Multiple mineral horizons in layered outcrops at Mawrth Vallis, Mars, signify changing geochemical environments on early Mars.

Refined calibrations of CRISM images are enabling identification of smaller deposits of unique aqueous materials on Mars that reveal changing environmental conditions at the region surrounding Mawrth Vallis. Through characterization of these clay-sulfate assemblages and their association with the layered, phyllosilicate units of this region, more details of the aqueous geochemical history can be gleaned. A stratigraphy including five distinct mineral horizons is mapped using compositional data from CRISM over CTX and HRSC imagery across 100s of km and from CRISM over HiRISE imagery across 100s of meters.

Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1-μm Absorptions on Mars.

Unlabelled: A previously unidentified artifact has been found in Compact Reconnaissance Imaging Spectrometer for Mars targeted I/F data. It exists in a small fraction (<0.05%) of pixels within 90% of images investigated and occurs in regions of high spectral/spatial variance.

Imaging Mercury's Polar Deposits during MESSENGER's Low-altitude Campaign.

Images obtained during MESSENGER's low-altitude campaign in the final year of the mission provide the highest-spatial-resolution views of Mercury's polar deposits. Images for distinct areas of permanent shadow within 35 north polar craters were successfully captured during the campaign. All of these regions of permanent shadow were found to have low-reflectance surfaces with well-defined boundaries.

Ancient aqueous environments at Endeavour crater, Mars.

Opportunity has investigated in detail rocks on the rim of the Noachian age Endeavour crater, where orbital spectral reflectance signatures indicate the presence of Fe(+3)-rich smectites. The signatures are associated with fine-grained, layered rocks containing spherules of diagenetic or impact origin. The layered rocks are overlain by breccias, and both units are cut by calcium sulfate veins precipitated from fluids that circulated after the Endeavour impact.

Subsurface water and clay mineral formation during the early history of Mars.

Clay minerals, recently discovered to be widespread in Mars's Noachian terrains, indicate long-duration interaction between water and rock over 3.7 billion years ago. Analysis of how they formed should indicate what environmental conditions prevailed on early Mars.

Hollows on Mercury: MESSENGER evidence for geologically recent volatile-related activity.

High-resolution images of Mercury's surface from orbit reveal that many bright deposits within impact craters exhibit fresh-appearing, irregular, shallow, rimless depressions. The depressions, or hollows, range from tens of meters to a few kilometers across, and many have high-reflectance interiors and halos. The host rocks, which are associated with crater central peaks, peak rings, floors, and walls, are interpreted to have been excavated from depth by the crater-forming process.

Flood volcanism in the northern high latitudes of Mercury revealed by MESSENGER.

MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury's high northern latitudes and occupies more than 6% of the planet's surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites.

Seasonal flows on warm Martian slopes.

Water probably flowed across ancient Mars, but whether it ever exists as a liquid on the surface today remains debatable. Recurring slope lineae (RSL) are narrow (0.5 to 5 meters), relatively dark markings on steep (25° to 40°) slopes; repeat images from the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment show them to appear and incrementally grow during warm seasons and fade in cold seasons.

Detection of hydrated silicates in crustal outcrops in the northern plains of Mars.

The composition of the ancient martian crust is a key ingredient in deciphering the environment and evolution of early Mars. We present an analysis of the composition of large craters in the martian northern plains based on data from spaceborne imaging spectrometers. Nine of the craters have excavated assemblages of phyllosilicates from ancient, Noachian crust buried beneath the plains' cover.

Distribution of mid-latitude ground ice on Mars from new impact craters.

New impact craters at five sites in the martian mid-latitudes excavated material from depths of decimeters that has a brightness and color indicative of water ice. Near-infrared spectra of the largest example confirm this composition, and repeated imaging showed fading over several months, as expected for sublimating ice. Thermal models of one site show that millimeters of sublimation occurred during this fading period, indicating clean ice rather than ice in soil pores.

The evolution of Mercury's crust: a global perspective from MESSENGER.

Mapping the distribution and extent of major terrain types on a planet's surface helps to constrain the origin and evolution of its crust. Together, MESSENGER and Mariner 10 observations of Mercury now provide a near-global look at the planet, revealing lateral and vertical heterogeneities in the color and thus composition of Mercury's crust. Smooth plains cover approximately 40% of the surface, and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically.

Orbital identification of carbonate-bearing rocks on Mars.

Geochemical models for Mars predict carbonate formation during aqueous alteration. Carbonate-bearing rocks had not previously been detected on Mars' surface, but Mars Reconnaissance Orbiter mapping reveals a regional rock layer with near-infrared spectral characteristics that are consistent with the presence of magnesium carbonate in the Nili Fossae region. The carbonate is closely associated with both phyllosilicate-bearing and olivine-rich rock units and probably formed during the Noachian or early Hesperian era from the alteration of olivine by either hydrothermal fluids or near-surface water.

