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The Mars Global Surveyor Mars Orbiter Camera has acquired data that establish the present-day impact cratering rate and document new deposits formed by downslope movement of material in mid-latitude gullies on Mars. Twenty impacts created craters 2 to 150 meters in diameter within an area of 21.5 x 10(6) square kilometers between May 1999 and March 2006. The values predicted by models that scale the lunar cratering rate to Mars are close to the observed rate, implying that surfaces devoid of craters are truly young and that as yet unrecognized processes of denudation must be operating. The new gully deposits, formed since August 1999, are light toned and exhibit attributes expected from emplacement aided by a fluid with the properties of liquid water: relatively long, extended, digitate distal and marginal branches, diversion around obstacles, and low relief. The observations suggest that liquid water flowed on the surface of Mars during the past decade.

Landforms representative of sedimentary processes and environments that occurred early in martian history have been recognized in Mars Global Surveyor Mars Orbiter Camera and Mars Odyssey Thermal Emission Imaging System images. Evidence of distributary, channelized flow (in particular, flow that lasted long enough to foster meandering) and the resulting deposition of a fan-shaped apron of debris indicate persistent flow conditions and formation of at least some large intracrater layered sedimentary sequences within fluvial, and potentially lacustrine, environments.

The Thermal Emission Imaging System (THEMIS) on Mars Odyssey has produced infrared to visible wavelength images of the martian surface that show lithologically distinct layers with variable thickness, implying temporal changes in the processes or environments during or after their formation. Kilometer-scale exposures of bedrock are observed; elsewhere airfall dust completely mantles the surface over thousands of square kilometers. Mars has compositional variations at 100-meter scales, for example, an exposure of olivine-rich basalt in the walls of Ganges Chasma. Thermally distinct ejecta facies occur around some craters with variations associated with crater age. Polar observations have identified temporal patches of water frost in the north polar cap. No thermal signatures associated with endogenic heat sources have been identified.

In this contribution we report measurements and interpretations of aeolian abrasion features observed along the Mars Science Laboratory rover traverse at Gale Crater on Mars. Aeolian abrasion of surface rocks has been documented by most missions that have landed on Mars, but attention has largely focused on relatively resistant rocks that display ventifact morphologies well‐known from Earth. The current study instead emphasizes abrasion features developed on mudstone surfaces, because aeolian abrasion textures are very common in the mudstone‐dominated Murray Formation at Gale Crater. These abrasion textures have been observed over a wide area, and they allow deductions about dominant past wind regimes that differ from those observed at present. On Earth, aeolian abrasion features in mudstone outcrops have minimal preservation potential because of the susceptibility of mudstones to moisture and aqueous erosion, even in dominantly arid surface environments. Because of this there is no literature on potential terrestrial analogs. In order to better understand aeolian abrasion morphologies in mudstones on Mars, laboratory experiments were conducted that exposed a range of mudstone samples and rock simulant samples to wind‐driven sand for periods of up to four months. These experiments produced a range of aeolian abrasion textures ('wind tails', fluting, differential 'etching' of laminae, stalks and pedestals) that can serve as analogs for features in the Murray Formation, increasing our confidence regarding aeolian origins for these and the deductions of formative, sand‐driving wind azimuths. Orientations of aeolian abrasion textures along the Mars Science Laboratory traverse were compared with active ripple orientations in rover images, and with previous analyses of orbital images of the study area, in order to evaluate whether the current, complex wind environment can explain the abrasion orientations observed in rover images, or if the abrasion record might also contain signatures of past wind regimes that differed from those of the present. Results show relatively coherent patterns of airflow and sand movement that vary in accordance with factors like local topography and upwind availability of sand for abrasion. Wind events blowing southwest that are primarily responsible for southwest migration of the Bagnold Dunes have also caused bedrock abrasion of similar orientation between these dunes. Less effective wind events (from an abrasion standpoint) blowing south‐southeast have broadly affected bedrock exposures elsewhere across the Mars Science Laboratory landing site away from the Bagnold Dunes. A notable exception are northeast abrasion textures on the more erosion‐resistant sandstones of the Naukluft Plateau. The latter are consistent with one of two abrasion directions measured in previous studies, prior to the rover's arrival at the Murray Formation mudstones, and probably reflect the preservation of an ancient sand‐driving wind regime that differs from that of the present. [ABSTRACT FROM AUTHOR]

