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Introduction: The global distribution of shortening structures on Mercury as well as geographic and temporal trends of any of their morphometric and structural parameters are key to better understand the planets’ history of contraction, tidal despinning, and lithologic/rheologic variations in its current crust [1–4]. Previous global cataloging of ~6000 shortening structures on Mercury by [1] was based on preliminary, ~250 m/pixel data by MESSENGER’s Mercury Dual Imaging System (MDIS) and included no morphometric analyses other than displacement-length (D/L) assessments on 665 m/pixel digital terrain models (DTMs) [5]. Furthermore, as recently elaborated on by [6], the association of shortening structures with larger tectonic complexes needs to be assessed to derive a more realistic inventory of actually implied faults. We provide such assessments without interpolating between geographically disconnected structures while maintaining optional association of structural complexes, thereby offering a more versatile dataset (Fig. 1). Data: Our mapping is carried out on high- and low-incidence angle, ~166 m/pixel mosaics of MDIS images as well as on 222 m/pixel digital terrain models (DTMs) derived from photogrammetric analysis of MDIS-Near-Angle Camera stereo images. While four quadrangles of these DTMs have been released [7–9], we have since finished the remaining 11 quadrangles. Hillshade models and slopemaps derived from these DTMs are also used for feature identification and mapping to minimize any residual illumination bias (especially at certain longitudinal bands) that likely affected previous efforts [1,6] that lacked fully global hi-res topographic data. Methodology: We established a thorough step-by-step procedure to map and parametrize each identified shortening structure: Digitization at scale 1:1,000,000 using streaming method at constant vertex placement every 2 km. Midpoint coordinates and lengths will be automatically recorded. Check whether feature had been (entirely or partially) recognized by [1]. Interpretive assignment to a complex (or none). Nomenclature "[LobateScarp or WrinkleRidge]_Quadrangle name_[closest named feature or official scarp name]#", e.g., "WR_H-03_Brahms01" (quadrangle based on where majority of complex is). All compound scarps associated with a wrinkle ridge are mapped as scarp-like slope breaks but assigned to an according complex with prefix WR. Measure maximum width and width at point of maximum offset, i.e., extent of scarp backarc until it reassumes background elevation or distance between front- and backscarp. Measure maximum relief (in steps of 50 m, which is the vertical accuracy of the DTMs) Determine shape of throw (topographic trend along scarp or ridge crest; only for lengths >20 km). Orientations are determined in bulk after completion of a quadrangle. Will be done per entire line feature (avg. orientation) and per 2 km-vertex segment (weighted orientation). Lobateness (i.e., ratio between line connecting endpoints and actual feature length) will be determined in bulk after completion of a quadrangle. Segmentation (i.e., sum of gap length divided by sum of segment lengths) will be determined per assigned complex. Depth of faulting, displacement-length ratios (D/L), and horizontal shortening will be derived per line feature according to methods by[10–15] in bulk after completion of a quadrangle. Maximum values per complex will be recorded. D/L per complex (i.e., max. offset/entire complex length) will also be offered. Minimum model ages will be dereived for subset of shortening structures from crater size-frequency distribution measurements on underlying units, e.g., sufficiently large smooth crater floors. Initial assessments indicate 5 to 10 such locations per quadrangle. Applications: The geographic and temporal distribution of the structural parameters listed above will be used to test a variety of predictions made by several hypotheses, e.g., regarding global contraction, tidal despinning, thermal weakening, mascon subsidence, as well as lithologic/rheologic variations [1,2,6,15–21]. Figure 1: Portion of H-11 quadrangle showing the region between Matisse and Rude craters as preliminary sample area of our structural mapping project. A) Current state of our structural map superposed on low incidence MDIS mosaic. Compared to the linework in (B), we were able to trace the scarp more coherently, also identifying segments on the floor of Rude crater and new structural features elsewhere. The blue-green arrow points to the feature highlighted in (E). B) Same as (A) but with the map by [1]. C) New color-coded DTM over derived hillshade model. D) Same as (C) but using topography data by the USGS [5]. E) Attribute table of our scarp-like slope break linework (blue lines in (A)), including the highlighted one in blue-green (marked by arrow in (A)). Columns show parameters for each line feature. Orientation and lobateness will be derived in bulk at a later point using ArcToolbox plugins. Ages will only be determined for scarps whose formation age can be constrained with CSFD measurements, e.g., when they transect a smooth, sufficiently large crater floor. References: [1] P. K. Byrne et al., Nat. Geosci. 1 (2014). [2] K. T. Crane and C. Klimczak, Icarus 317, 66 (2019). [3] M. E. Banks et al., J. Geophys. Res. Planets 122, 1010 (2017). [4] M. E. Banks et al., J. Geophys. Res. Planets 120, 1751 (2015).[5] K. J. Becker et al., in LPSC (2016), p. 2959.[6] T. R. Watters, Commun. Earth Environ. 2, 9 (2021).[7] F. Preusker et al., Planet. Space Sci. 142, 26 (2017).[8] A. Stark et al., in LPSC (2017), p. 2287.[9] J. Oberst et al., in LPSC (2017), p. 1442.[10] T. R. Watters et al., Geophys. Res. Lett. 29, 1542 (2002).[11] I. Egea-González et al., Planet. Space Sci. 60, 193 (2012).[12] G. A. Peterson et al., in LPSC (2017), p. 2315.[13] S. F. Wojtal, J. Struct. Geol. 18, 265 (1996).[14] T. R. Watters, R. A. Schultz, and M. S. Robinson, Geophys. Res. Lett. 27, 3659 (2000).[15] P. K. Byrne, C. Klimczak, and A. M. Celâl Şengör, in Mercur. - View after MESSENGER (2018), pp. 249–286.[16] T. R. Watters et al., Geophys. Res. Lett. 42, 3755 (2015).[17] S. A. Hauck et al., in Mercur. - View after MESSENGER (2018), pp. 516–543.[18] R. G. Strom, N. J. Trask, and J. E. Guest, J. Geophys. Res. 80, 2478 (1975).[19] T. R. Watters et al., Geophys. Res. Lett. 31, L04701 (2004).[20] R. J. Phillips et al., in Mercur. - View after MESSENGER (2018), pp. 52–84.[21] T. R. Watters et al., Earth Planet. Sci. Lett. 285, 309 (2009).

