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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.10.007 Byline: F. Poulet (a), R.E. Arvidson (b), J.-P. Bibring (a), B. Gondet (a), D. Jouglet (a), Y. Langevin (a), R.V. Morris (c) Keywords: Mars, Surface; Mineralogy; Spectroscopy Abstract: We present an analysis comparing observations acquired by the Mars Express Observatoire pour la Mineralogie l'Eau, les Glaces et l'Activite (OMEGA) and Phoenix lander measurements. Analysis of OMEGA data provides evidence for hydrous and ferric phases at the Phoenix landing site and the surrounding regions. The 3[mu]m hydration band deepens with increasing latitude, along with the appearance and deepening of a 1.9[mu]m H.sub.2O band as latitude increases [approximately equal to]60[degrees] polewards. A water content of 10-11% is derived from the OMEGA data for the optical surface at the Phoenix landing site compared to 1-2% derived for subsurface soil by Phoenix lander measurements. The hydration of these regions is best explained by surface adsorbed water onto soil grains. No evidence for carbonate or perchlorate-bearing phases is evident from OMEGA data, consistent with the relatively small abundances of these phases detected by Phoenix. The identification of spectral features consistent with hydrated phases (possibly zeolites) from OMEGA data covering regions outside the landing site and the ubiquitous ferric absorption edge suggest that chemical weathering may play a role in the arctic soils. Author Affiliation: (a) Institut d'Astrophysique Spatiale, CNRS/Universite Paris-Sud, Orsay, France (b) Department of Earth and Planetary Sciences, Washington University, St. Louis, MO, USA (c) NASA Johnson Space Center, Houston, TX 77058, USA Article History: Received 20 July 2009; Revised 20 September 2009; Accepted 2 October 2009

Carbonates are generally products of aqueous processes and may hold important clues about the history of liquid water on the surface of Mars. Calcium carbonate (approximately 3 to 5 weight percent) has been identified in the soils around the Phoenix landing site by scanning calorimetry showing an endothermic transition beginning around 725[degrees]C accompanied by evolution of carbon dioxide and by the ability of the soil to buffer pH against acid addition. Based on empirical kinetics, the amount of calcium carbonate is most consistent with formation in the past by the interaction of atmospheric carbon dioxide with liquid water films on particle surfaces.

High spatial and spectral resolution reflectance data acquired by the Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument reveal the presence of [H.sub.2]O-and SiOH-bearing phases on the Martian surface. The spectra are most consistent with opaline silica and glass altered to various degrees, confirming predictions based on geochemical experiments and models that amorphous silica should be a common weathering product of the basaltic Martian crust. These materials are associated with hydrated Fe sulfates, including [H.sub.3]O-bearing jarosite, and are found in finely stratified deposits exposed on the floor of and on the plains surrounding the Valles Marineris canyon system. Stratigraphic relationships place the formation age of these deposits in the late Hesperian or possibly the Amazonian, implying that aqueous alteration continued to be an important and regionally extensive process on Mars during that time.

