day variations are deviations from the uniform rotation rate of the planet. They are mainly related to the dynamics of the geophysical fluids of the system, mainly the atmosphere of Mars. The seasonal condensation/sublimation of the icecaps induces a large change in the length-of-day at the seasonal periods. These phenomena were previously estimated from the tracking data of the Viking (S-band) and of the Pathfinder (X-band) Landers as well as from the time variations of the gravity field of Mars determined from radioscience on spacecraft orbiting around Mars. The expected precision of LaRa will be at least a factor two better in the known quantities, even a factor four for the precession value for a very conservative 180 day mission life-time and better for a long stay at the surface of Mars. The improvement in the precision of the moment of inertia determination deduced from the precession observation realized by LaRa, even without further independent improvement in the mantle mineralogy, will reduce the uncertainty on the core radius. LaRa will thus provide more accurate values or new estimates of Mars' precession and nutations and Mars' length-of-day variations. Those geodetic quantities will substantially increase our knowledge on the interior structure of Mars and on the global circulation in the atmosphere related to the seasonal CO2 sublimation and condensation.
P53A-1431
Precise mass determination of the Mars moon Phobos from Mars Express close flybys
The mass of the Mars moon Phobos has been estimated several times from radio tracking data. The derived GM values (gravity constant times mass) of Phobos vary considerably between 0.585x10-3 km3/s2 (Smith et al., 1995) and 0.85x10-3 km3/s2 (Williams et al., 1988). The earliest estimates (Christensen et al., 1977; Tolson et al., 1977; Williams et al., 1988; Kolyuka et al., 1990) have been derived from radio tracking data gained during close flybys by the Viking 1 and 2 orbiters and Phobos-2 while the most recent estimates have been computed from distant encounters, however based on a large data base of mostly Mars Global Surveyor, Odyssey and Mars-Express tracking data (Yuan et al, 2001; Konopliv et al., 2006; Rosenblatt et al., 2008). MEX, however, is currently the only spacecraft at Mars which is able to perform close flybys at Phobos. The first two flybys have been used for the Phobos mass determination: the first one on 23rd March 2006 at a distance of 460 km and the second one on 17th July 2008 at a distance of 274 km. The tracking data of both flybys have been analysed by a high precision determination of the spacecraft orbit and by implementing relativistic corrections in the simulation of the Doppler signal. The tracking data were calibrated for the Earth ionosphere and troposphere and filtered in order to reduce the noise of the data. The mass of Phobos was determined by applying a least square technique when comparing the modelled and measured tracking data to be GM = 0.709 +/- 0.002x10-3 km3/s2. The mass solution is in good agreement with solutions obtained from recent estimates based on distant encounters and shows a very small statistical error. Depending on the choice of the volume estimate the bulk density of Phobos is estimated 1849 – 1870 kg/m3.
