ALBERT

Description: The cratered plains of Gusev traversed by Spirit are generally low-relief rocky plains dominated by impact and eolian processes. Ubiquitous shallow, soil-filled, circular depressions, called hollows, are modified impact craters. Rocks are dark, fine-grained basalts, and the upper 10 m of the cratered plains appears to be an impact-generated regolith developed over intact basalt flows. Systematic field observations across the cratered plains identified vesicular clasts and rare scoria similar to original lava flow tops, consistent with an upper inflated surface of lava flows with adjacent collapse depressions. Crater and hollow morphometry are consistent with most being secondaries. The size frequency distribution of rocks 〉0.1 m diameter generally follows exponential functions similar to other landing sites for total rock abundances of 5-35%. Systematic clast counts show that areas with higher rock abundance and more large rocks have higher thermal inertia. Plains with lower thermal inertia have fewer rocks and substantially more pebbles that are well sorted and evenly spaced, similar to a desert pavement or lag. Eolian bed forms (ripples and wind tails) have coarse surface lags, and many are dust covered and thus likely inactive. Deflation of the surface _5-25 cm likely exposed two-toned rocks and elevated ventifacts and transported fines into craters creating the hollows. This observed redistribution yields extremely slow average erosion rates of _0.03 nm/yr and argues for very little long-term net change of the surface and a dry and desiccating environment similar to today's since the Hesperian (or _3 Ga).

Description: We are using data from the Pancam and Microscopic Imager (MI) on the Opportunity rover to characterize the soil grains at Meridiani Planum. We have traced individual grains in all MI images of the soils using the software application ImageJ distributed by NIH, and subsequently derived size and shape properties about the grains. The resolution of the MI is 31 microns per pixel [1] so we limit our measurements to those grains larger than about 0.3 mm in size. In cases where the grain is partially or substantially buried by other grains or finer soil particles, we do not make a measurement. False-color composites from Pancam images that cover the same location imaged by MI are made from the Left 2,5,6 (753, 535, 482 nm) filters or Right 2,7,1 (753, 1009, 430 nm) filters [2] in the Red, Green, and Blue channels, respectively. These color images are then merged with the MI images to illustrate color properties of particular grains. Pancam spectra are also extracted from grains when there is sufficient spatial coverage. in diameter. Figure 2 illustrates the dominance of these small grains at this particular location, which happens to be on the southern wall of Eagle crater. The Pancam color merge with this MI image suggests that the small spherules are more consistent with the basalt grains than the blueberries (spherulitic concretions derived from outcrop rocks [7]). The resolution of Pancam images of this location is on the order of 0.5 mm so the grains are only barely resolved. A Mossbauer measurement taken on an adjacent soil (Sol 53 Vanilla) that is composed solely of these smaller spherules (Fig 1) is consistent with a basaltic composition for the grains. Their concentration at this particular location in a brighter, elongate patch along the southeastern wall compared to elsewhere inside Eagle crater suggests wind activity favored their transport and subsequent deposition here. Their spherical shape is also possibly the result of wind action rounding them during transport, though water action cannot be ruled out.

Description: The second MER rover (Opportunity) landed on Meridiani Planum on January 24, 2004 inside a shallow crater. The science rational for the selection of the landing site centered on detection of the mineral hematite from martian orbit by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) [1,2]. Other smaller occurrences of hematite are in Aram Chaos and several isolated spots in Valles Marineris. Proposed formation pathways for martian hematite include both aqueous (e.g., low temperature precipitation of Fe oxides/oxyhydroxides in a lacustrine environment, laterite-style weathering, and precipitation from fluids having a hydrothermal origin) and dry (e.g., oxidation of magnetite rich ash) processes [e.g., 1,2,3]. The crystallographic c-face of martian hematite must be exaggerated to account for the thermal emissions spectra and it must be gray in color so as to account for the absence of the characteristic spectral signature of red hematite at visible wavelengths

