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Results for “Journal of Geophysical Research E: Planets”

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At least 19 recordsLinked to original sources
USGS2011

Sources and physical processes responsible for OH/H2O in the lunarsoil as revealed by the Moon Mineralogy Mapper (M3)

Analysis of two absorption features near 3 μ m in the lunar reflectance spectrum, observed by the orbiting M 3 spectrometer and interpreted as being due to OH and H 2 O, is presented, and the results are used to discuss the processes producing these molecules. This analysis focuses on the dependence of the absorptions on lunar physical properties, including composition, illumination, latitude, and temperature. Solar wind proton-induced hydroxylation is proposed as the creation process, and its products could be a source for other reported types of hydrogen-rich material and water. The irregular and damaged fine-grained lunar soil seems especially adapted for trapping solar wind protons and forming OH owing to abundant dangling oxygen bonds. The M 3 data reveal that the strengths of the two absorptions are correlated and widespread, and both are correlated with lunar composition but in different ways. Feldspathic material seems richer in OH. These results seem to rule out water from the lunar interior and cometary infall as major sources. There appear to be correlations of apparent band strengths with time of day and lighting conditions. However, thermal emission from the Moon reduces the apparent strengths of the M 3 absorptions, and its removal is not yet completely successful. Further, many of the lunar physical properties are themselves intercorrelated, and so separating these dependencies on the absorptions is difficult, due to the incomplete M 3 data set. This process should also operate on other airless silicate surfaces, such as Mercury and Vesta, which will be visited by the Dawn spacecraft in mid-2011.

Journal of Geophysical Research E: Planets↗
USGS2008

First in situ investigation of a dark wind streak on Mars

Prominent low‐albedo wind streaks issue from embayments at the north end of Victoria crater in Meridiani Planum, the site of surface investigations by the Mars Exploration Rover Opportunity. Opportunity was sent to examine the darkest of these wind streaks and compare it to the adjacent soil in the brighter corridor between the streaks. Two nearby sites inside and outside of the streak were selected for visible and infrared remote sensing and for elemental abundance measurements and images of the microscopic soil morphology. The rover was next sent to study the interaction of the wind streak with an obstacle, a small rock near the source of the streak. Opportunity's observations suggest that the Victoria wind streaks are deposits of basaltic sand blown out of the crater from the dark dunes nestled below the crater rim, particularly at the base of the alcove leading up to the darkest streak. No local sources of sand have been identified within the Victoria crater wall rock, suggesting that the sand is supplied from outside the crater and is presently escaping its temporary topographic trap. This process presents a possible explanation for the serrated margin of Victoria crater through abrasion of the soft rock as trapped sands are blown out of the crater and carve alcoves under various seasonal winds. Some indication of the rate of activity of these wind streaks will be provided by continued monitoring of the erasure of the rover's tracks.

Journal of Geophysical Research E: Planets↗
USGS2009

Global distribution of bedrock exposures on Mars using THEMIS high-resolution thermal inertia

We investigate high thermal inertia surfaces using the Mars Odyssey Thermal Emission Imaging System (THEMIS) nighttime temperature images (100 m/pixel spatial sampling). For this study, we interpret any pixel in a THEMIS image with a thermal inertia over 1200 J m -2 K -1 s -1/2 as "bedrock" which represents either in situ rock exposures or rock-dominated surfaces. Three distinct morphologies, ranked from most to least common, are associated with these high thermal inertia surfaces: (1) valley and crater walls associated with mass wasting and high surface slope angles; (2) floors of craters with diameters >25 km and containing melt or volcanics associated with larger, high-energy impacts; and (3) intercrater surfaces with compositions significantly more mafic than the surrounding regolith. In general, bedrock instances on Mars occur as small exposures (less than several square kilometers) situated in lower-albedo (<0.18), moderate to high thermal inertia (>350 J m -2 K -1 s -1/2 ), and relatively dust-free (dust cover index <0.95) regions; however, there are instances that do not follow these generalizations. Most instances are concentrated in the southern highlands, with very few located at high latitudes (poleward of 45 o N and 58 o S), suggesting enhanced mechanical breakdown probably associated with permafrost. Overall, Mars has very little exposed bedrock with only 960 instances identified from 75 o S to 75 o N with likely <3500 km 2 exposed, representing???1% of the total surface area. These data indicate that Mars has likely undergone large-scale surface processing and reworking, both chemically and mechanically, either destroying or masking a majority of the bedrock exposures on the planet. Copyright 2009 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗
USGS2008