International audience; We report the aeolian changes observed in situ by NASA's InSight lander during the first 400 sols of operations: Granule creep, saltation, dust removal, and the formation of dark surface tracks. Aeolian changes are infrequent and sporadic. However, on sols, when they do occur, they consistently appear between noon to 3 p.m., and are associated with the passage of convective vortices during periods of high vortex activity. Aeolian changes are more frequent at elevated locations, such as the top surfaces of rocks and lander footpads. InSight observed these changes using, for the first time, simultaneous insitu and orbital imaging and high-frequency meteorological, seismological, and magnetic measurements. Seismometer measurements of ground acceleration constrain the timing and trajectory of convective vortex encounters, linking surface changes to source vortices. Magnetometer measurements show perturbations in magnetic field strength during the passage of convective vortices consistent with chargedparticle motion. Detachment of sand-scale particles occurs when high background winds and vortexinduced turbulence provide a peak surface friction wind speed above the classic saltation fluid threshold. However, detachment of dust-and granule-scale particles also occurred when the surface friction wind speed remained below this threshold. This may be explained by local enhancement of the surface roughness and other effects described here and further studied in Part 2 (Baker et al., 2021). The lack of saltation and bright dust-coated surfaces at the InSight landing site implies surface stability and the onset of particle motion may be suppressed by dust "cushioning." This differentiates the InSight landing site from other areas on Mars that exhibit more aeolian activity. Plain Language Summary Aeolian activity, the movement of dust and sand by the wind, is common on Earth and has been observed on other planets, including Mars. A new Mars lander, InSight, has for the first time monitored aeolian changes by combining imaging with weather, seismic and CHARALAMBOUS ET AL.

