Raymond E. Arvidson

Jarosite and Hematite at Meridiani Planum from Opportunity's Mossbauer Spectrometer

Mössbauer 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 hematite-rich 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 Opportunity Rover's Athena science investigation at Meridiani Planum, Mars

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.

H2O at the Phoenix Landing Site

Phoenix Ascending The Phoenix mission landed on Mars in March 2008 with the goal of studying the ice-rich soil of the planets northern arctic region. Phoenix included a robotic arm, with a camera attached to it, with the capacity to excavate through the soil to the ice layer beneath it, scoop up soil and water ice samples, and deliver them to a combination of other instruments—including a wet chemistry lab and a high-temperature oven combined with a mass spectrometer—for chemical and geological analysis. Using this setup, Smith et al. (p. 58) found a layer of ice at depths of 5 to 15 centimeters, Boynton et al. (p. 61) found evidence for the presence of calcium carbonate in the soil, and Hecht et al. (p. 64) found that most of the soluble chlorine at the surface is in the form of perchlorate. Together these results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past. The analysis revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars. Phoenix also carried a lidar, an instrument that sends laser light upward into the atmosphere and detects the light scattered back by clouds and dust. An analysis of the data by Whiteway et al. (p. 68) showed that clouds of ice crystals that precipitated back to the surface formed on a daily basis, providing a mechanism to place ice at the surface. A water ice layer was found 5 to 15 centimeters beneath the soil of the north polar region of Mars. The Phoenix mission investigated patterned ground and weather in the northern arctic region of Mars for 5 months starting 25 May 2008 (solar longitude between 76.5° and 148°). A shallow ice table was uncovered by the robotic arm in the center and edge of a nearby polygon at depths of 5 to 18 centimeters. In late summer, snowfall and frost blanketed the surface at night; H2O ice and vapor constantly interacted with the soil. The soil was alkaline (pH = 7.7) and contained CaCO3, aqueous minerals, and salts up to several weight percent in the indurated surface soil. Their formation likely required the presence of water.

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.

Perennial water ice identified in the south polar cap of Mars.

The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet. From the early mapping observation of the permanent ice caps on the martian poles, the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures, altimetry profiles, underlying buried hydrogen, and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide. Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2 km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap.

Evidence for Calcium Carbonate at the Mars Phoenix Landing Site

Phoenix Ascending The Phoenix mission landed on Mars in March 2008 with the goal of studying the ice-rich soil of the planets northern arctic region. Phoenix included a robotic arm, with a camera attached to it, with the capacity to excavate through the soil to the ice layer beneath it, scoop up soil and water ice samples, and deliver them to a combination of other instruments—including a wet chemistry lab and a high-temperature oven combined with a mass spectrometer—for chemical and geological analysis. Using this setup, Smith et al. (p. 58) found a layer of ice at depths of 5 to 15 centimeters, Boynton et al. (p. 61) found evidence for the presence of calcium carbonate in the soil, and Hecht et al. (p. 64) found that most of the soluble chlorine at the surface is in the form of perchlorate. Together these results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past. The analysis revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars. Phoenix also carried a lidar, an instrument that sends laser light upward into the atmosphere and detects the light scattered back by clouds and dust. An analysis of the data by Whiteway et al. (p. 68) showed that clouds of ice crystals that precipitated back to the surface formed on a daily basis, providing a mechanism to place ice at the surface. The action of liquid water may have helped to form the calcium carbonate found in the soils around the Phoenix landing site. 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°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.

Mössbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity's journey across sulfate-rich outcrop, basaltic sand and dust, and hematite lag deposits

Additonal co-authors: P Gutlich, E Kankeleit, T McCoy, DW Mittlefehldt, F Renz, ME Schmidt, B Zubkov, SW Squyres, RE Arvidson

Overview of the Spirit Mars Exploration Rover Mission to Gusev Crater: Landing site to Backstay Rock in the Columbia Hills

Spirit landed on the floor of Gusev Crater and conducted initial operations on soil-covered, rock-strewn cratered plains underlain by olivine-bearing basalts. Plains surface rocks are covered by wind-blown dust and show evidence for surface enrichment of soluble species as vein and void-filling materials and coatings. The surface enrichment is the result of a minor amount of transport and deposition by aqueous processes. Layered granular deposits were discovered in the Columbia Hills, with outcrops that tend to dip conformably with the topography. The granular rocks are interpreted to be volcanic ash and/or impact ejecta deposits that have been modified by aqueous fluids during and/or after emplacement. Soils consist of basaltic deposits that are weakly cohesive, relatively poorly sorted, and covered by a veneer of wind-blown dust. The soils have been homogenized by wind transport over at least the several kilometer length scale traversed by the rover. Mobilization of soluble species has occurred within at least two soil deposits examined. The presence of monolayers of coarse sand on wind-blown bedforms, together with even spacing of granule-sized surface clasts, suggests that some of the soil surfaces encountered by Spirit have not been modified by wind for some time. On the other hand, dust deposits on the surface and rover deck have changed during the course of the mission. Detection of dust devils, monitoring of the dust opacity and lower boundary layer, and coordinated experiments with orbiters provided new insights into atmosphere-surface dynamics.

Nature and origin of the hematite‐bearing plains of Terra Meridiani based on analyses of orbital and Mars Exploration rover data sets

The ~5 km of traverses and observations completed by the Opportunity rover from Endurance crater to the Fruitbasket outcrop show that the Meridiani plains consist of sulfate-rich sedimentary rocks that are largely covered by poorly-sorted basaltic aeolian sands and a lag of granule-sized hematitic concretions. Orbital reflectance spectra obtained by Mars Express OMEGA over this region are dominated by pyroxene, plagioclase feldspar, crystalline hematite (i.e., concretions), and nano-phase iron oxide dust signatures, consistent with Pancam and Mini-TES observations. Mossbauer Spectrometer observations indicate more olivine than observed with the other instruments, consistent with preferential optical obscuration of olivine features in mixtures with pyroxene and dust. Orbital data covering bright plains located several kilometers to the south of the landing site expose a smaller areal abundance of hematite, more dust, and a larger areal extent of outcrop compared to plains proximal to the landing site. Low-albedo, low-thermal-inertia, windswept plains located several hundred kilometers to the south of the landing site are predicted from OMEGA data to have more hematite and fine-grained olivine grains exposed as compared to the landing site. Low calcium pyroxene dominates spectral signatures from the cratered highlands to the south of Opportunity. A regional-scale model is presented for the formation of the plains explored by Opportunity, based on a rising ground water table late in the Noachian Era that trapped and altered local materials and aeolian basaltic sands. Cessation of this aqueous process led to dominance of aeolian processes and formation of the current configuration of the plains.

Two Years at Meridiani Planum: Results from the Opportunity Rover

The Mars Exploration Rover Opportunity has spent more than 2 years exploring Meridiani Planum, traveling ∼8 kilometers and detecting features that reveal ancient environmental conditions. These include well-developed festoon (trough) cross-lamination formed in flowing liquid water, strata with smaller and more abundant hematite-rich concretions than those seen previously, possible relict “hopper crystals” that might reflect the formation of halite, thick weathering rinds on rock surfaces, resistant fracture fills, and networks of polygonal fractures likely caused by dehydration of sulfate salts. Chemical variations with depth show that the siliciclastic fraction of outcrop rock has undergone substantial chemical alteration from a precursor basaltic composition. Observations from microscopic to orbital scales indicate that ancient Meridiani once had abundant acidic groundwater, arid and oxidizing surface conditions, and occasional liquid flow on the surface.