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitie) instrument (1,2), preserve a record of the [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.11.031 Byline: F. Poulet (a), R.E. Arvidson (b), C. Gomez (a), R.V. Morris (c), J.-P. Bibring (a), Y. Langevin (a), B. Gondet (a), J. Griffes (b)(d) Keywords: Mars; surface; Mineralogy; Spectroscopy Abstract: Analyses of Mars Express OMEGA hyperspectral data (0.4-2.7 [mu]m) for Terra Meridiani and western Arabia Terra show that the northern mantled cratered terrains are covered by dust that is spectrally dominated by nanophase ferric oxides. Dark aeolian dunes inside craters and dark streaks extending from the dunes into the intercrater areas in mantled cratered terrains in western Arabia Terra have similar pyroxene-rich signatures demonstrating that the dunes supply dark basaltic material to create dark streaks. The dissected cratered terrains to the south of the mantled terrains are dominated spectrally by both low-calcium and high-calcium pyroxenes with abundances of 20-30% each retrieved from nonlinear radiative transfer modeling. Spectra over the hematite-bearing plains in Meridiani Planum are characterized by very weak but unique spectral features attributed to a mixture of a dark and featureless component (possibly gray hematite) and minor olivine in some locations. Hydrated minerals (likely hydrous ferric sulfates and/or hydrous hydroxides) associated with poorly ferric crystalline phases are found in the etched terrains to the north and east of the hematite-bearing plains where erosion has exposed [approximately equal to]1km of section of layered outcrops with high thermal inertias. These materials are also found in numerous craters in the northern Terra Meridiani and may represent outliers of the etched terrain materials. A few localized spots within the etched terrain also exhibit the spectral signature of Fe-rich phyllosilicates. The ensemble of observations show that the evidence for aqueous processes detected by the Opportunity Rover in Meridiani Planum is widespread and confirms the extended presence of surface or near-surface water over this large region of Mars. The scenarios of formation of Terra Meridiani ('dirty' acidic evaporite, impact surge or weathering of volcanic ash) cannot satisfactorily explain the mineralogy derived from the OMEGA observations. The formation of the etched terrains is consistent with leaching of iron sulfides and formation of sulfates and hydrated iron oxides, either in-place or via transport and evaporation of aqueous fluids and under aqueous conditions less acidic than inferred from rocks examined by Opportunity. Author Affiliation: (a) Institut d'Astrophysique Spatiale, Universite Paris-Sud, 91405 Orsay Cedex, France (b) Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA (c) NASA Johnson Space Center, Houston, TX 77058, USA (d) Center for Earth and Planetary Studies, National Air & Space Museum, Smithsonian Institution, Washington, DC 20013, USA Article History: Received 25 January 2007; Revised 22 October 2007

The Spirit rover completed a 2.5 km traverse across gently sloping plains on the floor of Gusev crater from its location on the outer rim of Bonneville crater to the lower slopes of the Columbia Hills, Mars. Using the Athena suite of instruments in a transect approach, a systematic series of overlapping panoramic mosaics, remote sensing observations, surface analyses, and trenching operations documented the lateral variations in landforms, geologic materials, and chemistry of the surface throughout the traverse, demonstrating the ability to apply the techniques of field geology by remote rover operations. Textures and shapes of rocks within the plains are consistent with derivation from impact excavation and mixing of the upper few meters of basaltic lavas. The contact between surrounding plains and crater ejecta is generally abrupt and marked by increases in clast abundance and decimeter-scale steps in relief. Basaltic materials of the plains overlie less indurated and more altered rock types at a time-stratigraphic contact between the plains and Columbia Hills that occurs over a distance of one to two meters. This implies that regional geologic contacts are well preserved and that Earth-like field geologic mapping will be possible on Mars despite eons of overturn by small impacts. Keywords: Mars, planetary geology, rover, geotraverse, field geology.

The Mars Express Observatoire pour la Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) hyperspectral image data covering eastern Terra Meridiani indicate the ubiquitous presence of molecular water in etched terrain materials that disconformably overlie heavily cratered terrains and underlie the hematite-bearing plains explored by the Opportunity rover. Identification of crystalline water in kieserite (MgS[O.sub.4] x [H.sub.2]O) is linked to materials exposed in a valley and plateau to the north of hematite-bearing plains. The mineralogical similarities between the etched terrain deposits examined with OMEGA data and the layered rocks examined by Opportunity imply that the ancient aqueous environments inferred from analyses of the rover data extend over regional scales., The Mars Express instrument, OMEGA, has acquired hyperspectral imaging data of the martian atmosphere and surface since January 2004 (1). In this Report, we summarize analyses of OMEGA data from [...]

Mossbauer spectrometers provide quantitative information about the distribution of iron among its oxidation states, the identification of iron-bearing phases, and the distribution of iron among those phases. Characterizing the speciation [...], Mossbauer spectra measured by the Opportunity rover revealed four mineralogical components in Meridiani Planum at Eagle crater: jarosite- and hematite-rich outcrop, hematite-rich soil olivine-bearing basaltic soil, and a pyroxene-bearing basaltic rock (Bounce rock). Spherules, interpreted to be concretions, are hematiterich and dispersed throughout the outcrop. Hematitic soils both within and outside Eagle crater are dominated by spherules and their fragments. Olivine-bearing basaltic soil is present throughout the region. Bounce rock is probably an impact erratic. Because jarosite is a hydroxide sulfate mineral, its presence at Meridiani Planum is mineralogical evidence for aqueous processes on Mars, probably under acid-sulfate conditions.