P53A-1432
Unique Ground-Truthing of a Shergottitic Lithology as a Potential Orbital End-Member Provided by Mini-TES/MER and Laboratory TIR Data of Terrestrial Shocked Basalt
The majority of the equatorial regions of Mars are basaltic sands labeled Surface Type 1 (ST1). This MGS
TES orbital thermal infrared (TIR) end-member is often compared to laboratory TIR data of Deccan Trap
flood basalt from central India. Although subtle spectral differences exist (to be shown), the two likely have
similar abundances and composition of labradorite and clinopyroxenes augite and pigeonite. This has been
confirmed by recent works utilizing Mini-TES data of Meridiani sands and Gusev soils (both ST1) along with
data from other MER instruments. Further, the TIR spectrum of Deccan basalt from Lonar Crater, India that
was shocked 20-40 GPa is an exact match to the TIR spectrum of the Los Angeles shergottite in that both
contain 45% maskelynite and 35% augite/pigeonite. This strengthens the comparison of ST1 to Deccan
basalt and suggests that Los Angeles was ST1 bedrock before ejection. However, the TIR spectrum of ST1
and Deccan basalt both differ from the laboratory spectra of most basaltic shergottites, which contain more
clinopyroxene than plagioclase. Further, the shergottites contain shattered clinopyroxenes and maskelynite,
or plagioclase feldspar shock compressed to a diaplectic glass of plagioclase composition, that was created
in the impact event that launched them from Mars. Presumably, the original basaltic bedrock from which
shergottites originated contain the original plagioclase feldspar and unshattered clinopyroxenes. Whereas
there exists evidence for olivine and orthopyroxene on Mars, large occurrences of these CPX-rich
shergottites (not to be confused with pyroxenites or Nakhlites) have not been located using a laboratory TIR
spectrum as an orbital end-member. However, one particular MER-B target named Bounce Rock has APXS
and Mössbauer spectra similar to some shergottites such as Zagami. Mineral abundances resulting from the
deconvolution of the Mini-TES spectrum of Bounce Rock, where the effects of dust have been removed, are
similar to CPX-rich shergottites except that Bounce Rock appears to have spectral features attributed to
plagioclase and not maskelynite. A comparison of the dust-removed Mini-TES spectrum of Bounce Rock to
laboratory spectra of sherogottites Zagami and NWA 2986 share spectral features attributed to pigeonite and
augite but the position of the Christiansen Feature (from comparisons to both experimentally shocked basalt
and terrestrial shocked basalt) suggest that Bounce Rock is the unshocked version of these shergottites.
This is significant because an unshocked shergottite will never be available in meteorite collections or
traditional spectral libraries as all SNC's are shocked in their ejection event. Hence, the Mini-TES spectrum
of Bounce Rock, while not from a bedrock exposure in Meridiani, is an excellent end-member to search for
shergottite-like terrains on Mars. Spectral plots of the above comparisons between ST1, Deccan basalt,
shocked Lonar basalt, Bounce Rock, and several shergottites will be shown along with early results of the
orbital search for shergottite-like terrains.
http://www.lpi.usra.edu/meetings/lpsc2008/pdf/2330.pdf
P53A-1433
The Behavior of Metal-rich Meteorites in the Thermal Infrared – Implications for Dust Thickness, Spectral Anomalies, and the Continued Remote Sensing of Meteorites on Mars Using Mini-TES.
The scientific value of meteorites resting on the surface of Mars (or any other planetary body) lies in their
hypersensitivity to chemical (aqueous) alteration. Reduced iron metal, present in some 88 percent of all
terrestrial falls, will oxidize in water-contact to generate secondary iron oxide and oxyhydroxide products. The
metal content ranges from <1 % for L ordinary chondrites [e.g. 1], to more than 20 % for H ordinary
chondrites [e.g. 2], subequal volumes with silicate phases in the case of many stony-irons and CB chondrites
[e.g. 3 & 4], to close to 100 % for irons (less sulfides, occasional silicates, and other trace non-metal
fractions). Weathered meteorites thus assist in the assessment of paleoclimatic history and habitability
potential at the locations where they are found. Understanding the behavior of metals in the infrared would
be useful for interpreting spectra of these meteorite classes. TIR spectra of metal-rich meteorites are
therefore being included in the thermal emission spectral library reported in [5]. Metal present in large
volumes within such rocks appears to have both predictable and counterintuitive effects on spectral patterns,
requiring further evaluation. The Miniature Thermal Emission Spectrometer (Mini-TES) instrument on the
Mars Exploration Rovers has been effective in remote sensing iron meteorites because of their highly
reflective behavior in the thermal infrared [6]. The radiance of an ideal metal is that of a graybody,
demonstrating greatly reduced emissivity with greatly enhanced reflectivity. This can lead to a reflected
spectrum of the environment. On Mars, reflected features tend to be those of the downwelling atmosphere.