Found on all terrestrial planets, wrinkle ridges are anticlines formed by thrust faulting and folding resulting from crustal shortening. The MErcury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft's orbital phase returned high resolution images and topographic data of the previously unimaged northern high latitudes of Mercury where there are large expanses of smooth plains deformed by wrinkle ridges. Concurrently, the Lunar Reconnaissance Orbiter (LRO) is obtaining high resolution images and topographic data covering lunar mare wrinkle ridges. These data allow quantitative comparison of the scale of wrinkle ridges in smooth plains volcanic units on Mercury with mare wrinkle ridges. We evaluate the topographic relief of 300 wrinkle ridges within and outside of mascon basins on the Moon and Mercury. Measured wrinkle ridges range from ~112 to 776 m in relief with a mean of ~350 m (median = ~340 m, n = 150) on Mercury and from ~47 to 678 m in relief with a mean of ~198 m (median = ~168 m, n = 150) on the Moon. Wrinkle ridges on Mercury thus are approximately twice as large in mean relief compared to their counterparts on the Moon. The larger scale of Mercury's wrinkle ridges suggests that their formation can be attributed, in part, to global contraction. As global contraction on the Moon is estimated to be an order of magnitude smaller than on Mercury, the smaller scale of lunar wrinkle ridges suggests they most likely form primarily by load induced subsidence of the mare basalt. Wrinkle ridges located in lunar mascon basins and in the Caloris mascon on Mercury are not statistically significantly different in relief than ridges in non-mascon regions, suggesting comparable levels of contractional strain. The fact that mascon basins do not host wrinkle ridges with greater structural relief relative to non-mascon units may indicate the critical role lithospheric thickness plays in controlling subsidence and contraction of thick volcanic sequences on the Moon and Mercury.

Nearside basin-related extensional tectonism on the Moon was thought to have ended by about 3.6 billion years ago and mare basin-localized contractional deformation ended by about 1.2 billion years ago. Lunar Reconnaissance Orbiter Camera (LROC) high resolution (50–200 cm/pixel) images show the Moon's surface in unprecedented detail and have enabled us to find many previously unidentified tectonic landforms, forcing a re-assessment of our views of tectonism in the maria. The morphology and stratigraphic relationships of these newly identified populations of tectonic landforms imply a more complex and longer-lasting history of deformation. We selected Mare Frigoris as an ideal location to perform a mapping survey where excellent imaging conditions, abundant tectonic landforms, and restricted mascon allow us to unravel the mare's tectonic evolution. Similar to previous surveys, we find that tectonism in the eastern portion of Mare Frigoris was controlled by ancient mascon induced flexure. In the western portion, however, we identify a parallel set of ancient compressional wrinkle ridges across the mare that is inconsistent with an origin by mare basin-centric mascon flexure or influence from the Mare Imbrium mascon. Instead, our results imply an ancient, regional, non-isotropic stress field over western Mare Frigoris. We also identify young wrinkle ridges and show that they have likely been active within the last 1 billion years, and some ridges as recently as within 40 million years. Finally, we identify a 300 km long series of lobate scarps coincident with one of the shallow moonquakes recorded during Apollo and use geodetic strain from the mapped global population of young lobate scarps to predict a level of seismicity consistent with the shallow moonquakes recorded during Apollo. In tandem with similarly young lobate scarps and small graben, as well as recorded shallow moonquakes, these young wrinkle ridges imply that some tectonism in and around Mare Frigoris has occurred in the geologically recent past and likely still continues today.

This study maps and measures assorted properties of new dated crater clusters that formed recently when impactors fragmented in the atmosphere of Mars. We report these statistics for 77 clusters: number of craters, size of cluster, dispersion of cluster, direction (azimuth) from which the impactor approached, and an estimate of the angle from vertical of the impact. Clusters range from a few to hundreds of craters, with most containing tens of craters. They are most commonly dispersed over hundreds of meters, with extents ranging from a few meters to a few kilometers. We find that dispersion generally does not correlate with topographic elevation. However, when the highest elevations are disregarded, clusters are more dispersed at lower elevations, as expected. Impact azimuths are randomly distributed and do not express a clear directionality of incoming meteoroids. Results suggest impacts occur closer to horizontal than expected, which could be due to observational effects. The characteristics we report here provide important constraints for future work in understanding atmospheric fragmentation processes; properties of the impactors themselves, such as density and orbital parameters; and the seismic detectability of impacts. These are critical aspects to understand, as approximately half of current impacts are observed to be clusters.