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., The Mini-TES has provided remote measurements of mineral abundances and compositions, thermophysical properties, atmospheric temperature profiles, and atmospheric dust and ice opacities at the Opportunity rover landing site in Meridiani [...]

Sedimentary rocks at Eagle crater in Meridiani Planum are composed of fine-grained siliciclastic materials derived from weathering of basaltic rocks, sulfate minerals (including magnesium sulfate and jarosite) that constitute several tens of percent of the rock by weight, and hematite. Cross-stratification observed in rock outcrops indicates eolian and aqueous transport. Diagenetic features include hematite-rich concretions and crystal-mold vugs. We interpret the rocks to be a mixture of chemical and siliciclastic sediments with a complex diagenetic history. The environmental conditions that they record include episodic inundation by shallow surface water, evaporation, and desiccation. The geologic record at Meridiani Planum suggests that conditions were suitable for biological activity for a period of time in martian history., The primary objective of the Mars Exploration Rover mission is to search for evidence in the martian geologic record of environmental conditions that might once have been suitable for life. [...]

The location of the Opportunity landing site was determined to better than 10-m absolute accuracy from analyses of radio tracking data. We determined Rover locations during traverses with an error as small as several centimeters using engineering telemetry and overlapping images. Topographic profiles generated from rover data show that the plains are very smooth from meter- to centimeter-length scales, consistent with analyses of orbital observations. Solar cell output decreased because of the deposition of airborne dust on the panels. The lack of dust-covered surfaces on Meridiani Planum indicates that high velocity winds must remove this material on a continuing basis. The low mechanical strength of the evaporitic rocks as determined from grinding experiments, and the abundance of coarse-grained surface particles argue for differential erosion of Meridian Planum., The Mars Exploration Rover (MER) Mission required accurate tracking of the location of Opportunity to ensure efficient drives and to place measurements in proper geological context, e.g., associating a rock [...]

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., Martian soils (1) are remnants of their source rocks and thus provide insight into geologic processes and history on local, regional, and global scales. Whether soils are actively changing or [...]

The Mars Exploration Rover Opportunity has investigated the landing site in Eagle crater and the nearby plains within Meridiani Planum. The soils consist of fine-grained basaltic sand and a surface lag of hematite-rich spherules, spherule fragments, and other granules. Wind ripples are common. Underlying the thin soil layer, and exposed within small impact craters and troughs, are flat-lying sedimentary rocks. These rocks are finely laminated, are rich in sulfur, and contain abundant sulfate salts. Small-scale cross-lamination in some locations provides evidence for deposition in flowing liquid water. We interpret the rocks to be a mixture of chemical and siliciclastic sediments formed by episodic inundation by shallow surface water, followed by evaporation, exposure, and desiccation. Hematite-rich spherules are embedded in the rock and eroding from them. We interpret these spherules to be concretions formed by postdepositional diagenesis, again involving liquid water., The Mars Exploration Rover Opportunity landed in Eagle crater on Meridiani Planum on 24 January 2004 UTC, 21 days after the landing of Spirit at Gusev crater (1). Both vehicles [...]

Panoramic Camera (Pancam) images from Meridiani Planum reveal a low-albedo, generally flat, and relatively rock-free surface. Within and around impact craters and fractures, laminated outcrop rocks with higher albedo are observed. Fine-grained materials include dark sand, bright ferric iron-rich dust, angular rock clasts, and millimeter-size spheroidal granules that are eroding out of the laminated rocks. Spectra of sand, clasts, and one dark plains rock are consistent with mafic silicates such as pyroxene and olivine. Spectra of both the spherules and the laminated outcrop materials indicate the presence of crystalline ferric oxides or oxyhydroxides. Atmospheric observations show a steady decline in dust opacity during the mission. Astronomical observations captured solar transits by Phobos and Deimos and time-lapse observations of sunsets., On 24 January 2004 UTC, the Mars Exploration Rover Opportunity landed on Mars within the classical low-albedo Noachian terrain of Meridiani Planum. The landing region was previously identified in orbital [...]