Geomorphic evidence for former lobate debris aprons at low latitudes on Mars: Indicators of the Martian paleoclimate

Circumferential depressions enclosing mesas and plateaus in the northern Kasei Valles and in the Tartarus Colles regions of Mars are interpreted as indicators of the former extent of lobate debris aprons, thought to be mixtures of ice and elastic particles. These former lobate debris aprons existed about 1 Ga ago and were embayed by lavas or other flow deposits. After the lobate debris aprons had been removed by sublimation and deflation, topographic depressions with a depth of 50 m and a width of several kilometers were left behind between the mesa or plateau scarp and the solidified flow materials. These depressions or moats are located equatorward of ??30?? at significantly lower latitudes than generally observed for occurrences of modem, intact lobate debris aprons. This observation provides evidence that the paleoclimate at that time was different than today, probably due to a higher averaged obliquity of the planet's rotational axis. Copyright 2008 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗
USGS1999

Photogrammetric analysis of horizon panoramas: The Pathfinder landing site in Viking orbiter images

Tiepoint measurements, block adjustment techniques, and sunrise/sunset pictures were used to obtain precise pointing data with respect to north for a set of 33 IMP horizon images. Azimuth angles for five prominent topographic features seen at the horizon were measured and correlated with locations of these features in Viking orbiter images. Based on this analysis, the Pathfinder line/sample coordinates in two raw Viking images were determined with approximate errors of 1 pixel, or 40 m. Identification of the Pathfinder location in orbit imagery yields geological context for surface studies of the landing site. Furthermore, the precise determination of coordinates in images together with the known planet-fixed coordinates of the lander make the Pathfinder landing site the most important anchor point in current control point networks of Mars.

Journal of Geophysical Research E: Planets↗
USGS2001

Mars Global Surveyor observations of Martian fretted terrain

The Martian fretted terrain between latitudes 30° and 50°N and between 315° and 360°W has been reexamined in light of new Mars Orbiter Camera (MOC) and Mars Orbiter Laser Altimeter (MOLA) data from Mars Global Surveyor. Much of the terrain in the 30°–50° latitude belt in both hemispheres has a characteristic stippled or pitted texture at MOC (1.5 m) scale. The texture appears to result from partial removal of a formerly smooth, thin deposit as a result of sublimation and deflation. A complex history of deposition and exhumation is indicated by remnants of a former, thicker cover of layered deposits. In some hollows and on some slopes, particularly those facing the pole, are smooth textured deposits outlined by an outward facing escarpment. Throughout the study area are numerous escarpments with debris flows at their base. The escarpments typically have slopes in the 20°–30° range. At the base of the escarpment is commonly a deposit with striae oriented at right angles to the escarpment. Outside this deposit is the main debris apron with a surface that typically slopes 2°–3° and complex surface textures suggestive of compression, sublimation, and deflation. The presence of undeformed impact craters indicates that the debris flows are no longer forming. Fretted valleys contain lineated fill and are poorly graded. They likely form from fluvial valleys that were initially like those elsewhere on the planet but were subsequently widened and filled by the same mass-wasting processes that formed the debris aprons. Slope reversals indicate that downvalley flow of the lineated fill is minor. The ubiquitous presence of breaks in slope formed by mass wasting and the complex surface textures that result from mass wasting, deflation, and sublimation decreases the recognizability of the shorelines formerly proposed for this area.