The Mars Science Laboratory Curiosity rover conducted a reconnaissance traverse across the Pahrump Hills outcrop within Gale crater from Sols 780–797. During the traverse, the Mars Descent Imager (MARDI) acquired a continuous imaging record of primary and secondary sedimentary features throughout the outcrop. The characteristics of the features (laminae, resistant features, fractures, gray clasts) and their spatial distribution provide insight into the processes that contributed to the formation of Pahrump Hills. Thin, regular laminae (mm-scale) are ubiquitous in the bedrock, implying that depositional processes at that scale did not change appreciably during deposition of the mudstone succession at Pahrump Hills. Higher bedrock slopes correlate with undulatory bedrock surfaces, bedrock with elevated Mg contents, and fractures exhibiting wide, raised edges. These collective features are consistent with increased erosional resistance caused by greater quantities of erosionally-resistant, Mg-bearing cement within the bedrock permitted by coarser grain sizes. Resistant features exhibit a range of morphologies, elevated Mg contents, and do not deflect laminae within the bedrock. Their characteristics implicate the involvement of Mg-enriched fluids in a late diagenetic overprint affecting the bedrock. The variations of fracture fill and edge morphologies and chemistries further suggest repeated fracturing and fluid interaction events within the strata exposed at Pahrump Hills. Gray clasts strongly resemble fragments eroded from sandstone horizons interspersed throughout the Pahrump Hills outcrop. • A MARDI sidewalk mosaic was used to systematically map the Pahrump Hills outcrop. • MARDI identified primary and secondary sedimentary features throughout the outcrop. • MARDI determined the spatial distribution of the sedimentary features. • The features record the depositional and diagenetic history of Pahrump Hills. [ABSTRACT FROM AUTHOR]

This paper characterizes the detailed sedimentology of a fluvial sandbody on Mars for the first time and interprets its depositional processes and palaeoenvironmental setting. Despite numerous orbital observations of fluvial landforms on the surface of Mars, ground-based characterization of the sedimentology of such fluvial deposits has not previously been possible. Results from the NASA Mars Science Laboratory Curiosity rover provide an opportunity to reconstruct at fine scale the sedimentary architecture and palaeomorphology of a fluvial environment on Mars. This work describes the grain size, texture and sedimentary facies of the Shaler outcrop, reconstructs the bedding architecture, and analyses cross-stratification to determine palaeocurrents. On the basis of bedset geometry and inclination, grain-size distribution and bedform migration direction, this study concludes that the Shaler outcrop probably records the accretion of a fluvial barform. The majority of the outcrop consists of large-scale trough cross-bedding of coarse sand and granules. Palaeocurrent analyses and bedform reconstruction indicate that the beds were deposited by bedforms that migrated towards the north-east, across the surface of a bar that migrated south-east. Stacked cosets of dune cross-bedding suggest aggradation of multiple bedforms, which provides evidence for short periods of sustained flow during Shaler deposition. However, local evidence for aeolian reworking and the presence of potential desiccation cracks within the outcrop suggest that fluvial deposition may have been intermittent. The uppermost strata at Shaler are distinct in terms of texture and chemistry and are inferred to record deposition from a different sediment dispersal system with a contrasting provenance. The outcrop as a whole is a testament to the availability of liquid water on the surface of Mars in its early history. [ABSTRACT FROM AUTHOR]

Sedimentary rocks exposed in the Meridiani Planum region of Mars record aqueous and eolian deposition in ancient dune and interdune playa-like environments that were arid, acidic, and oxidizing. On Earth, microbial populations have repeatedly adapted to low pH and both episodic and chronic water limitation, suggesting that, to a first approximation, the Meridiani plain may have been habitable during at least part of the interval when deposition and early diagenesis took place. On the other hand, the environmental conditions inferred for Meridiani deposition would have posed a challenge for prebiotic chemical reactions thought to have played a role in the origin of life on Earth. Orbital observations suggest that the combination of sulfate minerals and hematite found in Meridiani rocks may be unusual on the martian surface; however, there is reason to believe that acidity, aridity, and oxidizing conditions were broadly distributed on ancient Mars. When these conditions were established and how much environmental heterogeneity existed on early Mars remain to be determined. Because sulfates and iron oxides can preserve detailed geochemical records of environmental history as well as chemical, textural and microfossil signatures of biological activity, Meridiani Planum is an attractive candidate for Mars sample return., Organismic and Evolutionary Biology