Description: Erosion rates derived from the Gusev cratered plains and the erosion of weak sulfates by saltating sand at Meridiani Planum are so slow that they argue that the present dry and desiccating environment has persisted since the Early Hesperian. In contrast, sedimentary rocks at Meridiani formed in the presence of groundwater and occasional surface water, and many Columbia Hills rocks at Gusev underwent aqueous alteration during the Late Noachian, approximately coeval with a wide variety of geomorphic indicators that indicate a wetter and likely warmer environment. Two-toned rocks, elevated ventifacts, and perched and undercut rocks indicate localized deflation of the Gusev plains and deposition of an equivalent amount of sediment into craters to form hollows, suggesting average erosion rates of approx.0.03 nm/yr. Erosion of Hesperian craters, modification of Late Amazonian craters, and the concentration of hematite concretions in the soils of Meridiani yield slightly higher average erosion rates of 1-10 nm/yr in the Amazonian. These erosion rates are 2-5 orders of magnitude lower than the slowest continental denudation rates on Earth, indicating that liquid water was not an active erosional agent. Erosion rates for Meridiani just before deposition of the sulfate-rich sediments and other eroded Noachian areas are comparable with slow denudation rates on Earth that are dominated by liquid water. Available data suggest the climate change at the landing sites from wet and likely warm to dry and desiccating occurred sometime between the Late Noachian and the beginning of the Late Hesperian (3.7-3.5 Ga).

Description: Laser-induced breakdown spectroscopy (LIBS) uses pulses of laser light to ablate a material from the surface of a sample and produce an expanding plasma. The optical emission from the plasma produces a spectrum which can be used to classify target materials and estimate their composition. The ChemCam instrument on the Mars Science Laboratory (MSL) mission will use LIBS to rapidly analyze targets remotely, allowing more resource- and time-intensive in-situ analyses to be reserved for targets of particular interest. ChemCam will also be used to analyze samples that are not reachable by the rover's in-situ instruments. Due to these tactical and scientific roles, it is important that ChemCam-derived sample compositions are as accurate as possible. We have compared the results of partial least squares (PLS), multilayer perceptron (MLP) artificial neural networks (ANNs), and cascade correlation (CC) ANNs to determine which technique yields better estimates of quantitative element abundances in rock and mineral samples. The number of hidden nodes in the MLP ANNs was optimized using a genetic algorithm. The influence of two data preprocessing techniques were also investigated: genetic algorithm feature selection and averaging the spectra for each training sample prior to training the PLS and ANN algorithms. We used a ChemCam-like laboratory stand-off LIBS system to collect spectra of 30 pressed powder geostandards and a diverse suite of 196 geologic slab samples of known bulk composition. We tested the performance of PLS and ANNs on a subset of these samples, choosing to focus on silicate rocks and minerals with a loss on ignition of less than 2 percent. This resulted in a set of 22 pressed powder geostandards and 80 geologic samples. Four of the geostandards were used as a validation set and 18 were used as the training set for the algorithms. We found that PLS typically resulted in the lowest average absolute error in its predictions, but that the optimized MLP ANN and the CC ANN often gave results comparable to PLS. Averaging the spectra for each training sample and/or using feature selection to choose a small subset of wavelengths to use for predictions gave mixed results, with degraded performance in some cases and similar or slightly improved performance in other cases. However, training time was significantly reduced for both PLS and ANN methods by implementing feature selection, making this a potentially appealing method for initial, rapid-turn-around analyses necessary for Chemcam's tactical role on MSL. Choice of training samples has a strong influence on the accuracy of predictions. We are currently investigating the use of clustering algorithms (e.g. k-means, neural gas, etc.) to identify training sets that are spectrally similar to the unknown samples that are being predicted, and therefore result in improved predictions

Description: Visible and Near Infrared (VNIR) multispectral observations of rocks made by the Mars Exploration Rover Spirit s Panoramic camera (Pancam) have been analysed using a spectral mixture analysis (SMA) methodology. Scenes have been examined from the Gusev crater plains into the Columbia Hills. Most scenes on the plains and in the Columbia Hills could be modeled as three endmember mixtures of a bright material, rock, and shade. Scenes of rocks disturbed by the rover s Rock Abrasion Tool (RAT) required additional endmembers. In the Columbia Hills there were a number of scenes in which additional rock endmembers were required. The SMA methodology identified relatively dust-free areas on undisturbed rock surfaces, as well as spectrally unique areas on RAT abraded rocks. Spectral parameters from these areas were examined and six spectral classes were identified. These classes are named after a type rock or area and are: Adirondack, Lower West Spur, Clovis, Wishstone, Peace, and Watchtower. These classes are discriminable based, primarily, on near-infrared (NIR) spectral parameters. Clovis and Watchtower class rocks appear more oxidized than Wishstone class rocks and Adirondack basalts based on their having higher 535 nm band depths. Comparison of the spectral parameters of these Gusev crater rocks to parameters of glass-dominated basaltic tuffs indicates correspondence between measurements of Clovis and Watchtower classes, but divergence for the Wishstone class rocks which appear to have a higher fraction of crystalline ferrous iron bearing phases. Despite a high sulfur content, the rock Peace has NIR properties resembling plains basalts.