The precise location and relative elevation of Spirit during its traverses from the Columbia Memorial station to Bonneville crater were determined with bundle-adjusted retrievals from rover wheel turns, suspension and tilt angles, and overlapping images. Physical properties experiments show a decrease of 0.2% per Mars solar day in solar cell output resulting from deposition of airborne dust, cohesive soil-like deposits in plains and hollows, bright and dark rock coatings, and relatively weak volcanic rocks of basaltic composition. Volcanic, impact, aeolian, and water-related processes produced the encountered landforms and materials., During the first few Mars solar days (sols) (1) of operations, we determined the landed location in inertial coordinates by analyzing Spirit-to-Earth two-way X-band Doppler transmissions and two passes of [...]

The Spirit landing site in Gusev Crater on Mars contains dark, fine-grained, vesicular rocks interpreted as lavas. Pancam and Mini-Thermal 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 ≤ 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. Mossbauer, 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., Rocks at the Spirit landing site are fine-grained with irregular vesicles and vugs, suggesting a volcanic origin. The rocks are angular and strewn across the surface, which suggests they were [...]

The Miniature Thermal Emission Spectrometer (Mini-TES) on Spirit has studied the mineralogy and thermophysical properties at Gusev crater. Undisturbed soil spectra show evidence for minor carbonates and bound water. Rocks are olivine-rich basalts with varying degrees of dust and other coatings. Dark-toned soils observed on disturbed surfaces may be derived from rocks and have derived mineralogy (± 5 to 10%) of 45% pyroxene (20% Ca-rich pyroxene and 25% pigeonite), 40% sodic to intermediate plagioclase, and 15% olivine (forsterite 45% ± 5 to 10). Two spectrally distinct coatings are observed on rocks, a possible indicator of the interaction of water, rock, and airfall dust. Diurnal temperature data indicate particle sizes from 40 to 80 µm in hollows to ~0.5 to 3 mm in soils., The Miniature Thermal Emission Spectrometer (Mini-TES) has provided remote measurements of mineralogy, thermophysical properties, and atmospheric temperature profile and composition of the scene surrounding the Spirit rover. The mineralogy of [...]

Mossbauer spectra measured on Mars by the Spirit rover during the primary mission are characterized by two ferrous iron doublets (olivine and probably pyroxene) and a ferric iron doublet (tentatively associated to nanophase ferric iron oxide). Two sextets resulting from nonstoichiometric magnetite are also present, except for a coating on the rock Mazatzal, where a hematite-like sextet is present. Greater proportions of ferric-bearing phases are associated with undisturbed soils and rock surfaces as compared to fresh rock surfaces exposed by grinding. The ubiquitous presence of olivine in soil suggests that physical rather than chemical weathering processes currently dominate at Gusev crater., Miniaturized Mossbauer spectrometers (MIMOS II) (1) are part of the science payload of each Mars Exploration Rover (MER) (2-4). Mossbauer spectrometers provide quantitative information about the distribution of iron among [...]

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., On 4 January 2004 universal time coordinated, the Mars Exploration Rover, Spirit, landed on Mars at 14.5692°S, 175.4729°E, within the crater Gusev, a 160-km-diameter Noachian-age impact crater. Previous orbital remote [...]

The Mars Exploration Rover Spirit and its Athena science payload have been used to investigate a landing site in Gusev crater. Gusev is hypothesized to be the site of a former lake, but no clear evidence for lacustrine sedimentation has been found to date. Instead, the dominant lithology is basalt, and the dominant geologic processes are impact events and eolian transport. Many rocks exhibit coatings and other characteristics that may be evidence for minor aqueous alteration. Any lacustrine sediments that may exist at this location within Gusev apparently have been buried by lavas that have undergone subsequent impact disruption., Spirit landed in Gusev crater on 4 January 2004 UTC. It was followed 21 days later by Opportunity, which landed on Meridiani Planum. Both vehicles landed using a variant of [...]