Journal of Geophysical Research E: Planets↗
USGS2012

Venus cartography

The entire surface of the planet Venus is being mapped at global and regional scales (1:50,000,000 through 1:1,500,000) with synthetic aperture radar (SAR), radar altimeter, and radiometer measurements of physical properties from the Magellan spacecraft. The mapping includes SAR image mosaics, shaded relief maps, and topographic contour overlays made from altimetry data and by radargrammetric methods. Methods used include new techniques of radar image processing that became operational as a result of the Magellan mission. The area being mapped (at a resolution of 75 m/pixel) is roughly equivalent to that of Earth, including seafloors. The mapping is designed to support geologic and geophysical investigations.

Journal of Geophysical Research E: Planets↗
USGS2003

Oceans on Mars: An assessment of the observational evidence and possible fate

If the large Late Hesperian outflow channels were eroded by extensive floods, as appears likely, then large bodies of water must have once occupied the northern plains during that period. Previous estimates of the sizes of bodies of water in the northern lowlands range up to 3 ?? 108 km3. Several contacts have been previously mapped around the edges of the northern plains and interpreted to be shorelines remaining from these former standing bodies of water. We examine the elevations and geologic relations along these contacts in detail and find little support for their interpretation as shorelines. Some contacts are clearly of volcanic origin, and all have significant variations in elevation. Better support for the former presence of water over large parts of the northern plains is provided by the Vastitas Borealis Formation (VBF). Most of the post-Noachian fill within the northern basin is ridged plains of Lower Hesperian age, interpreted to be volcanic in origin. Overlying the ridged plains is the VBF, a thin veneer of material of Upper Hesperian age. The VBF may have been deposited from large floods. Support for this interpretation is the similarity in age between the outflow channels and the VBF, the presence of the VBF at the lower ends of the outflow channels, and identification of numerous features in the outcrop areas of the VBF that are suggestive of basal melting of an ice sheet. To cover all the area over which the VBF is exposed would require ???2.3 ?? 107 km3 of water. Spread over the entire surface of Mars, this volume is equal to a global layer (global equivalent layer, or GEL) ???156 m deep. We find no support for the larger estimates of ocean volumes that range up to 3 ?? 108 km3 and which imply comparable amounts of water per unit area as are currently present on the surface of the Earth. Under present climatic conditions on Mars an ocean would freeze in a geologically short time period (???104 years), then would sublimate away at rates strongly dependent on the presence or absence of debris on the ice surface. The present VBF is interpreted as a sublimation residue from the ponded outflow channel effluents. The fate of a volume of water thought to have been emplaced by the outflow channels (???2.3 ?? 107 km3) is largely accounted for by the presence of other existing reservoirs on the planet. An approximately 20-30 m GEL of water is estimated to be in the present polar caps, and a 50 m GEL may have escaped to space since the Hesperian, leaving ???80 m GEL unaccounted for. This amount may be partly trapped in other volatile-rich deposits on the surface, and a significant amount could have reentered the groundwater system by south polar basal melting and been progressively cold-trapped at the base of a growing cryosphere. On the basis of our assessment of the Hesperian-aged deposits, we predict that testing of the Clifford and Parker [2001] hypothesis that a Noachian-aged ocean covered up to one third of the surface of Mars will be made very difficult by the enhanced degradation rates in the Noachian and subsequent geological events in the northern lowlands.

