The presence of hydrated phases in the soil and near‐surface bedrock of Gale Crater is thought to be direct evidence for water‐rock interaction in the crater in the ancient past. Layered sediments ...over the Gale Crater floor are thought to have formed in past epochs due to sediment transport, accumulation, and cementation through interaction with fluids, and the observed strata of water‐bearing minerals record the history of these episodes. The first data analysis of the Dynamic Albedo of Neutrons (DAN) investigation on board the Curiosity rover is presented for 154 individual points of active mode measurements along 1900 m of the traverse over the first 361 Martian solar days in Gale crater. It is found that a model of constant water content within subsurface should be rejected for practically all tested points, whereas a two‐layer model with different water contents in each layer is supported by the data. A so‐called direct two‐layer model (water content increasing with depth) yields acceptable fits for odometry ranges between 0 and 455 m and beyond 638 m. The mean water (H2O) abundances of the top and bottom layers vary from 1.5 to 1.7 wt % and from 2.2 to 3.3 wt %, respectively, while at some tested spots the water content is estimated to be as high as ~5 wt %. The data for odometry range 455–638 m support an inverse two‐layer model (water content decreasing with depth), with an estimated mean water abundance of 2.1 ± 0.1 wt % and 1.4 ± 0.04 wt % in the top and bottom layers, respectively.
Key Points
First analysis of active neutron data from DAN instrument on board MSL roverEstimations of water distribution along MSL rover traverse by DAN instrumentEstimations of chlorine abundance along MSL rover traverse by DAN instrument
Compositional mapping of Mars at the 100-metre scale with the Mars Odyssey Thermal Emission Imaging System (THEMIS) has revealed a wide diversity of igneous materials. Volcanic evolution produced ...compositions from low-silica basalts to high-silica dacite in the Syrtis Major caldera. The existence of dacite demonstrates that highly evolved lavas have been produced, at least locally, by magma evolution through fractional crystallization. Olivine basalts are observed on crater floors and in layers exposed in canyon walls up to 4.5 km beneath the surface. This vertical distribution suggests that olivine-rich lavas were emplaced at various times throughout the formation of the upper crust, with their growing inventory suggesting that such ultramafic (picritic) basalts may be relatively common. Quartz-bearing granitoid rocks have also been discovered, demonstrating that extreme differentiation has occurred. These observations show that the martian crust, while dominated by basalt, contains a diversity of igneous materials whose range in composition from picritic basalts to granitoids rivals that found on the Earth.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Spirit landing site in Gusev Crater on Mars contains dark, fine-grained, vesicular rocks interpreted as lavas. Pancam and MiniâThermal Emission Spectrometer (Mini-TES) spectra suggest that all ...of these rocks are similar but have variable coatings and dust mantles. Magnified images of brushed and abraded rock surfaces show alteration rinds and veins. Rock interiors contain â¤25% megacrysts. Chemical analyses of rocks by the Alpha Particle X-ray Spectrometer are consistent with picritic basalts, containing normative olivine, pyroxenes, plagioclase, and accessory FeTi oxides. MoÌssbauer, Pancam, and Mini-TES spectra confirm the presence of olivine, magnetite, and probably pyroxene. These basalts extend the known range of rock compositions composing the martian crust.
On Mercury, high-reflectance, flat-floored depressions called hollows are observed nearly globally within low-reflectance material, one of Mercury’s major color units. Hollows are thought to be ...young, or even currently active, features that form via sublimation, or a “sublimation-like” process. The apparent abundance of sulfides within LRM combined with spectral detections of sulfides associated with hollows suggests that sulfides may be the phase responsible for hollow formation. Despite the association of sulfides with hollows, it is still not clear whether sulfides are the hollow-forming phase. To better understand which phase(s) might be responsible for hollow formation, we calculated sublimation rates for 57 candidate hollow-forming volatile phases from the surface of Mercury and as a function of depth beneath regolith lag deposits of various thicknesses. We found that stearic acid (C18H36O2), fullerenes (C60, C70), and elemental sulfur (S) have the appropriate thermophysical properties to explain hollow formation. Stearic acid and fullerenes are implausible hollow-forming phases because they are unlikely to have been delivered to or generated on Mercury in high enough volume to account for hollows. We suggest that S is most likely the phase responsible for hollow formation based on its abundance on Mercury and its thermophysical properties. We discuss the possibility that S is the phase responsible for hollow formation within the hollow-formation model framework proposed by Blewett et al. (2013). However, several potential limitations with that model lead us to suggest an alternative hollow-formation model: a subsurface heat source (most often impact-induced) generates thermal systems that drive sulfur-rich fumaroles in which S and other phases accumulate on and within the surface at night and sublimate during the day to create hollows. We call this hollow-formation model “Sublimation Cycling Around Fumarole Systems” (SCArFS). We suggest that thermal decomposition of sulfides within LRM is a main contributor to S and S-bearing gases within the proposed fumarole systems and that (re-)precipitation of sulfides may occur at the surface along hollow floors and rims.