Highlights • Long-term monitoring reveals seasonal storm trends, generally repeatable each year. • Syrtis sites peak storm frequency occurs during autumn. • Columbia Hills peak storm frequency occurs during winter. • Dust storms pose little concern during EDL season (L s = 345–20°), probability ≤ 3.2%. Abstract MRO-MARCI daily global mapping images from 16 November 2006 (L s = 111.4°) through 15 July 2017 (L s = 33.5°), covering portions of seven consecutive Martian years (MY 28 – MY 34), were used in a survey of dust storm activity within 2000 km radius of the final three candidate sites for the 2020 Mars Rover mission (Columbia Hills at 14.59° S 175.53° E, North East Syrtis at 17.89° N 77.16° E, and Jezero at 18.39° N 77.54° E). A total of 1321 dust storms (1228 local, 93 regional) were observed. Storms were observed in all seasons, but showed distinct seasonal trends. Columbia Hills monitoring area experienced eastward traversing polar hood/seasonal cap edge storms during southern autumn and winter, with a gradual transition during southern spring to northern hemisphere storms along the Elysium and Arcadia-Amazonis tracks with winter dominated by the northern hemisphere storms. Syrtis sitess were dominated by cap edge storms during northern spring, transitioning to subtropical (Syrtis Major and north of Hellas Basin) storms in both hemispheres during northern summer, with autumn bringing continued storm activity in the southern subtropics and increased activity along the Utopia cross-equatorial storm-track, followed in winter by a transition back to north polar hood/seasonal cap edge storms. Peak storm frequency occurs during autumn for the Syrtis sites and winter for Columbia Hills site. During the entry, descent, and landing (EDL) season, L s = 345–25°, the daily probability of dust storm activity at the landing sites ranged from ≤ 1.6% for the Columbia Hills to ≤ 3.2% for Syrtis sites. For EDL sol (L s ∼ 6°), storm probabilities drop to approximately 0.0% for Columbia Hills and ranged from 0.6–1.2% for the other sites, indicating that storms are of minimum concern for EDL. [ABSTRACT FROM AUTHOR]

Launched on August 12, 2005, the Mars Reconnaissance Orbiter (MRO) entered Mars orbit on March 10, 2006. Following a period of aerobraking, MRO completed its entry into its primary science orbit in September 2006, initiating a program of systematic observations of Mars that still continues. Five providers, including the Italian Space Agency, provided 6 instruments for flight, observing the surface, atmosphere, and subsurface of Mars with greater spatial resolution and systematic coverage than ever before. Two investigations utilized the spacecraft accelerometers and tracking of the orbiter via the Deep Space Network to study upper atmosphere densities and the gravity field of the planet. The data acquired by MRO have revealed a dynamic planet whose change from an ancient wetter climate to the drier climate of today was a complex transition and not a simple "drying out". Furthermore, that climate continues to change even today. The diversity of the early habitable environments, the ice ages recorded in the polar cap layering and subsurface ice deposits, the repeating patterns of dust storms, and the revelation of new features at the limit of resolution are all part of the scientific return from MRO during nearly a decade of Mars years. The story of that mission, of the evolution of its capabilities, and its contributions to our current understanding of Mars are the subject of nearly two dozen papers in the Icarus special issue, MRO: Sixteen Years at Mars. Three papers describe in more detail the evolution of instrument operations and data products over the mission; several papers describe new analysis techniques for the radar, including construction of 3-dimensional views, and for the atmospheric sounder, enabling better retrievals in a dusty lower atmosphere. Other papers report on recent research including, but not limited to, dune movement, the roles of water and carbon dioxide ice in surface change, and attempts to understand the formation and fading of the enigmatic recurring slope lineae. This paper describes general aspects of the MRO spacecraft, payload, and mission as context for the special issue papers; it also summarizes scientific results and mission support events on a mission phase by mission phase basis to give a time history of discovery and effort. • Diverse potentially habitable environments have been identified from orbit. • Evidence found for climate change throughout Mars history, including today. • Recurring slope lineae were discovered; origins are still being investigated. • Internal layering of the Mars polar caps appears linked to orbital cycles. • Mid-latitude, near-surface water ice is exposed in cliffs and new impact craters. [ABSTRACT FROM AUTHOR]