Journal of Geophysical Research E: Planets↗
USGS2001

Mars Color Imager (MARCI) on the Mars Climate Orbiter

The Mars Color Imager, or MARCI, experiment on the Mars Climate Orbiter (MCO) consists of two cameras with unique optics and identical focal plane assemblies (FPAs), Data Acquisition System (DAS) electronics, and power supplies. Each camera is characterized by small physical size and mass (∼6×6×12 cm, including baffle; <500 g), low power requirements (<2.5 W, including power supply losses), and high science performance (1000×1000 pixel, low noise). The Wide Angle (WA) camera will have the capability to map Mars in five visible and two ultraviolet spectral bands at a resolution of better than 8 km/pixel under the worst case downlink data rate. Under better downlink conditions the WA will provide kilometer-scale global maps of atmospheric phenomena such as clouds, hazes, dust storms, and the polar hood. Limb observations will provide additional detail on atmospheric structure at 1/3 scale-height resolution. The Medium Angle (MA) camera is designed to study selected areas of Mars at regional scale. From 400 km altitude its 6° FOV, which covers ∼40 km at 40 m/pixel, will permit all locations on the planet except the poles to be accessible for image acquisitions every two mapping cycles (roughly 52 sols). Eight spectral channels between 425 and 1000 nm provide the ability to discriminate both atmospheric and surface features on the basis of composition. The primary science objectives of MARCI are to (1) observe Martian atmospheric processes at synoptic scales and mesoscales, (2) study details of the interaction of the atmosphere with the surface at a variety of scales in both space and time, and (3) examine surface features characteristic of the evolution of the Martian climate over time. MARCI will directly address two of the three high-level goals of the Mars Surveyor Program: Climate and Resources. Life, the third goal, will be addressed indirectly through the environmental factors associated with the other two goals.

Journal of Geophysical Research E: Planets↗
USGS2016

Stratigraphy of the north polar layered deposits of Mars from high-resolution topography

The stratigraphy of the layered deposits of the polar regions of Mars is theorized to contain a record of recent climate change linked to insolation changes driven by variations in the planet's orbital and rotational parameters. In order to confidently link stratigraphic signals to insolation periodicities, a description of the stratigraphy is required based on quantities that directly relate to intrinsic properties of the layers. We use stereo Digital Terrain Models (DTMs) from the High Resolution Imaging Science Experiment (HiRISE) to derive a characteristic of North Polar Layered Deposits (NPLD) strata that can be correlated over large distances: the topographic protrusion of layers exposed in troughs, which is a proxy for the layers&rsquo; resistance to erosion. Using a combination of image analysis and a signal-matching algorithm to correlate continuous depth-protrusion signals taken from DTMs at different locations, we construct a stratigraphic column that describes the upper ~500&thinsp;m of at least 7% of the area of the NPLD, and find accumulation rates that vary by factors of up to two. We find that, when coupled with observations of exposed layers in orbital images, the topographic expression of the strata is consistently continuous through large distances in the top 300 &ndash; 500&thinsp;m of the NPLD, suggesting it is better related to intrinsic layer properties than brightness alone.

Journal of Geophysical Research E: Planets↗
USGS2005

Three decades of Martian surface changes

The surface of Mars has changed dramatically during the three decades spanned by spacecraft exploration. Comparisons of Mars Global Surveyor images with Viking and Mariner 9 pictures suggest that more than one third of Mars' surface area has brightened or darkened by at least 10%. Such albedo changes could produce significant effects on solar heating and the global circulation of winds across the planet. All of the major changes took place in areas of moderate to high thermal inertia and rock abundance, consistent with burial of rocky surfaces by thin dust layers deposited during dust storms and subsequent exposure of the rocky surfaces by aeolian erosion. Several distinct mechanisms contribute to aeolian erosion on Mars. Prevailing winds dominate erosion at low latitudes, producing diffuse albedo boundaries and elongated wind streaks generally oriented in the direction of southern summer winds. Dust devils darken the mid to high latitudes from 45 to 70 degrees during the summer seasons, forming irregular albedo patterns consisting of dark linear tracks. Dust storms produce regional albedo variations with distinct but irregular margins. Dark sand duties in southern high latitudes appear to be associated with regional darkening that displays diffuse albedo boundaries. No surface changes were observed to repeat regularly on an annual basis, but many of the changes took place in areas that alternate episodically between high- and low-albedo states as thin mantles of dust are deposited and later stripped off. Hence the face of Mars remains recognizable after a century of telescopic observations, in spite of the enormous extent of alteration that has taken place during the era of spacecraft exploration.

