Michael Bruce Wyatt

Mineralogy at Meridiani Planum from the Mini-TES experiment on the opportunity rover

The Miniature Thermal Emission Spectrometer (Mini-TES) on Opportunity investigated the mineral abundances and compositions of outcrops, rocks, and soils at Meridiani Planum. Coarse crystalline hematite and olivine-rich basaltic sands were observed as predicted from orbital TES spectroscopy. Outcrops of aqueous origin are composed of 15 to 35% by volume magnesium and calcium sulfates [a high-silica component modeled as a combination of glass, feldspar, and sheet silicates (∼20 to 30%)], and hematite; only minor jarosite is identified in Mini-TES spectra. Mini-TES spectra show only a hematite signature in the millimeter-sized spherules. Basaltic materials have more plagioclase than pyroxene, contain olivine, and are similar in inferred mineral composition to basalt mapped from orbit. Bounce rock is dominated by clinopyroxene and is close in inferred mineral composition to the basaltic martian meteorites. Bright wind streak material matches global dust. Waterlain rocks covered by unaltered basaltic sands suggest a change from an aqueous environment to one dominated by physical weathering.

Spectral evidence for weathered basalt as an alternative to andesite in the northern lowlands of Mars

Mineral abundances derived from the analysis of remotely sensed thermal emission data from Mars have been interpreted to indicate that the surface is composed of basalt (Surface Type 1) and andesite (Surface Type 2). The global distribution of these rock types is divided roughly along the planetary dichotomy which separates ancient, heavily cratered crust in the southern hemisphere (basalt) from younger lowland plains in the north (andesite). But the existence of such a large volume of andesite is difficult to reconcile with our present understanding of the geological evolution of Mars. Here we reinterpret martian surface rock lithologies using mineral abundances from previous work and new mineralogies derived from a spectral end-member set representing minerals common in unaltered and low-temperature aqueously altered basalts. Our results continue to indicate the dominance of unaltered basalt in the southern highlands, but reveal that the northern lowlands can be interpreted as weathered basalt as an alternative to andesite. The coincidence between locations of such altered basalt and a suggested northern ocean basin implies that lowland plains material may be composed of basalts weathered under submarine conditions or weathered basaltic sediments transported into this depocentre.

Characterization and petrologic interpretation of olivine‐rich basalts at Gusev Crater, Mars

Additional co-authors: PR Christensen, BC Clark, JA Crisp, DJ DesMarais, T Economou, JD Farmer, W Farrand, A Ghosh, M Golombek, S Gorevan, R Greeley, VE Hamilton, JR Johnson, BL Joliff, G Klingelhofer, AT Knudson, S McLennan, D Ming, JE Moersch, R Rieder, SW Ruff, PA de Souza Jr, SW Squyres, H Wnke, A Wang, A Yen, J Zipfel

Diviner lunar radiometer observations of cold traps in the moon's south polar region

Watering the Moon About a year ago, a spent upper stage of an Atlas rocket was deliberately crashed into a crater at the south pole of the Moon, ejecting a plume of debris, dust, and vapor. The goal of this event, the Lunar Crater Observation and Sensing Satellite (LCROSS) experiment, was to search for water and other volatiles in the soil of one of the coldest places on the Moon: the permanently shadowed region within the Cabeus crater. Using ultraviolet, visible, and near-infrared spectroscopy data from accompanying craft, Colaprete et al. (p. 463; see the news story by Kerr; see the cover) found evidence for the presence of water and other volatiles within the ejecta cloud. Schultz et al. (p. 468) monitored the different stages of the impact and the resulting plume. Gladstone et al. (p. 472), using an ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), detected H2, CO, Ca, Hg, and Mg in the impact plume, and Hayne et al. (p. 477) measured the thermal signature of the impact and discovered that it had heated a 30 to 200 square-meter region from ∼40 kelvin to at least 950 kelvin. Paige et al. (p. 479) mapped cryogenic zones predictive of volatile entrapment, and Mitrofanov et al. (p. 483) used LRO instruments to confirm that surface temperatures in the south polar region persist even in sunlight. In all, about 155 kilograms of water vapor was emitted during the impact; meanwhile, the LRO continues to orbit the Moon, sending back a stream of data to help us understand the evolution of its complex surface structures. A controlled spacecraft impact into a crater in the lunar south pole plunged through the lunar soil, revealing water and other volatiles. Diviner Lunar Radiometer Experiment surface-temperature maps reveal the existence of widespread surface and near-surface cryogenic regions that extend beyond the boundaries of persistent shadow. The Lunar Crater Observation and Sensing Satellite (LCROSS) struck one of the coldest of these regions, where subsurface temperatures are estimated to be 38 kelvin. Large areas of the lunar polar regions are currently cold enough to cold-trap water ice as well as a range of both more volatile and less volatile species. The diverse mixture of water and high-volatility compounds detected in the LCROSS ejecta plume is strong evidence for the impact delivery and cold-trapping of volatiles derived from primitive outer solar system bodies.

Evidence for magmatic evolution and diversity on Mars from infrared observations

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.

Soils of Eagle Crater and Meridiani Planum at the Opportunity Rover Landing Site

The soils at the Opportunity site are fine-grained basaltic sands mixed with dust and sulfate-rich outcrop debris. Hematite is concentrated in spherules eroded from the strata. Ongoing saltation exhumes the spherules and their fragments, concentrating them at the surface. Spherules emerge from soils coated, perhaps from subsurface cementation, by salts. Two types of vesicular clasts may represent basaltic sand sources. Eolian ripples, armored by well-sorted hematite-rich grains, pervade Meridiani Planum. The thickness of the soil on the plain is estimated to be about a meter. The flatness and thin cover suggest that the plain may represent the original sedimentary surface.