Wind-abraded rocks, ripples, drifts, and other deposits of windblown sediments are seen at the Columbia Memorial Station where the Spirit rover landed. Orientations of these features suggest formative winds from the north-northwest, consistent with predictions from atmospheric models of afternoon winds in Gusev Crater. Cuttings from the rover Rock Abrasion Tool are asymmetrically distributed toward the south-southeast, suggesting active winds from the north-northwest at the time (midday) of the abrasion operations. Characteristics of some rocks, such as a two-toned appearance, suggest that they were possibly buried and exhumed on the order of 5 to 60 centimeters by wind deflation, depending on location., In the current environment of Mars, wind appears to be the most frequent agent of surface modification, resulting in albedo patterns that change on time scales as short as a [...]

The dielectric constants for various features on the Ovda Regio on planet Venus are separated from textural effects by developing a model based on radar specific cross section and polarized emissivity. The horizontal and vertical random specific cross section and polarized emissivity are obtained from observations from the Magellan spacecraft. The separation is performed by the model by assuming that surface emission is prominent in the signatures produced by diffuse scale processes.

Correlation between Arabian and Nubian Neoproterozoic outcrops helps determine the new constraints and mechanics of Red Sea rifting. The use of a mosaic of 23 Landsat thematic mapper scenes and geodata facilitates the location and mapping of lithologic units, terranes and mobile belts in the Arabian and Nubian Shields. Field observations reveal the alignment of geological features if Arabia is rotated relative to Africa by 6. 7 degrees around a pole at a latitude of 34.6 degrees North and a longitude of 18.1 degrees East. The motion between Arabia and Nubia is parallel to that along the Dead Sea transform.

Digital Landsat Thematic Mapper (TM) observations, multi-frequency polarimetric AIRSAR radar data, microtopographic profiles derived from helicopter-borne photography, and field measurements were analyzed over the Pisgah and Cima volcanic fields, and Lavic Lake playa, Mojave Desert, California. Surface radiance factors were extracted from TM data using radiative transfer procedures to model the atmosphere and surface simultaneously. AIRSAR data were calibrated to backscatter specific cross sections using corner reflectors deployed at a number of locations. Analyses focused on the use of key field sites to understand compositional and textural controls on TM band-5 (1.55 to 1.75 micrometer) radiance factors and C- (5.6 cm), L- (24 cm), and P- (68 cm) band specific cross sections. For the Pisgah and Lavic Lake sites, the TM data are explained using a linear mixing model of basalt and fine-grained sediment (clay to sand-sized grains of clay minerals, quartz, feldspar, carbonate, and lithic fragments) radiance factors, with the addition of shadows for the rough a'a flow. On the other hand, AIRSAR data are shown to be primarily controlled by surface roughness, as there is a strong correlation between specific cross section and surface height standard deviation. For the elongate flows at the Cima volcanic field, which exhibit a mix of pahoehoe and a'a properties, we find a positive linear relationship between TM band-5 radiance factors and flow ages for the 3 m.y. age range represented. Specific cross sections remain constant until about 0.14 Ma and then decrease with age up to about 0.56 Ma. On the other hand, data for the two older flows (0.70 and 0.85 Ma) show increasing specific cross sections with increasing age. We interpret the increase in TM band-5 radiance factors (from 0.05 to 0.25) with increasing flow age and the decrease in specific cross sections (8 dB change at 35 degrees incidence angle for L band) for flows up to 0.56 Ma in age as due to smoothing by mass wasting and infill by fine-grained aeolian deposits. The increased specific cross sections for older flows are interpreted to be a consequence of surface roughness generated by fluvial dissection.

Whether the Red Sea floor is underlain mostly by oceanic or extended continental crustal material is a controversial topic. To test between the two hypotheses, we used a digital color mosaic of 23 Landsat thematic mapper (TM) scenes with field, geochemical, and geochronological data to identify and correlate crosscutting geologic features on the African and Arabian sides. Faults, shear zones, sutures, granitic complexes, volcano-sedimentary units, and dike swarms align if Arabia is rotated relative to Africa by 6.7 [degrees] around a pole at lat 34.6 [degrees] N, long 18.1 [degrees] E. This solution implies that the amount of continental crust underlying the Red Sea is small because the restored Red Sea coasts are typically juxtaposed.