Journal of Geophysical Research E: Planets↗
USGS2012

The geology and distribution of impact craters on Venus: What are they telling us?

Magellan has revealed an ensemble of impact craters on Venus that is unique in many important ways. We have compiled a data base describing the 842 craters on 89% of Venus' surface mapped through orbit 2578. (The craters range in diameter from 1.5 to 280 km.) We have studied the distribution, size‐density, morphology, geology, and associated surface properties of these craters both in the aggregate and, for some craters, in greater detail. We find that (1) the spatial distribution of craters is highly uniform; (2) the size‐density distribution of large craters (diameters ≥35 km) is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0.5 Ga (based on the estimated population of Venus‐crossing asteroids); (3) unlike the case on other planets, the density of small craters (diameters ≤35 km) declines rapidly with decreasing diameters because of atmospheric filtering; (4) the spectrum of crater modification differs greatly from that on other planets: 62% of all craters are pristine, only 4% are embayed by lavas, and the remainder are affected by tectonism, but none are severely and progressively depleted (as extrapolated from the size‐density distribution of larger craters); (5) large craters have a progression of morphologies generally similar to those on other planets, but small craters are typically irregular or multiple rather than bowl shaped; (6) diffuse radar‐bright or ‐dark features surround some craters, and 367 similar diffuse “splotches” with no central crater are observed; and (7) other crater features unique to Venus include radar‐bright or ‐dark parabolic arcs opening westward and extensive outflows originating in crater ejecta. The first three of these observations are entirely unexpected. We interpret them as indicating that the planet's cratering record was erased by a global resurfacing event or events, the latest ending about 0.5 Ga, after which volcanic activity declined (but did not cease entirely). Since the last resurfacing event, a maximum of 10% of the planet has been resurfaced and only about 4% of the craters have been obliterated. Convective thermal evolution models support this interpretation (Arkani‐Hamed and Toksoz, 1984). Observations 3–7 confirm quantitatively the expectation that the dense atmosphere of Venus has strongly affected the production of craters. Large impactors have been relatively unaffected, intermediate‐sized ones have been fragmented and have produced overlapping or multiple craters, a narrow size range has produced shock‐induced “splotches” but no craters, and the smallest bodies have had no observable effect on the surface. The number of craters eliminated by the “atmospheric filter” is enormous, about 98% of the craters between 2 and 35 km in diameter that Magellan might have observed on a hypothetical airless Venus. Unique crater‐related features such as parabolas and outflow deposits demonstrate the roles of Venus' high atmospheric density and temperature in modifying the crater formation process. Finally, heavily fractured craters and lava‐embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting Aphrodite Terra and Atla, Beta, Themis, and Phoebe regiones. These craters thus provide physical evidence for recent volcanic and tectonic activity at a low level.

Journal of Geophysical Research E: Planets↗
USGS2002

Thermal infrared spectroscopy of experimentally shocked anorthosite and pyroxenite: Implications for remote sensing of Mars

The feldspar and pyroxene mineralogies on Mars revealed by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor likely record a variety of shock effects, as suggested by petrologic analyses of the Martian meteorites and the abundance of impact craters on the planet's surface. To study the effects of shock pressures on thermal infrared spectra of these minerals, we performed shock recovery experiments on orthopyroxenite and anorthosite samples from the Stillwater Complex (Montana) over peak pressures from 17 to 63 GPa. We acquired emissivity and hemispherical reflectance spectra (350–1400 cm −1 ; ∼7–29 μm) of both coherent chips and fine-grained powders of shocked and unshocked samples. These spectra are more directly comparable to remotely sensed data of Mars (e.g., TES) than previously acquired absorption or transmission spectra of shocked minerals. The spectra of experimentally shocked feldspar show systematic changes with increasing pressure due to depolymerization of the silica tetrahedra. For the spectra of chips, this includes the disappearance of small bands in the 500–650 cm −1 region and a strong band at 1115 cm −1 , and changes in positions of a strong band near 940 cm −1 and the Christiansen feature near 1250 cm −1 . Spectra of the shocked powders show the gradual disappearance of a transparency feature near 830 cm −1 . Fewer changes are observed in the pyroxene spectra at pressures as high as 63 GPa. Spectra of experimentally shocked minerals will help identify more precisely the mineralogy of rocks and soils not only from TES but also from Mars instruments such as miniTES and THEMIS.