Analysis of terrestrial and martian volcanic compositions using thermal emission spectroscopy: 1. Determination of mineralogy, chemistry, and classification strategies

We have examined and applied existing classification schemes for volcanic rocks and developed new schemes using thermal emission spectra of terrestrial volcanic rocks. Laboratory thermal infrared spectra (5–25 μm, at 2 cm−1 spectral sampling), deconvolved modal mineralogies, and derived mineral and bulk rock chemistries were used to distinguish basalt, basaltic andesite, andesite, and dacite. Modal mineralogies derived from linear deconvolution of terrestrial volcanic rocks were compared to modes measured by an electron microprobe phase mapping technique to determine the accuracy of linear deconvolution in modeling specific mineral abundances. One σ standard deviations of the absolute differences between modeled and measured mineral abundances range from 2.4 to 12.2 vol %, with an average standard deviation of 4.8 vol % being in agreement with average uncertainties calculated in previous studies. Weighted average compositions of feldspars in the deconvolution generally overlap the measured ranges of plagioclase compositions and the presence of low-calcium and high-calcium pyroxenes was correctly identified. Bulk chemistries of volcanic rocks were derived with a relatively high degree of accuracy (1σ standard deviations ranging from 0.4 to 2.6 vol %) by combining the compositions of spectrally modeled phases in proportion to their relative abundances in a particular sample. These data were collectively used to examine existing and develop new volcanic rock classification schemes. However, no single classification scheme was effective in accurately classifying all samples. Multiple steps of classification were required to distinguish volcanic rocks, reflecting the mineralogic diversity and continuum of compositions that exists in volcanic rock types. In a companion paper [Hamilton et al., this issue] these schemes are applied to the classification of Martian surface compositions.

Highly Silicic Compositions on the Moon

Lunar Reconnaissance The Lunar Reconnaissance Orbiter reached lunar orbit on 23 June 2009. Global data acquired since then now tell us about the impact history of the Moon and the igneous processes that shaped it. Using the Lunar Orbiter Laser Altimeter, Head et al. (p. 1504; see the cover) provide a new catalog of large lunar craters. In the lunar highlands, large-impact craters have obliterated preexisting craters of similar size, implying that crater counts in this region cannot be used effectively to determine the age of the underlying terrain. Crater counts based on the global data set indicate that the nature of the Moons impactor population has changed over time. Greenhagen et al. (p. 1507) and Glotch et al. (p. 1510) analyzed data from the Diviner Lunar Radiometer Experiment, which measures emitted thermal radiation and reflected solar radiation at infrared wavelengths. The silicate mineralogy revealed suggests the existence of more complex igneous processes on the Moon than previously assumed. Remote thermal emission spectroscopy reveals the existence of complex igneous processes on the Moon. Using data from the Diviner Lunar Radiometer Experiment, we show that four regions of the Moon previously described as “red spots” exhibit mid-infrared spectra best explained by quartz, silica-rich glass, or alkali feldspar. These lithologies are consistent with evolved rocks similar to lunar granites in the Apollo samples. The spectral character of these spots is distinct from surrounding mare and highlands material and from regions composed of pure plagioclase feldspar. The variety of landforms associated with the silicic spectral character suggests that both extrusive and intrusive silicic magmatism occurred on the Moon. Basaltic underplating is the preferred mechanism for silicic magma generation, leading to the formation of extrusive landforms. This mechanism or silicate liquid immiscibility could lead to the formation of intrusive bodies.

Global Silicate Mineralogy of the Moon from the Diviner Lunar Radiometer

Lunar Reconnaissance The Lunar Reconnaissance Orbiter reached lunar orbit on 23 June 2009. Global data acquired since then now tell us about the impact history of the Moon and the igneous processes that shaped it. Using the Lunar Orbiter Laser Altimeter, Head et al. (p. 1504; see the cover) provide a new catalog of large lunar craters. In the lunar highlands, large-impact craters have obliterated preexisting craters of similar size, implying that crater counts in this region cannot be used effectively to determine the age of the underlying terrain. Crater counts based on the global data set indicate that the nature of the Moons impactor population has changed over time. Greenhagen et al. (p. 1507) and Glotch et al. (p. 1510) analyzed data from the Diviner Lunar Radiometer Experiment, which measures emitted thermal radiation and reflected solar radiation at infrared wavelengths. The silicate mineralogy revealed suggests the existence of more complex igneous processes on the Moon than previously assumed. Remote thermal emission spectroscopy reveals the existence of complex igneous processes on the Moon. We obtained direct global measurements of the lunar surface using multispectral thermal emission mapping with the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment. Most lunar terrains have spectral signatures that are consistent with known lunar anorthosite and basalt compositions. However, the data have also revealed the presence of highly evolved, silica-rich lunar soils in kilometer-scale and larger exposures, expanded the compositional range of the anorthosites that dominate the lunar crust, and shown that pristine lunar mantle is not exposed at the lunar surface at the kilometer scale. Together, these observations provide compelling evidence that the Moon is a complex body that has experienced a diverse set of igneous processes.

Global geologic context for rock types and surface alteration on Mars

Petrologic interpretations of thermal emission spectra from Mars orbiting spacecraft indicate the widespread occurrence of surfaces having basaltic and either andesitic or partly altered basalt compositions. Global concentration of ice-rich mantle deposits and near-surface ice at middle to high latitudes and their spatial correlation with andesitic or partly altered basalt materials favor the alteration hypothesis. We propose the formation of these units through limited chemical weathering from basalt interactions with icy mantles deposited during periods of high obliquity. Alteration of sediments in the northern lowlands depocenter may have been enhanced by temporary standing bodies of water and ice.