We report the Pb isotopic compositions for alkali feldspars separated from 34 granitic rocks from the late Proterozoic assemblages of the Eastern Desert and the Sinai Peninsula (Egypt) in the Nubian segment of the Arabian-Nubian Shield. The Eastern Desert shows a range of initial Pb isotopic compositions ([Pb.sup.206/Pb.sup.204]) = 17.375-19.176; [Pb.sup.207/Pb.sup.204] = 15.462-15.629; [Pb.sup.208/Pb.sup.204] = 37.023-38.349) that extend from model mantle toward upper-crustal values. Initial Pb isotopic compositions do not define geographic provinces, except at Aswan and surrounding areas, where the most radiogenic values were obtained (Aswan, [Pb.sup.207/Pb.sup.204] = 15.611; Gebel El Hudi, [Pb.sup.207/Pb.sup.204] = 15.629; and Wadi Mariya, [Pb.sup.207/Pb.sup.204] = 15.615). Samples from these areas are separated from the less radiogenic samples to the east by trails of serpentinites that we interpret from Landsat thematic mapper images as the extension of the Allaqi-Heiani ophiolitic belt. Our data support previous interpretations of the belt as a suture location and suggest that its western margin might coincide with the boundary between the Nubian Shield and the old African continent. We interpret the Pb isotopic compositions from the Eastern Desert and the previously identified Group I and II Pb (Stacey and others, 1980) from the Arabian-Nubian Shield to indicate various degrees of mixing between mantle-derived, juvenile oceanic component(s) and pre-Pan-African crustal material. On [Pb.sup.207/Pb.sup.204-Pb.sup.208/Pb.sup.204] plots, the feldspar and galena display variations characteristic of modern island-arc magmas, consistent with models that relate Shield assembly to accretion of island arcs. The crustal Pb isotopic signature of samples from Aswan and surrounding areas is best explained by interaction of arc magmas with an overriding crustal plate, whereas the less radiogenic nature of the Eastern Desert Pb's is probably related to subduction of sediments derived from adjacent continent or deposition of these sediments in near-arc basins. Accounting for recyled pre-Pan-African crustal material in the Shield results in normal growth rates ([0.44 km.sup.3/yr]), using reasonable assumptions for initial Pb isotopic compositions and concentrations.

Field observations in the Meatiq Dome, Eastern Desert, Egypt, show that amphibolite weathers to form massive outcrops and dense desert pavements that are extensively varnished. Laboratory-based reflectance spectra of representative samples (covering 5 [cm.sup.2]) and spectra extracted from Landsat thematic mapper data (covering 2.25 x [10.sup.4] [m.sup.2]) confirm that varnish dominates the reflectance properties of amphibolite outcrops. Field, laboratory, and Landsat data indicate that granite gneiss weathers to expose mainly bare rock, whereas granitic outcrops evolve to a mix of rock and locally generated sediment. Penetrative foliation in quartz phyllonite leads to production of slab-shaped clasts that cover the surface. The data show that mass wasting of the slabs results in a mix of rock, varnish, and sediment exposures. Outcrop reflectance properties for arid regions are thus dependent on both rock mineralogy and texture, because both parameters modulate bedrock weathering and erosion.

Initial Magellan observations reveal a planet with high dielectric constant materials exposed preferentially in elevated regions with high slopes, ejecta deposits extending up to 1000 kilometers to the west of [...]

The conclusions reached by Sultan et al on their studies about the nature of the Red Sea Crust, based from a compilation of remote-sensing, geochemical and geochronological data, is criticized. The arguments are based from the presence of a continental basement in the southern tip of the Red Sea and the Gulf of Suez. Reply reconfirmed the conclusions on the alignment of geologic features along two thirds of the Red Sea with the Arabian and Nubian coastlines within 14 km of juxtaposition.