J. A. Grant

A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars

The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements; by inference, phosphorus is assumed to have been available. The environment probably had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars.

Deposition, exhumation, and paleoclimate of an ancient lake deposit, Gale crater, Mars

Ancient lake system at Gale crater Since 2012, the Curiosity rover has been diligently studying rocky outcrops on Mars, looking for clues about past water, climate, and habitability. Grotzinger et al. describe the analysis of a huge section of sedimentary rocks near Gale crater, where Mount Sharp now stands (see the Perspective by Chan). The features within these sediments are reminiscent of delta, stream, and lake deposits on Earth. Although individual lakes were probably transient, it is likely that there was enough water to fill in low-lying depressions such as impact craters for up to 10,000 years. Wind-driven erosion removed many of these deposits, creating Mount Sharp. Science, this issue p.10.1126/science.aac7575, see also p. 167 Mount Sharp now stands where there was once a large intercrater lake system. [Also see Perspective by Chan] INTRODUCTION Remote observational data suggest that large bodies of standing water existed on the surface of Mars in its early history. This would have required a much wetter climate than that of the present, implying greater availability of water on a global basis and enhanced potential for global habitability. However, based on assumptions of a vast water inventory and models of atmospheric erosion, theoretical studies suggest a climate that was wetter but not by enough to sustain large lakes, even in depressions such as impact craters. RATIONALE The Mars Science Laboratory mission’s rover, Curiosity, provides the capability to test hypotheses about Mars’s past climate. The focus of the mission is the exploration of a ~5-km-high mountain, Aeolis Mons (informally known as Mount Sharp), located near the center of the ~140-km-wide Gale impact crater. Mount Sharp is underlain by hundreds of meters of sedimentary rock strata deposited ~3.6 billion to 3.2 billion years ago. These sediments accumulated in aqueous environments, recording the history of Mars’s ancient climate. Because of Curiosity’s ability to study these strata where they are exposed near the base of Mount Sharp, we can directly test the hypothesis that large impact craters were capable of accumulating and storing water as lakes for substantial periods of time. RESULTS Over the course of 2 years, Curiosity studied dozens of outcrops distributed along a ~9-km transect that also rose ~75 m in elevation. Image data were used to measure the geometry and grain sizes of strata and to survey the textures associated with sediment deposition and diagenesis. Erosion of Gale’s northern crater wall and rim generated gravel and sand that were transported southward in shallow streams. Over time, these stream deposits advanced toward the crater interior, transitioning downstream into finer-grained (sand-sized), southward-advancing delta deposits. These deltas marked the boundary of an ancient lake where the finest (mud-sized) sediments accumulated, infilling both the crater and its internal lake basin. After infilling of the crater, the sedimentary deposits in Gale crater were exhumed, probably by wind-driven erosion, creating Mount Sharp. The ancient stream and lake deposits are erosional remnants of superimposed depositional sequences that once extended at least 75 m, and perhaps several hundreds of meters, above the current elevation of the crater floor. Although the modern landscape dips northward away from Mount Sharp, the ancient sedimentary deposits were laid down along a profile that projected southward beneath Mount Sharp and indicate that a basin once existed where today there is a mountain. CONCLUSION Our observations suggest that individual lakes were stable on the ancient surface of Mars for 100 to 10,000 years, a minimum duration when each lake was stable both thermally (as liquid water) and in terms of mass balance (with inputs effectively matching evaporation and loss of water to colder regions). We estimate that the stratigraphy traversed thus far by Curiosity would have required 10,000 to 10,000,000 years to accumulate, and even longer if overlying strata are included. Though individual lakes may have come and gone, they were probably linked in time through a common groundwater table. Over the long term, this water table must have risen at least tens of meters to enable accumulation of the delta and lake deposits observed by Curiosity in Gale crater. Inclined strata in the foreground dip southward toward Mount Sharp and represent ancient delta deposits. These deposits transition into strata in the mid-field that were deposited in ancient lakes. The buttes and mesas in the background contain younger deposits that overlie and postdate the lake deposits beneath Mount Sharp. The outcrop in the foreground is about 6 m wide, and the buttes and mesas in the background are hundreds of meters wide and tens of meters high. The image has been white-balanced. [Credit: NASA/Caltech/JPL/MSSS] The landforms of northern Gale crater on Mars expose thick sequences of sedimentary rocks. Based on images obtained by the Curiosity rover, we interpret these outcrops as evidence for past fluvial, deltaic, and lacustrine environments. Degradation of the crater wall and rim probably supplied these sediments, which advanced inward from the wall, infilling both the crater and an internal lake basin to a thickness of at least 75 meters. This intracrater lake system probably existed intermittently for thousands to millions of years, implying a relatively wet climate that supplied moisture to the crater rim and transported sediment via streams into the lake basin. The deposits in Gale crater were then exhumed, probably by wind-driven erosion, creating Aeolis Mons (Mount Sharp).