Although simulating this behavior has been difficult in the laboratory for natural surfaces because they tend
to scatter the downwelling radiance, the effect has been recreated for a polished slab of the Canyon Diablo
iron meteorite using an artificial quartz-based control. We have collected TIR measurements in concert with
incremental Mars dust analog loading and determined that approximately one order of magnitude less dust is
required to obscure the effect in the thermal infrared than in visible light. A maximum thickness of only 14
microns of dust is sufficient to completely obscure reflective effects evident on an undusted, polished slab
surface. The result has implications for dust thicknesses coating the surfaces of the iron meteorite Meridiani
Planum, found by the Opportunity rover at Meridiani Planum; and Alan Hills and Zhong Shan, found at Gusev
Crater by the Spirit rover. Future evaluations of Mini-TES spectra will incorporate these findings as the
search for weathered meteorites continues. References: [1] Pun A. et al. (1990) Meteoritics 25, 237-239. [2]
McCoy T.J. (1990) Meteoritics 25, 77-79. [3] Weisberg M.K. et al. (2002) LPSC XXXIII, abstract 1551. [4]
Ivanova M.A. et al. (2005) 68th Annual Meteoritical Society Meeting, abstract 5073. [5] Ashley J.W. and
Christensen P.R. (2007) AGU Fall Meeting abstract 12506. [6] Schroder, C., et al. (2008), JGR 113, E06S22,
doi:10.1029/2007JE002990.
http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1750.pdf
P53A-1434
Constraints on Aeolian Degradation Rates on Mars from Erasure of Rover Tracks
The wheel tracks left by the MER rovers Spirit and Opportunity are unique artificial markings on the surface of Mars. The tracks stretch several kilometers across diverse terrain in two widely separated regions of the planet. The initial appearance and characteristics of the tracks were well documented by the science and navigation cameras aboard the vehicles at the time the tracks were formed. Orbital observations by Mars Global Surveyor and now Mars Reconnaissance Orbiter document the erasure of the tracks over a period of more than two Mars years. We are studying the erasure of the rover tracks as a means to better understand the mechanisms and time-scales of aeolian degradation on Mars. Spirit's wheels left conspicuous dark tracks in the dust-mantled soil of Gusev crater. Low albedo sand and gravel were exposed from beneath the bright dust mantle along Spirit's traverse from its landing site across the Columbia Hills. High albedo, silica-rich soils were ploughed up near Home Plate by the rover's stuck right front wheel. MOC images show that the tracks near the landing site began to fade gradually in the weeks after they were made. Atmospheric dust fallout and summertime dust-devils rendered Spirit's tracks invisible before the arrival of MRO. Only two small sections of tracks from Spirit's traverse to Home Plate can still be seen in recent HiRISE images, on the southern flanks of Husband Hill and in the corridor west of Mitcheltree Ridge. Spirit witnessed the episodic erasure of its tracks at the height of the global dust storm of 2007, when strong surface winds blew away the tracks and shifted the soil surrounding the rover over a period of just days. Opportunity's tracks across Meridiani Planum to Victoria crater were barely visible at the time that they were made, and had largely disappeared before the arrival of MRO. Opportunity's tracks are slightly brighter than the undisturbed surface because the wheels press the darker hematite spherules into the soft soil. Only two short sections of pre-2006 tracks remain visible in recent HiRISE images, near the craters Fram and Erebus. However, the rover left conspicuous bright tracks in the smooth annulus surrounding Victoria crater as she traversed the crater's northern rim. Surprisingly, these tracks were scarcely altered by the 2007 dust storm, despite clear changes in the pattern of wind streaks emanating from the crater. Understanding the time-scales and mechanisms of erasure of the rover tracks is important for constraining the age of the many pristine impact craters that have been discovered by MRO. In turn, this knowledge lets us place bounds on the present day bombardment rate of Mars.