Lithology and physical properties of strata exposed at the Earth''s surface have direct influence on the erosion and geomorphic expression of landforms. While this is well known on our planet, examples on Mars are just coming to light among the tens of thousands of airphoto-quality images (resolutions 1.5–12 m/pixel) acquired since 1997 by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC). Specific examples occur among martian north polar layered materials, which MOC images reveal are divided into two distinct stratigraphic units: a lower, dark-toned layered unit and a younger, upper, lighter-toned layered unit. The lower unit is less resistant to wind erosion than the upper unit. The upper unit most likely consists of stratified dust and ice, while the lower unit contains abundant, poorly cemented sand. Sand is more easily mobilized by wind than dust; the lower resistance to erosion of the lower unit results from the presence of sand. Where wind erosion in polar troughs has penetrated to the lower unit, geomorphic change has proceeded more rapidly: sand has been liberated from the lower unit, and arcuate scarps have formed as the upper unit has been undermined. Wind erosion of the lower unit thus influences the geomorphology of the north polar region; this result likely explains the genesis of the large polar trough, Chasma Boreale, and the relations between dunes and arcuate scarps that have puzzled investigators for nearly three decades. The properties of the stratigraphic units suggest that the upper limit for the amount of water contained in the north polar layered materials may be 30–50% less than previously estimated. [Copyright &y& Elsevier]

Observations by the Mars Color Imager (MARCI) onboard the Mars Reconnaissance Orbiter (MRO) in the ultraviolet (UV, Band 7; 320 nm) are used to characterize the spatial and temporal behavior of atmospheric water ice over a period of 6 Mars Years. Exploiting the contrast of the bright ice clouds to the low albedo surface, a radiative transfer-based retrieval algorithm is developed to derive the column-integrated optical depth of the ice (τ ice). Several relatively unique input products are created as part of the retrieval development process, including a zonal dust climatology based on emission phase function (EPFs) sequences from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), a spatially variable UV-reflectance model for Band 7 (as well as for Band 6, 260 nm), and a water ice scattering phase function based on a droxtal ice habit. Taking into account a radiometric precision of 7%, an error analysis estimates the uncertainty in τ ice to be ∼0.03 (excluding particle size effects, which are discussed separately). Zonal trends are analyzed over the full temporal extent of the observations, looking at both diurnal and interannual variability. The main (zonal) features are the aphelion cloud belt (ACB) and the polar hoods. For the ACB, there can be an appreciable diurnal change in τ ice between the periods of 14h30–15h00 and 15h00–15h30 Local True Solar Time (LTST). The amplitude of this effect shows relatively large interannual variability, associated mainly with changes in the earlier time block. When averaged over the interval 14h00–16h00 LTST, the interannual differences in the ACB structure are appreciably smaller. When the MARCI τ ice are compared to those from the Thermal Emission Spectrometer (TES), there is a good correlation of features, with the most significant difference being the seasonal (L S) evolution of the ACB. For TES, the ACB zonal profile is relative symmetric about L S = 90°. In the MARCI data, this profile is noticeably asymmetric, with the centroid shifted to later in the northern summer season (L S = 120°). The MARCI behavior is consistent with that observed by several other instruments. The correspondence of MARCI τ ice zonal and meridional behaviors with that predicted by two Global Circulation Models (GCM) is good. Each model captures the general behavior seen by MARCI in the ACB, the polar hoods, and the major orographic/topographic cloud features (including Valles Mariners). However, the mismatches between GCM results and MARCI reinforce the challenging nature of water ice clouds for dynamical models. The released τ ice are being archived at Malin Space Science Systems at https://www.msss.com/mro_marci_iceclouds/. • The zonal behavior is dominated by aphelion cloud belt (ACB) and polar hood structures. • The MARCI ACB can demonstrate appreciable diurnal change and annual variability. • MARCI ACB observations show differences with those of Thermal Emission Spectrometer. • Dynamical models can capture the general behavior seen by the MARCI ACB and polar hoods. [ABSTRACT FROM AUTHOR]