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.

In situ radiometric and exposure age dating of the martian surface.

We determined radiogenic and cosmogenic noble gases in a mudstone on the floor of Gale Crater. A K-Ar age of 4.21 ± 0.35 billion years represents a mixture of detrital and authigenic components and confirms the expected antiquity of rocks comprising the crater rim. Cosmic-ray–produced 3He, 21Ne, and 36Ar yield concordant surface exposure ages of 78 ± 30 million years. Surface exposure occurred mainly in the present geomorphic setting rather than during primary erosion and transport. Our observations are consistent with mudstone deposition shortly after the Gale impact or possibly in a later event of rapid erosion and deposition. The mudstone remained buried until recent exposure by wind-driven scarp retreat. Sedimentary rocks exposed by this mechanism may thus offer the best potential for organic biomarker preservation against destruction by cosmic radiation.

Ancient Impact and Aqueous Processes at Endeavour Crater, Mars

Martian Veins After more than 7 years of traveling across the Meridiani Planum region of Mars, the Mars Exploration rover Opportunity reached the Endeavour Crater, a 22-km-impact crater made of materials older than those previously investigated by the rover. Squyres et al. (p. 570) present a comprehensive analysis of the rim of this crater. Localized zinc enrichments that provide evidence for hydrothermal alteration and gypsum-rich veins that were precipitated from liquid water at a relatively low temperature provide a compelling case for aqueous alteration processes in this area at ancient times. Analysis of data from the Mars Exploration Rover Opportunity provides evidence for past water flow near an ancient crater. The rover Opportunity has investigated the rim of Endeavour Crater, a large ancient impact crater on Mars. Basaltic breccias produced by the impact form the rim deposits, with stratigraphy similar to that observed at similar-sized craters on Earth. Highly localized zinc enrichments in some breccia materials suggest hydrothermal alteration of rim deposits. Gypsum-rich veins cut sedimentary rocks adjacent to the crater rim. The gypsum was precipitated from low-temperature aqueous fluids flowing upward from the ancient materials of the rim, leading temporarily to potentially habitable conditions and providing some of the waters involved in formation of the ubiquitous sulfate-rich sandstones of the Meridiani region.

Spirit Mars Rover Mission: Overview and selected results from the northern Home Plate Winter Haven to the side of Scamander crater

Spirit Mars Rover Mission : Overview and selected results from the northern Home Plate Winter Haven to the side of Scamander crater

Assessment of Mars Exploration Rover landing site predictions.

Comprehensive analyses of remote sensing data during the three-year effort to select the Mars Exploration Rover landing sites at Gusev crater and at Meridiani Planum correctly predicted the atmospheric density profile during entry and descent and the safe and trafficable surfaces explored by the two rovers. The Gusev crater site was correctly predicted to be a low-relief surface that was less rocky than the Viking landing sites but comparably dusty. A dark, low-albedo, flat plain composed of basaltic sand and haematite with very few rocks was expected and found at Meridiani Planum. These results argue that future efforts to select safe landing sites based on existing and acquired remote sensing data will be successful. In contrast, geological interpretations of the sites based on remote sensing data were less certain and less successful, which emphasizes the inherent ambiguities in understanding surface geology from remotely sensed data and the uncertainty in predicting exactly what materials will be available for study at a landing site.