Journal of Geophysical Research E: Planets↗
USGS2010

Gone with the wind: Eolian erasure of the Mars Rover tracks

The wheel tracks left by the Mars Exploration 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 Mars Reconnaissance Orbiter document the erasure of the tracks over a period of more than 2 Mars years. Close‐up examinations of track crossings, where the rovers encountered tracks made hundreds of Martian solar days earlier, provide insights into the mechanisms and time scales of eolian alteration on Mars. These observations suggest that fallout of atmospheric dust plays only a minor role in obscuring rover tracks over time. Instead, track erasure is dominated by sediment that is transported by surface winds. Both deposition and erosion act to erase the rover tracks. The length scales for eolian sediment transport are hundreds of meters at least, much larger than the size of the tracks. Gradual processes such as dust devils and sand saltation have minor effects that can nonetheless erase rover tracks over long time periods. However, short‐lived strong wind events associated with seasonal dust storms have much more pronounced effects, significantly altering the tracks on time scales of days. These episodic strong winds tend to occur annually during the perihelion season. The time scale for track erasure is typically only 1 Martian year.

Journal of Geophysical Research E: Planets↗
USGS2014

Igneous mineralogy at Bradbury Rise: the first ChemCam campaign at Gale crater

Textural and compositional analyses using ChemCam Remote Micro Imager (RMI) and Laser Induced Breakdown Spectroscopy (LIBS) have been performed on 5 float rocks and coarse gravels along the first 100 meters of the Curiosity traverse at Bradbury rise. ChemCam, the first LIBS instrument sent to another planet, offers the opportunity to assess mineralogic diversity at grain-size scales (∼100μm) and, from this, lithologic diversity. Depth profiling indicates that targets are relatively free of surface coatings. One type of igneous rock is volcanic and includes both aphanitic (Coronation) and porphyritic (Mara) samples. The porphyritic sample shows dark grains that are likely pyroxene megacrysts in a finegrained mesostasis containing andesine needles. Both types have magnesium-poor basaltic compositions and in this respect are similar to the evolved Jake-Matijevic rock [Stolper et al., 2013] analyzed further along the Curiosity traverse both with APXS and ChemCam instruments. The second rock-type encountered is a coarse-grained intrusive rock (Thor Lake) showing equigranular texture with mm size crystals of feldspars and Fe-Ti oxides. Such a rock is not unique at Gale as the surrounding coarse gravels (such as Beaulieu) and the conglomerate Link are dominated by feldspathic (andesine-bytownite) clasts. Finally, alkali feldspar compositions associated with a silica polymorph have been analyzed in fractured filling material of Preble rock and in Stark, a putative pumice or an impact melt. These observations document magmatic diversity at Gale and describe the first fragments of feldspar-rich lithologies (possibly an anorthosite) that may be ancient crust transported from the crater rim and now forming float rocks, coarse gravel or conglomerate clasts.