Imaging during and after the landing of the Mars Science Laboratory (MSL) rover in 2012 provides a means to examine two transitory phenomena for the first time: the settling of the plume of material raised by the powered terminal descent, and the possible dispersal of 140 kg of hydrazine into the atmosphere as fine-grained solid carbazic acid. The peri-landing images, acquired by the Mars Descent Imager (MARDI) and the rover hazard cameras (Hazcams), allow the first comparison of post-landing geological assessment of surface deflation with the plume itself. Examination of the Hazcam images acquired over a period of 4011 s shows that only a small fraction (350–1000 kg) of the total mass of fine-grained surface material displaced by the landing (4000 kg) remained in the atmosphere for this duration. Furthermore, a large component of this dust occurs as particles for which the characteristic optical radius is 20–60 μm, preventing them from being substantially mixed with the atmospheric column by eddy diffusion. Examination of the MARDI record over 225 s post-landing reveals a rapidly settling component that comprised approximately 1800–2400 kg and had a larger particle size with an optical radius of 360–470 μm. The possible release of hydrazine by the sky crane stage also may have created particles of carbazic acid that would, analogous to the dust, spread through eddy diffusivity and settle to the ground. Peri-landing Hazcam images of the plume created during sky crane destruction constrains the particle radius to be either less than 23 μm or greater than 400 μm. When combined with a Lagrangian model of the atmosphere, such particle sizes suggest that the carbazic acid was either deposited very near the sky crane crash site, or was widely dispersed as small particles which would have been quickly photodissociated to volatile ammonia and carbon dioxide. Surfaces visited by the MSL rover, Curiosity, would have received at most <0.2 ppb of carbazic acid and levels of sky crane related organics would have fallen well below the detection threshold of the Sample Analysis at Mars (SAM) instruments within 4–6 sols, well before the rover acquired its first samples over 60 sols into the mission. [ABSTRACT FROM AUTHOR]

Since November of 2006, The Mars Color Imager (MARCI) onboard the Mars Reconnaissance Orbiter (MRO) has obtained multiple-filter daily global images of Mars centered upon a local time (LT) of 3 pm. Ultraviolet imaging bands placed within (260 nm) and longward (320 nm) of Hartley band (240–300 nm) ozone (O 3 ) absorption support retrievals of atmospheric ozone columns, with detection limits (∼1 μm-atm) appropriate to mapping elevated O 3 abundances at low latitudes around Mars aphelion, and over mid-to-high latitudes during fall/winter/spring seasons. MARCI O 3 maps for these regions reveal the detailed spatial (∼1° lat/long, for 8 × 8 pixel binned resolution) and temporal (daily, with substantial LT coverage at pole) behaviors of water vapor saturation conditions that force large variations in water vapor photolysis products (HO x -OH, HO 2 , and H) responsible for the catalytic destruction of O 3 in the Mars atmosphere. A detailed description of the MARCI O 3 data set, including measurement and retrieval characteristics, is provided in conjunction with comparisons to Mars Express SPICAM ozone measurements (Perrier, S. et al. [2006]. J. Geophys. Res. (Planets) 111) and LMD GCM simulated O 3 abundances (Lefèvre, F. [2004]. J. Geophys. Res. (Planets) 109). Presented aspects of the MARCI ozone mapping data set include aphelion increases in low latitude O 3 , dynamically evolving high latitude O 3 maxima associated with planetary waves and weather fronts during northern early spring, and distinctive winter/spring O 3 and CO increases within the Hellas Basin associated with transport of condensation-enhanced south polar air mass. Comparisons of coincident MARCI measurements and LMD simulations for ice cloud and O 3 columns are considered in the context of potential heterogeneous photochemical processes (Lefèvre, F. [2008]. Nature 454, 971–975), which are not strongly evidenced in the MARCI observations. Modest interannual variations are exhibited, most notably a 20% reduction in aphelion low latitude O 3 columns following the 2007 perihelic global dust storm. [ABSTRACT FROM AUTHOR]