Spirit Mars Rover Mission to the Columbia Hills, Gusev Crater: Mission overview and selected results from the Cumberland Ridge to Home Plate

This paper summarizes the Spirit rover operations in the Columbia Hills of Gusev Crater from sols 513 to 1476 and provides an overview of selected findings that focus on synergistic use of the Athena Payload and comparisons to orbital data. Results include discovery of outcrops (Voltaire) on Husband Hill that are interpreted to be altered impact melt deposits that incorporated local materials during emplacement. Evidence for extensive volcanic activity and aqueous alteration in the Inner Basin is also detailed, including discovery and characterization of accretionary lapilli and formation of sulfate, silica, and hematite-rich deposits. Use of Spirits data to understand the range of spectral signatures observed over the Columbia Hills by the Mars Reconnaissance Orbiters Compact Reconnaissance Imaging Spectrometer (CRISM) hyperspectral imager (0.4–4 μm) is summarized. We show that CRISM spectra are controlled by the proportion of ferric-rich dust to ferrous-bearing igneous minerals exposed in ripples and other wind-blown deposits. The evidence for aqueous alteration derived from Spirits data is associated with outcrops that are too small to be detected from orbital observations or with materials exposed from the shallow subsurface during rover activities. Although orbital observations show many other locations on Mars with evidence for minerals formed or altered in an aqueous environment, Spirits data imply that the older crust of Mars has been altered even more extensively than evident from orbital data. This result greatly increases the potential that the surface or shallow subsurface was once a habitable regime.

Crater gradation in Gusev crater and Meridiani Planum, Mars

[1]xa0The Mars Exploration Rovers investigated numerous craters in Gusev crater and Meridiani Planum during the first ∼400 sols of their missions. Craters vary in size and preservation state but are mostly due to secondary impacts at Gusev and primary impacts at Meridiani. Craters at both locations are modified primarily by eolian erosion and infilling and lack evidence for modification by aqueous processes. Effects of gradation on crater form are dependent on size, local lithology, slopes, and availability of mobile sediments. At Gusev, impacts into basaltic rubble create shallow craters and ejecta composed of resistant rocks. Ejecta initially experience eolian stripping, which becomes weathering-limited as lags develop on ejecta surfaces and sediments are trapped within craters. Subsequent eolian gradation depends on the slow production of fines by weathering and impacts and is accompanied by minor mass wasting. At Meridiani the sulfate-rich bedrock is more susceptible to eolian erosion, and exposed crater rims, walls, and ejecta are eroded, while lower interiors and low-relief surfaces are increasingly infilled and buried by mostly basaltic sediments. Eolian processes outpace early mass wasting, often produce meters of erosion, and mantle some surfaces. Some small craters were likely completely eroded/buried. Craters >100 m in diameter on the Hesperian-aged floor of Gusev are generally more pristine than on the Amazonian-aged Meridiani plains. This conclusion contradicts interpretations from orbital views, which do not readily distinguish crater gradation state at Meridiani and reveal apparently subdued crater forms at Gusev that may suggest more gradation than has occurred.

Large wind ripples on Mars: A record of atmospheric evolution

Wind blowing over sand on Earth produces decimeter-wavelength ripples and hundred-meter– to kilometer-wavelength dunes: bedforms of two distinct size modes. Observations from the Mars Science Laboratory Curiosity rover and the Mars Reconnaissance Orbiter reveal that Mars hosts a third stable wind-driven bedform, with meter-scale wavelengths. These bedforms are spatially uniform in size and typically have asymmetric profiles with angle-of-repose lee slopes and sinuous crest lines, making them unlike terrestrial wind ripples. Rather, these structures resemble fluid-drag ripples, which on Earth include water-worked current ripples, but on Mars instead form by wind because of the higher kinematic viscosity of the low-density atmosphere. A reevaluation of the wind-deposited strata in the Burns formation (about 3.7 billion years old or younger) identifies potential wind-drag ripple stratification formed under a thin atmosphere.