Phyllosilicate diversity and past aqueous activity revealed at Mawrth Vallis, Mars.

Observations by the Mars Reconnaissance Orbiter/Compact Reconnaissance Imaging Spectrometer for Mars in the Mawrth Vallis region show several phyllosilicate species, indicating a wide range of past aqueous activity. Iron/magnesium (Fe/Mg)-smectite is observed in light-toned outcrops that probably formed via aqueous alteration of basalt of the ancient cratered terrain. This unit is overlain by rocks rich in hydrated silica, montmorillonite, and kaolinite that may have formed via subsequent leaching of Fe and Mg through extended aqueous events or a change in aqueous chemistry.

Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument.

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars's history.

Geology of the Caloris basin, Mercury: a view from MESSENGER.

The Caloris basin, the youngest known large impact basin on Mercury, is revealed in MESSENGER images to be modified by volcanism and deformation in a manner distinct from that of lunar impact basins. The morphology and spatial distribution of basin materials themselves closely match lunar counterparts. Evidence for a volcanic origin of the basin's interior plains includes embayed craters on the basin floor and diffuse deposits surrounding rimless depressions interpreted to be of pyroclastic origin.

Volcanism on Mercury: evidence from the first MESSENGER flyby.

The origin of plains on Mercury, whether by volcanic flooding or impact ejecta ponding, has been controversial since the Mariner 10 flybys (1974-75). High-resolution images (down to 150 meters per pixel) obtained during the first MESSENGER flyby show evidence for volcanic vents around the Caloris basin inner margin and demonstrate that plains were emplaced sequentially inside and adjacent to numerous large impact craters, to thicknesses in excess of several kilometers. Radial graben and a floor-fractured crater may indicate intrusive activity.

Reflectance and color variations on Mercury: regolith processes and compositional heterogeneity.

Multispectral images of Mercury obtained by the MESSENGER spacecraft reveal that its surface has an overall relatively low reflectance with three large-scale units identified on the basis of reflectance and slope (0.4 to 1.0 micrometer).

Spectroscopic observations of Mercury's surface reflectance during MESSENGER's first Mercury flyby.

During MESSENGER's first flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer made simultaneous mid-ultraviolet to near-infrared (wavelengths of 200 to 1300 nanometers) reflectance observations of the surface. An ultraviolet absorption (<280 nanometers) suggests that the ferrous oxide (Fe2+) content of silicates in average surface material is low (less than 2 to 3 weight percent). This result is supported by the lack of a detectable 1-micrometer Fe2+ absorption band in high-spatial-resolution spectra of mature surface materials as well as immature crater ejecta, which suggests that the ferrous iron content may be low both on the surface and at depth.

Return to Mercury: a global perspective on MESSENGER's first Mercury flyby.

In January 2008, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft became the first probe to fly past the planet Mercury in 33 years. The encounter revealed that Mercury is a dynamic system; its liquid iron-rich outer core is coupled through a dominantly dipolar magnetic field to the surface, exosphere, and magnetosphere, all of which interact with the solar wind. MESSENGER images confirm that lobate scarps are the dominant tectonic landform and record global contraction associated with cooling of the planet.

A closer look at water-related geologic activity on Mars.

Water has supposedly marked the surface of Mars and produced characteristic landforms. To understand the history of water on Mars, we take a close look at key locations with the High-Resolution Imaging Science Experiment on board the Mars Reconnaissance Orbiter, reaching fine spatial scales of 25 to 32 centimeters per pixel. Boulders ranging up to approximately 2 meters in diameter are ubiquitous in the middle to high latitudes, which include deposits previously interpreted as finegrained ocean sediments or dusty snow.

The nature of ponded deposits on Eros.

One of the surprises of the NEAR-Shoemaker mission was that Eros's surface exhibits a wide variety of landforms, which are indicative of a global covering of loose fragmental debris. At one extreme in roughness is the Shoemaker Regio area, which is characterized by a high density of boulders up to 100 m across, slumps, slides, and finer blanketing material. At the other extreme are distinctive, flat deposits that appear smooth down to a resolution of 1.

Shoemaker crater as the source of most ejecta blocks on the asteroid 433 Eros.

The loose material--regolith--on the surfaces of asteroids is thought to represent ballistically emplaced ejecta from impacts but the identification of source craters and the detailed study of the regolith modification have been hampered by the limited spatial resolution and area coverage of the few asteroids imaged by spacecraft. Here we report the results of global mapping of the asteroid 433 Eros from high-resolution images obtained by the NEAR-Shoemaker spacecraft. Based on the images and ejecta-emplacement models, we suggest that most large ejecta blocks on Eros originate from a relatively young 7.