P53A-1435
Degradation of Victoria Crater, Mars
The approximately 750 m diameter and 75 m deep Victoria crater in Meridiani Planum, Mars, presents evidence for significant degradation including a low, serrated, raised rim characterized by alternating alcoves and promontories, a surrounding low relief annulus, and a floor partially covered by dunes. The amount and processes of degradation responsible for the modified appearance of Victoria crater were evaluated using images obtained in situ by the Mars Exploration Rover Opportunity in concert with a digital elevation model (DEM) created using orbital HiRISE images. Opportunity traversed along the north and northwest rim and annulus and sufficiently characterized features visible in the DEM, thereby enabling detailed measurements of rim relief, ejecta thickness, and wall slopes around the entire degraded, primary impact structure. Victoria retains a 5 m raised rim consisting of 1 to 2 m of uplifted rocks overlain by 3 m of ejecta at the rim crest. The rim is 120 to 220 m wide and is surrounded by a dark annulus reaching an average of 590 m beyond the raised rim. Comparison between observed morphology and that expected for pristine craters 500 to 750 m across indicate the original, pristine crater was close to 600 m in diameter. Hence, the crater has been erosionally widened by approximately 150 m and infilled by about 50 m of sediments. Eolian processes are responsible for modification at Victoria, but lesser contributions from mass wasting or other processes cannot be ruled out. Erosion by prevailing winds is most significant along the exposed rim and upper walls and accounts for roughly 50 m widening across a WNW to ESE diameter. The volume of material eroded from the crater walls and rim is about 20 percent less than the volume of sediments partially filling the crater, indicating eolian infilling from sources outside the crater over time. The annulus formed when less than 1 m deflation of the ejecta created a lag of more resistant hematite spherules that trapped darker, regional basaltic sands.
P53A-1436
Diversity of Soil Textures Along Spirit's Traverse in Gusev Crater
Since landing, Spirit has documented a diversity of bedforms i.e. dunes, ripples, drifts, and unstructured soils. Mineralogy and chemistry show basaltic soils in the Plains with limited water alteration to localized sulfate-rich soils in the Columbia Hills. While these analyses provide data about the nature of the soils, the texture of individual particles constrain their formation and the various processes affecting their evolution. Here, we present an example of quantitative techniques applied to the Microscopic Imager (MI) for analyses performed on all undisturbed soils up to Sol 1085, which represents 3,140 particles in 31 soils. Particle-size distribution, elongation, roundness/angularity, sorting, and morphology were quantified. These characteristics are indicative of soil dynamics and provide clues to understanding whether and how particles have been mobilized. Samples in the Plains Unit and Columbia Hills appear as contrasting textural domains: One is heterogeneous, with a continuum of angular-to-rounded particles of fine sand to pebble sizes and is generally dust covered and locally cemented. The second shows the effect of a dominant and ongoing dynamic aeolian process redistributing a uniform population of 270 µm-size sand. Results are consistent with volcanic, aeolian, impact, and water processes. A systematic textural classification of soils is critical for the understanding of geologic processes and their evolution on Mars and this analysis represents a first attempt in that direction. Results can also be used to more precisely model past/present erosional and depositional activity, in particular as it relates to aqueous (chemical alteration, cementation) and aeolian (wind strength, particle transport, erosion, saltation) processes over a broad range of spatial scales. The detailed evaluation of grain-size distribution is also key to link albedo to fine-grained dust deposits and thermal inertia determined from orbit.