•A thermophysical model was developed to test the viability of 57 candidate hollow-forming phases.•The thermophysical properties of elemental sulfur and its abundance on Mercury make it the most likely hollow-forming phase.•We propose a novel hollow-formation model: sublimation cycling around fumarole systems (SCArFS).
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).
The Miniature Thermal Emission Spectrometer (Mini‐TES) on board the Mars Exploration Rover Spirit is part of a payload designed to investigate whether a lake once existed in Gusev Crater. Mini‐TES ...has observed hundreds of rocks along the rover's traverse into the Columbia Hills, yielding information on their distribution, bulk mineralogy, and the potential role of water at the site. Although dust in various forms produces contributions to the spectra, we have established techniques for dealing with it. All of the rocks encountered on the plains traverse from the lander to the base of the Columbia Hills share common spectral features consistent with an olivine‐rich basaltic rock known as Adirondack Class. Beginning at the base of the West Spur of the Columbia Hills and across its length, the rocks are spectrally distinct from the plains but can be grouped into a common type called Clovis Class. These rocks, some of which appear as in‐place outcrop, are dominated by a component whose spectral character is consistent with unaltered basaltic glass despite evidence from other rover instruments for significant alteration. The northwest flank of Husband Hill is covered in float rocks known as Wishstone Class with spectral features that can be attributed uniquely to plagioclase feldspar, a phase that represents more than half of the bulk mineralogy. Rare exceptions are three classes of basaltic “exotics” found scattered across Husband Hill that may represent impact ejecta and/or float derived from local intrusions within the hills. The rare outcrops observed on Husband Hill display distinctive spectral characteristics. The outcrop called Peace shows a feature attributable to molecular bound water, and the outcrop that hosts the rock called Watchtower displays a dominant basaltic glass component. Despite evidence from the rover's payload for significant alteration of some of the rocks, no unambiguous detection of crystalline phyllosilicates or other secondary silicates has been observed by Mini‐TES. The mineralogical results supplied by Mini‐TES provide no clear evidence that a lake once existed in Gusev Crater.
Rocks on the floor of Gusev crater are basalts of uniform composition and mineralogy. Olivine, the only mineral to have been identified or inferred from data by all instruments on the Spirit rover, ...is especially abundant in these rocks. These picritic basalts are similar in many respects to certain Martian meteorites (olivine‐phyric shergottites). The olivine megacrysts in both have intermediate compositions, with modal abundances ranging up to 20–30%. Associated minerals in both include low‐calcium and high‐calcium pyroxenes, plagioclase of intermediate composition, iron‐titanium‐chromium oxides, and phosphate. These rocks also share minor element trends, reflected in their nickel‐magnesium and chromium‐magnesium ratios. Gusev basalts and shergottites appear to have formed from primitive magmas produced by melting an undepleted mantle at depth and erupted without significant fractionation. However, apparent differences between Gusev rocks and shergottites in their ages, plagioclase abundances, and volatile contents preclude direct correlation. Orbital determinations of global olivine distribution and compositions by thermal emission spectroscopy suggest that olivine‐rich rocks may be widespread. Because weathering under acidic conditions preferentially attacks olivine and disguises such rocks beneath alteration rinds, picritic basalts formed from primitive magmas may even be a common component of the Martian crust formed during ancient and recent times.