Daily, global wide angle imaging of Mars clouds in MARCI (MARs Color Imager, (Malin et al., 2008)) ultraviolet and visible bands reveals the spatial/seasonal distributions and physical characteristics of perihelion cloud trails (PCT); a class of high altitude (40–50 km), horizontally extended (200–1000 km, trending W to WSW) water ice clouds formed over specific southern low-to-mid latitude (5S-40S), mesoscale (∼ 50 km) locations during the Mars perihelion, southern summer season. PCT were first reported in association with rim regions of Valles Marineris (Clancy et al., 2009). The current study employs MARCI 2007–2011 imaging to sample the broader distributions and properties of PCT; and indicates several distinct locations of peak occurrences, including SW Arsia Mons, elevated regions of Syria, Solis, and Thaumasia Planitia, along Valles Marineris margins, and the NE rim of Hellas Basin. PCT are present over Mars solar longitudes ( L S ) of 210–310 ° , in late morning to mid afternoon hours (10 am–3 pm), and are among the brightest and most distinctive clouds exhibited during the perihelion portion of the Mars orbit. Their locations (i.e. , eastern margin origins) correspond to strong local elevation gradients, and their timing to peak solar heating conditions (perihelion, subsolar latitudes and midday local times). They occur approximately on a daily basis among all locations identified (i.e. , not daily at a single location). Based on cloud surface shadow analyses, PCT form at 40–50 km aeroid altitudes, where water vapor is generally at near-saturation conditions in this perihelion period (e.g. Millour et al., 2014). They exhibited notable absences during periods of planet encircling and regional dust storm activity in 2007 and 2009, respectively, presumably due to reduced water saturation conditions above 35–40 km altitudes associated with increased dust heating over the vertically extended atmosphere (e.g. Neary et al., 2019). PCT exhibit smaller particle sizes (R e f f =0.2-0.5 μ m) than typically exhibited in the lower atmosphere, and incorporate significant fractions of available water vapor at these altitudes. PCT ice particles are inferred to form continuously (over ∼ 4 h) at their PCT eastern origins, associated with localized updrafts, and are entrained in upper level zonal/meridional winds (towards W or WSW with ∼ 50 m/s speeds at 40–50 km altitudes) to create long, linear cloud trails. PCT cloud formation is apparently forced in the lower atmosphere (≤ 10–15 km) by strong updrafts associated with distinctive topographic gradients, such as simulated in mesoscale studies (e.g., Tyler and Barnes, 2015) and indicated by the surface-specific PCT locations. These lower scale height updrafts are proposed to generate vertically propagating gravity waves (GW), leading to PCT formation above ∼ 40 km altitudes where water vapor saturation conditions promote vigorous cloud ice formation. Recent mapping of GW amplitudes at ∼ 25 km altitudes, from Mars Climate Sounder 15 μ m radiance variations (Heavens et al., 2020) in fact demonstrates close correspondences to the detailed spatial distributions of observed PCT, relative to other potential factors such as surface albedo and surface elevation (or related boundary layer depths). • A distinct Mars perihelion cloud form. • Long, E–W oriented linear clouds tied to specific surface regions. • Eastern margin cloud plume formation at 45 km altitudes. • Forced by topographic winds generating gravity waves. [ABSTRACT FROM AUTHOR]

• Two new cosmogenic isotope analyses were performed on rocks from Mount Sharp. • Low 36Ar abundance suggests recent (≲1 Ga) water-rock interactions. • These rocks likely were exposed >1 Ga, compared to <100 Ma on the crater floor. • Episodic intense erosion likely drives landscape change in Gale crater. • Ongoing erosion rates suggest Mount Sharp formed during the Hesperian. The erosion rates and mechanisms operating on Mount Sharp in Gale crater, Mars were assessed via experiments performed by the SAM instrument to determine the cosmogenic noble gas contents of Murray mudstone formation samples Mojave 2 and Quela. Previous measurements of samples from the Aeolis Palus depression between Mount Sharp and the north rim of Gale crater indicate that scarp retreat-generated surfaces formed within the last 100 Ma. In contrast, Mojave 2 yielded exposure ages of 1,320±240 (3He), 910±420 (21Ne), and 310±60 Ma (36Ar). Quela gave a 3He age of 1,460±200 Ma; 21Ne and 36Ar from this sample could not be quantified due to isobaric interferences. The discordant and young 36Ar exposure age in Mojave 2 is likely the result of interaction with water which dissolved the chlorine-bearing host phases of this nuclide. The most probable exposure scenario is that both Mojave 2 and Quela have been at the surface for the most recent ∼1 Ga after the overlying few meters of rock were removed in a geologically rapid exhumation episode. Based on local geomorphology, scarp retreat is the most likely mechanism for the exposure at these two sites. The exposure ages measured throughout Curiosity's traverse indicate that the net removal of rock has proceeded more recently on Aeolis Palus than on the lower slopes of Mount Sharp. The implied differential erosion rate is insufficient to explain how Mount Sharp formed, even over billions of years. Instead, given that the surfaces on Mount Sharp have existed for >1 Ga, the mountain must have formed early, likely during the Hesperian. This study provides direct quantitative support for inferences based on crater counts that Mount Sharp had eroded to close to its current form before onset of the Amazonian. [ABSTRACT FROM AUTHOR]