P53A-1437
High-Silica Rocks and Soils at Gusev Crater, Mars: Distribution, Spectra, and Implications for Past Hydrothermal Activity
The Mars Exploration Rover (MER) Spirit has discovered surprisingly high concentrations of amorphous silica in the Inner Basin of the Columbia Hills. As described by Squyres et al. (2008, Science, 320, 1063), within a topographic lowland called Eastern Valley, Spirit's Alpha Particle X-Ray Spectrometer (APXS) measured a composition of >90 wt.% silica at the soil feature "Gertrude Weise", a record high for Mars. The Mössbauer spectrum of this target is featureless. APXS measurements of light-toned nodular outcrops also show high silica concentrations (up to ~72 wt.%), which in some locations co-exist with sulfur-rich soils. Miniature Thermal Emission Spectrometer (Mini-TES) results from the soils and nodules are consistent with opal-A. These deposits have been found adjacent to "Home Plate", a layered plateau interpreted as the product of explosive volcanism. The silica-rich soils and nodules are consistent with sinters and/or residues formed in a hydrothermal system, and may be related to the same hydrovolcanic activity that produced Home Plate. We have begun to map the distribution of high-silica materials in Gusev Crater more extensively using remote sensing, in order to understand the regional extent of possible hydrothermal activity. Spirit's Pancam instrument has collected visible to near-infrared relative reflectance spectra of the region in 11 unique wavelengths. We find that a distinct absorption feature at the longest Pancam wavelength (1009 nm) appears to be characteristic of the high-silica soils and nodules. By mapping the occurrence of this feature with other spectral parameters in Pancam images, we can remotely identify potential amorphous silica deposits elsewhere in the Columbia Hills. Here we present a map with our proposed regional distribution of silica-rich materials within the rover's Gusev Crater traverse area. The mineralogic origin of the 1009nm feature is enigmatic; reflectance spectra of amorphous silica are typically featureless in near-infrared wavelengths (as are spectra of TiO2, which was also detected by the MER team in APXS data of the Gusev high-silica soils). For comparison, we have performed laboratory analyses of silica sinters and residues from hydrothermal sites at Wairakei, New Zealand and Geysir, Iceland. Based on spectral mixing models with these and other mineral spectra from digital spectral libraries, we hypothesize that the presence of water or OH, either free (trapped in pore spaces) or bound in a mineral structure (perhaps from a minor species produced together with the high-silica materials), is responsible for the spectral feature observed by Pancam.
P53A-1438
Evidence for Relatively Recent Hydrothermal Activity Due to an Impact within the Syrtis Major
Based on detailed morphological and mineralogical analyses, we have identified a relatively fresh impact crater on the northern edge of the Syrtis Major Volcanic Plains (71.8E, 17.0N) with indications of hydrothermal activity. The crater is 40 km in diameter and 2 km depth, and has a well preserved ejecta blanket, terraced walls, a few superposed impact craters and a well developed central peak with a central pit. The maximum age of this impact crater is constrained to the mid/late Hesperian by the formation age of the Syrtis Major Volcanic Plains. CRISM observations were used to identify extensive hydrated silicates outcrops. When combined with altimetry (MOLA) and imaging (Hirise and HRSC) data, these outcrops are associated with the top and rim of the central peak, and the northern part of the crater floor. The mineralogy is dominated by Fe/Mg-phyllosilicates and chlorite, a suite of minerals that frequently results from hydrothermal alteration in terrestrial craters. Lesser abundances of Al-phyllosilicates are also present. Impact shock decomposition analyses indicate that this suite of minerals postdate the impact event. In this study we present the mineralogy of this fresh crater and propose a nominal stratigraphy as evidence for impact triggered hydrothermal activity in a relatively late stage of the evolution of Mars.