Description: The Mars Exploration Rover Spirit has traversed a fairly flat, rock-strewn terrain whose surface is shaped primarily by impact events, although some of the landscape has been altered by eolian processes.Impacts ejected basaltic rocks that probably were part of locally formed lava flows from at least 10 meters depth.Some rocks have been textured and/or partially buried by windblown sediments less than 2 millimeters in diameter that concentrate within shallow, partially filled, circular impact depressions referred to as hollows.The terrain traversed during the 90-sol (martian solar day) nominal mission shows no evidence for an ancient lake in Gusev crater.

Description: Iron-rich spherules (〉90% Fe2O3 from electron microprobe analyses) approx.10-100 microns in diameter are found within sulfate-rich rocks formed by aqueous, acid-sulfate alteration of basaltic tephra on Mauna Kea volcano, Hawaii. Although some spherules are nearly pure Fe, most have two concentric compositional zones, with the core having a higher Fe/Al ratio than the rim. Oxide totals less than 100% (93-99%) suggest structural H2O and/or /OH. The transmission Moessbauer spectrum of a spherule-rich separate is dominated by a hematite (alpha-Fe2O3) sextet whose peaks are skewed toward zero velocity. Skewing is consistent with Al(3+) for Fe(3+) substitution and structural H2O and/or /OH. The grey color of the spherules implies specular hematite. Whole-rock powder X-ray diffraction spectra are dominated by peaks from smectite and the hydroxy sulfate mineral natroalunite as alteration products and plagioclase feldspar that was present in the precursor basaltic tephra. Whether spherule formation proceeded directly from basaltic material in one event (dissolution of basaltic material and precipitation of hematite spherules) or whether spherule formation required more than one event (formation of Fe-bearing sulfate rock and subsequent hydrolysis to hematite) is not currently constrained. By analogy, a formation pathway for the hematite spherules in sulfate-rich outcrops at Meridiani Planum on Mars (the Burns formation) is aqueous alteration of basaltic precursor material under acid-sulfate conditions. Although hydrothermal conditions are present on Mauna Kea, such conditions may not be required for spherule formation on Mars if the time interval for hydrolysis at lower temperatures is sufficiently long.

Description: Rocks on the ejecta blanket of Bonneville crater and along Spirit s traverse over the Gusev plains towards the Columbia Hills are angular and strewn across the surface. They have a basaltic composition [1,2], and their Mossbauer spectra are dominated by an olivine doublet [1]. The ubiquitous presence of abundant olivine in rocks and in surrounding soil suggests that physical rather than chemical weathering processes currently dominate the plains at Gusev crater [1]. However, MB spectra of rocks and outcrops in the Columbia Hills suggest more aggressive alteration processes have occurred. Ascending into the hills, Spirit encountered outcrop and rocks exhibiting layered structures. Some scattered rocks at the foot of the Columbia Hills appeared "rotten" or highly altered by physical and/or chemical processes (fig. 1). Mossbauer spectra of those rocks show a decrease in olivine accompanied by an increase in the Fe-oxides magnetite, hematite, and nanophase Fe3+ -oxides (fig. 2), suggesting that chemical weathering processes in the presence of water have altered these rocks and outcrops.

Description: Onboard the Mars Science Laboratory (MSL) Curiosity rover, the ChemCam instrument consists of :(1) a Laser-Induced Breakdown Spectrometer (LIBS) for elemental analysis of the targets [1;2] and (2) a Remote Micro Imager (RMI), for the imaging context of laser analysis [3]. Within the Gale crater, Curiosity traveled from Bradbury Landing through the Rocknest region and into Yellowknife Bay (YB). In the latter, abundant light-toned fracture-fill material were seen [4;5]. ChemCam analysis demonstrate that those fracture fills consist of calcium sulfates [6].