Journal of Geophysical Research E: Planets↗
USGS2014

HiRISE observations of new impact craters exposing Martian ground ice

Twenty small new impact craters or clusters have been observed to excavate bright material inferred to be ice at mid and high latitudes on Mars. In the northern hemisphere, the craters are widely distributed geographically and occur at latitudes as low as 39°N. Stability modeling suggests that this ice distribution requires a long-term average atmospheric water vapor content around 25 precipitable microns, more than double the present value, which is consistent with the expected effect of recent orbital variations. Alternatively, near-surface humidity could be higher than expected for current column abundances if water vapor is not well-mixed with atmospheric CO 2 , or the vapor pressure at the ice table could be lower due to salts. Ice in and around the craters remains visibly bright for months to years, indicating that it is clean ice rather than ice-cemented regolith. Although some clean ice may be produced by the impact process, it is likely that the original ground ice was excess ice (exceeding dry soil pore space) in many cases. Observations of the craters suggest small-scale heterogeneities in this excess ice. The origin of such ice is uncertain. Ice lens formation by migration of thin films of liquid is most consistent with local heterogeneity in ice content and common surface boulders, but in some cases nearby thermokarst landforms suggest large amounts of excess ice that may be best explained by a degraded ice sheet.

Journal of Geophysical Research E: Planets↗
USGS2014

Trace element geochemistry (Li, Ba, Sr, and Rb) using Curiosity's ChemCam: early results for Gale crater from Bradbury Landing Site to Rocknest

The ChemCam instrument package on the Mars rover, Curiosity, provides new capabilities to probe the abundances of certain trace elements in the rocks and soils on Mars using the laser-induced breakdown spectroscopy technique. We focus on detecting and quantifying Li, Ba, Rb, and Sr in targets analyzed during the first 100 sols, from Bradbury Landing Site to Rocknest. Univariate peak area models and multivariate partial least squares models are presented. Li, detected for the first time directly on Mars, is generally low (<15 ppm). The lack of soil enrichment in Li, which is highly fluid mobile, is consistent with limited influx of subsurface waters contributing to the upper soils. Localized enrichments of up to ~60 ppm Li have been observed in several rocks but the host mineral for Li is unclear. Bathurst_Inlet is a fine-grained bedrock unit in which several analysis locations show a decrease in Li and other alkalis with depth, which may imply that the unit has undergone low-level aqueous alteration that has preferentially drawn the alkalis to the surface. Ba (~1000 ppm) was detected in a buried pebble in the Akaitcho sand ripple and it appears to correlate with Si, Al, Na, and K, indicating a possible feldspathic composition. Rb and Sr are in the conglomerate Link at abundances >100 ppm and >1000 ppm, respectively. These analysis locations tend to have high Si and alkali abundances, consistent with a feldspar composition. Together, these trace element observations provide possible evidence of magma differentiation and aqueous alteration.

Journal of Geophysical Research E: Planets↗
USGS2014

Sands at Gusev Crater, Mars

Processes, environments, and the energy associated with the transport and deposition of sand at Gusev Crater are characterized at the microscopic scale through the comparison of statistical moments for particle size and shape distributions. Bivariate and factor analyses define distinct textural groups at 51 sites along the traverse completed by the Spirit rover as it crossed the plains and went into the Columbia Hills. Fine-to-medium sand is ubiquitous in ripples and wind drifts. Most distributions show excess fine material, consistent with a predominance of wind erosion over the last 3.8 billion years. Negative skewness at West Valley is explained by the removal of fine sand during active erosion, or alternatively, by excess accumulation of coarse sand from a local source. The coarse to very coarse sand particles of ripple armors in the basaltic plains have a unique combination of size and shape. Their distribution display significant changes in their statistical moments within the ~400 m that separate the Columbia Memorial Station from Bonneville Crater. Results are consistent with aeolian and/or impact deposition, while the elongated and rounded shape of the grains forming the ripples, as well as their direction of origin, could point to Ma'adim Vallis as a possible source. For smaller particles on the traverse, our findings confirm that aeolian processes have dominated over impact and other processes to produce sands with the observed size and shape patterns across a spectrum of geologic (e.g., ripples and plains soils) and aerographic settings (e.g., wind shadows).

Journal of Geophysical Research E: Planets↗