P53A-1439
Constraining the development of the upper martian crust using orbital and in-situ thermophysical, mineralogical, and geochemical observations
Measurements from the Mars Exploration Rovers (MER) indicate distinct compositional differences between rock interiors, rock surfaces, and soil that may indicate a close relationship between mechanical and chemical weathering [Hurowitz et al., 2006, Ming et al., 2006, Morris et al., 2006]. This is most clearly indicated by olivine-rich Adirondack Class basalts and relatively olivine-poor dark soils in the plains of Gusev crater, consistent with aqueous alteration in an water-limited acidic environment [e.g. Tosca et al., 2004; Hurowitz et al., 2006]. Similar spectral and thermophysical relationships can be observed from orbit with the Thermal Emission Spectrometer (TES) and the Thermal Emission Imaging System (THEMIS) [ Bandfield and Rogers, 2008]. The detailed rover observations combined with the large spatial coverage of the orbital observations are highly complimentary, allowing for global inferences to be made from more detailed local observations. The combined set of observations may give an indication of the extent of chemical and mechanical weathering of the martian crust. Mafic, olivine-rich compositions are highly correlated with high inertia rocky surfaces and are rare elsewhere. Because olivine is highly sensitive to dissolution with limited amounts of liquid water, its presence or absence may be a proxy for the amount of chemical weathering that has occurred. This is the case even where spectroscopic measurements do not otherwise show clear evidence for significant alteration throughout martian dark regions. High thermal inertia surfaces, consistent with relatively unprocessed rocky materials, are not common on Mars [Edwards et al., 2005]. Even where several kilometers of canyon or crater wall are exposed, high inertia materials are commonly absent except in isolated layers. This is also the case for materials exposed at the MER landing sites [Fergason et al., 2006]. The high inertia materials that are present are relatively unaltered blocks. Materials that appear to be altered (including in-place units) have relatively low thermal inertia values. Mechanical and chemical weathering are highly correlated on Mars as on Earth and unweathered materials are not commonly exposed. The martian near surface may have experienced significantly more alteration than previously assumed and our understanding of martian igneous processes and aqueous history based on orbital remote sensing may need to be revised. Bandfield, J.L. and A.D. Rogers (2008) Geology, 10.1130/G24724A. Edwards, C.S., et al. (2005) Eos Trans. AGU, 86, P21C-0158. Fergason, R.L., et al. (2006) J. Geophys. Res., 10.1029/2005JE002583. Hurowitz, J.A., et al. (2006) J. Geophys. Res., 10.1029/2005JE002515. Ming, D.W., et al. (2006) J. Geophys. Res., 10.1029/2005JE002560. Morris, R.V., et al. (2006) J. Geophys. Res., 10.1029/2005JE002584. Tosca, N.J., et al. (2004) J. Geophys. Res., 10.1029/2003JE002218.
P53A-1440
Surface units of MER landing sites as seen by Mars Express/HRSC color data
Imaging spectrometry and color data of the High Resolution Stereo Camera (HRSC, [1]) onboard Mars
Express complement each other for mapping outcrop types. The HRSC dataset is the only one that currently
combines extensivee coverage of the planet (75% and more in the future), 200 m/pixel spatial resolution or
better and 5-band radiation filtering. Our current work shows the potential and limitations of HRSC color data
as a visual support and as multispectral images. The MER landing sites provide us the opportunity to
compare HRSC image analysis to in-situ observations[2, 3]. HRSC has five color filters in the visible and
near-infrared that are designed for visual interpretation and the mapping of various surface units [1]. HRSC
is sensitive to materials with absorption features in the visible and near-infrared up to 1 μm. Thus, oxide-
rich red dust and basalts (pyroxenes) can be mapped, as well as very bright components like water ice [4, 5].
All broadband channels are acquired by separated cameras oriented at varying anglea from the normal to
the surface. This implies that a given spectrum results from different proportions of shade at each
wavelength. Thus, shade affects the shape of HRSC spectra in a different way from pixel to pixel. This
contribution has to be considered when analyzing HRSC spectra in order to provide compositional and
surface property information. We perform Spectral Mixing Analysis (SMA, [6, 7]) of HRSC data by using the
Multiple-Endmember Linear Spectral Unmixing Model (MELSUM) [8, 9, 10]. Results for shade and residuals
are related to topography, surface roughness, aerosol scattering, the geometry of illumination/observation
and instrumental noise [9]. Comparisons with surface roughness-specific studies [11,12, 13] are used to
better discriminate the aforementionned effects. References [1] Neukum G. et al. (2004) ESA SP, 90,
1151–1154. [2] Johnson et al., (2006) JGR 112. [3] Johnson et al. (2007) GRL 34. [3].[4] McCord T. B. et al.