Panoramic Camera images at Gusev crater reveal a rock-strewn surface interspersed with high- to moderate-albedo fine-grained deposits occurring in part as drifts or in small circular swales or ...hollows. Optically thick coatings of fine-grained ferric iron-rich dust dominate most bright soil and rock surfaces. Spectra of some darker rock surfaces and rock regions exposed by brushing or grinding show near-infrared spectral signatures consistent with the presence of mafic silicates such as pyroxene or olivine. Atmospheric observations show a steady decline in dust opacity during the mission, and astronomical observations captured solar transits by the martian moons, Phobos and Deimos, as well as a view of Earth from the martian surface.
Wind‐related features observed by the rover Spirit in Gusev crater, Mars, include patches of soil on the surface, some of which are organized into bed forms. Windblown grains include dust (inferred ...to be <3 μm in diameter), sands (up to a few hundred μm in diameter), and granules (>2 mm in diameter). Microscopic Imager data show the sands and granules to be rounded and relatively spherical, typical of grains transported long distances by the wind. The interior of bed forms exposed by rover operations suggests the infiltration of dust among the grains, indicating that these sands are not currently experiencing saltation. Orientations of 1520 features (such as bed forms and ventifacts) along Spirit's traverse from the landing site (the Columbia Memorial Station) to West Spur in the Columbia Hills suggest primary formative winds from the north‐northwest, which correlate with measurements of features seen in orbiter images and is consistent with afternoon winds predicted by atmospheric models. A secondary wind from the southeast is also suggested, which correlates with predictions for nighttime/early morning winds. Wind abrasion is indicated by ventifacts in the form of facets and grooves cut into rocks, the orientations of which also indicate prevailing winds from the north‐northwest. Orientations of many aeolian features in the West Spur area, however, have more scatter than elsewhere along the traverse, which is attributed to the influence of local topography on the patterns of wind. Active dust devils observed on the floor of Gusev from the Columbia Hills demonstrate that dust is currently mobile. Sequential images of some dust devils show movement as rapid as 3.8 m/s, consistent with wind velocities predicted by atmospheric models for the afternoon, when most of the dust devils were observed. Sands accumulated on the rover deck in the same period suggest that some sands in the Columbia Hills experience active saltation. “Two‐toned” rocks having a light band coating at their bases are considered to represent partial burial by soils and subsequent exposure, while “perched” rocks could represent materials lowered onto other rocks by deflation of supporting soils. Measurements of the heights of the light bands and the perched rocks range from <1 cm to 27 cm, indicating local deflation by as much as 27 cm.
Emissivity spectra of particulate mineral samples are highly dependent on particle size when that size is comparable to the wavelength of light emitted (5-50 micrometers for the midinfrared). Proper ...geologic interpretation of data from planetary infrared spectrometers will require that these particle size effects be well understood. To address this issue, samples of quartz powders were produced with narrow, well-characterized particle size distributions. Mean particle diameters in these samples ranged from 15 to 227 micrometers. Emission spectra of these powders allow the first detailed comparison of the complex spectral variations with particle size observed in laboratory data with the predictions of radiative transfer models. Four such models are considered here. Hapke's relectance theory (converted to emissivity via Kirchoff's law) is the first model tested. Hapke's more recently published emission theory is also employed. The third model, the 'Mie/Conel' model, uses Mie single scattering with a two-stream approximation for multiple scattering. This model, like the first, is a converted reflec- tance model. Mie scattering assumes particles are both spherical and well separated, which is not true for the quartz powders, but includes diffraction effects. The fourth model uses the Mie solution for single scattering by spheres and inputs those results into the multiple scattering formalism of Hapke's emission theory. The results of the four models are considered in relation to the values of the optical constants n and k. We have grouped these as class 1 (k large), class 2 (k moderate, n is approximately 2), class 3 (k small, n is approximately 2), and class 4 (k small, n is approximately 1). In general, the Mie/Hapke hybrid model does best at predicting variations with grain size. In particular, it predicts changes of the correct pattern, although incorrect magnitude, for class 1 bands, where large increases in emissivity with decreasing grain size are observed. This model also does an excellent job on moderate (class 2) and very weak and intraband (class 3) regions, and correctly predicts the emission maximum and its invariance with grain size near the Christiansen frequency (class 4). The Mie/Hapke hybrid model also has the fewest free parameters of the four models examined, while maintaining the most physical treatment of the radiative transfer. The Mie/Conel model performs as well as the Mie/Hapke hybrid model in strong bands (class 1) but does not accurately model the behavior of moderate (class 2) and very weak (class 3) bands.
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