(2007) JGR 112. [5] McCord T. B. et al. (2006) LPSC 1757. [6] Adams et al. (1986), JGR 91. [7] Adams and
Gillespie, Cambridge Univ. Press. [8] Combe J.-Ph. et al. (2008) PSS, 54/7. [9] Combe et al. (2008), LPSC
2381. [10] Combe J.-Ph. et al., submitted to Earth and Planet. Sci. Let.[11]Mushkin and Gillespie (2006), GRL
33. [12] Pinet et al. (2006) LPSC 1120. [13] Jehl et al. (2008), JGR 113. Work supported by the HRSC team
and the NASA Mars Data Analysis Program NN-X07AZ37G.
http://www.bearfightcenter.com
P53A-1441
Measurement of Mars Atmosphere Argon Density with the APXS on the Opportunity site
Using the APXS on board the Opportunity rover on MER mission, we were able to measure the argon density variation in the martian atmosphere as a function of seasons. The freezing and melting of the CO2 in the martian atmosphere at the poles creates local atmospheric lows/highs that moves the air mass. The argon, however, never freezes and stays in the air. An enhancement of Ar/CO2 mixing ratio by a factor of six at the martian South pole during the winter has been observed by the GRS onboard the Odyssey orbiter. In order to study this effect from the ground at the Opportunity site on MER mission, we have been making dedicated APXS measurements of the Ar in the atmosphere. In this case, the only real peak in the X-ray spectrum is the Ar line. To a good approximation, the APXS count rate is proportional to the number of argon atoms in the sensing volume, and hence measures the atmospheric density of argon, ρAr. If the atmosphere were perfectly mixed, the argon partial pressure would be constant. If we define the Local Mix- ing Ratio (MRlocal) as equal to the ratio of the local Ar partial pressure to the global average, then the measurement PAr/T is proportional to the local mixing ratio. The APXS Ar experiment thus gives a direct measurement of the local mixing ratio at the MER landing sites and it is a direct probe of the global circulation between the polar CO2 resource/sink and the equatorial regions. We have found that the Ar amount in the martian atmosphere at the Opportunity landing site is not constant and it is changing with the changing seasons. The change of the Ar in the atmos-phere follows the overall change in the atmospheric pressure but it is not in phase with it. There is a delay of many months between the maximum in Ar/CO2 mixing ratio and the pressure maximum. In addition, we have even seen the asymmetry between the south and north martian poles due to a different degree of CO2 contribution from both poles during the martian year. The actual Ar abundance that is realized at the near equatorial location of the rovers is controlled principally by the efficiency with which the atmosphere can mix away the Ar abundance gradients that occur from the localized condensation of CO2 at the poles. While we understand the overall condensation/sublimation cycles of CO2 in Mars" atmosphere, we do not have a good understanding of the meridional mixing that controls the equatorial abundance of Ar that we are measuring on the rovers with their APXS instruments. We expect that (at least) two factors are important in controlling this mixing that smooth the Ar gradients. The first is the Hadley-cell type circulation of the Martian atmosphere that moves the bulk of the atmosphere between the regions it is heated (near the equator) and more poleward latitudes where it is cooled. However, the Hadley cell meridional circulation only extends up to ~60 degrees latitude near the winter pole. At this point, a polar vortex of fast zonal winds exists, and the Hadley cell circulation is closed off from the winter pole.Transport across Mars' polar vortex is not well understood, and indeed not only important for Ar abundance on Mars, but also the transport of H2O and dust to the polar regions of Mars. Thus, by studying the Ar abundance at the equator on Mars, we will have some insight into the meridional circulation and mixing present on Mars, not only in the organized Hadley cell, but also across the polar vortex.