Change detection analyses of aeolian bedforms (dunes and ripples), using multitemporal images acquired by the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE), can reveal migration of bedforms on Mars. Here we investigated bedform mobility (evidence of wind-driven migration or activity), from analysis of HiRISE temporal image pairs, and dune field modification (i.e., apparent presence/lack of changes or degradation due to nonaeolian processes) through use of a dune stability index or SI (1–6; higher numbers indicating increasing evidence of stability/modification). Combining mobility data and SI for 70 dune fields south of 40°S latitude, we observed a clear trend of decreasing bedformmobility with increasing SI and latitude. Both dunes and ripples were more commonly active at lower latitudes, although some high-latitude ripples are migrating. Most dune fields with lower SIs (≤3) were found to be active while those with higher SIs were primarily found to be inactive. A shift in prevalence of active to apparently inactive bedforms and to dune fields with SI ≥ 2 occurs at ~60°S latitude, coincident with the edge of high concentrations ofH2O-equivalent hydrogen observed by the Mars Odyssey Neutron Spectrometer. This result is consistent with previous studies suggesting that stabilizing agents, such as ground ice, likely stabilize bedforms and limit sediment availability. Observations of active dune fields with morphologies indicative of stability (i.e., migrating ripples in SI = 3 dune fields) may have implications for episodic phases of reworking or dune building, and possibly geologically recent activation or stabilization corresponding to shifts in climate.

Mars Polar Science is an integrated, compelling system that serves as a nearby analogue to numerous other planets, supports human exploration, and habitability. Mars possesses the closest and most easily accessible layered ice deposits outside of Earth, and accessing those layers to read the climate record would be a triumph for planetary science.

Orbital and surface observations demonstrate that aeolian activity is occurring on Mars. Here we report the aeolian changes observed in situ by NASA's InSight lander during the first 400 sols of op...

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in western Elysium Planitia on November 26, 2018. Because of its stationary position and a multi-instrument package, InSight offers the unique opportunity of detecting changes induced by aeolian activity and constraining the atmospheric conditions responsible for particle motion.In this work, we present the most significant changes from aeolian activity as detected by the InSight lander during its first 400 Martian days of operations. We will show that particle entrainment by wind activity around InSight is a subtle process and report simultaneous measurements observed across multiple instruments. The changes observed are episodic and are seen correlated with excursions in both seismic and magnetic signals, which will be discussed further. Our observations show that all aeolian movements are consistent with the passage of deep convective vortices between noon to 3 pm local time. These vortices may be the primary initiators for aeolian transportation at InSight, inducing episodic particulate motion of grains up to 3 mm in diameter.

Mercury’s images obtained by the 1974 Mariner 10 flybys show extensive cratered landscapes degraded into vast knob fields, known as chaotic terrain (AKA hilly and lineated terrain). For nearly half a century, it was considered that these terrains formed due to catastrophic quakes and ejecta fallout produced by the antipodal Caloris basin impact. Here, we present the terrains’ first geologic examination based on higher spatial resolution MESSENGER (MErcury Surface Space ENvironment GEochemistry and Ranging) imagery and laser altimeter topography. Our surface age determinations indicate that their development persisted until ~1.8 Ga, or ~2 Gyrs after the Caloris basin formed. Furthermore, we identified multiple chaotic terrains with no antipodal impact basins; hence a new geological explanation is needed. Our examination of the Caloris basin’s antipodal chaotic terrain reveals multi-kilometer surface elevation losses and widespread landform retention, indicating an origin due to major, gradual collapse of a volatile-rich layer. Crater interior plains, possibly lavas, share the chaotic terrains’ age, suggesting a development associated with a geothermal disturbance above intrusive magma bodies, which best explains their regionality and the enormity of the apparent volume losses involved in their development. Furthermore, evidence of localized, surficial collapse, might reflect a complementary, and perhaps longer lasting, devolatilization history by solar heating.

The following dataset consists of the seismic waveforms created for studying impact crater clusters on Mars., We investigated the seismic signatures of recent impact crater clusters on Mars that would be recorded by the InSight Seismic Experiment for Interior Structure (SEIS) seismometers. We used a database of 77 measured and dated impact sites, with craters with effective diameters between 2.1 and 33.8 m, along with inferred impact angle, bolide trajectory, and varying target material properties to empirically scale for the momentum, expected seismic source function, and radiation pattern of impacts. The impact source is simulated in a local 3-D finite difference wave propagation code and coupled to teleseismic distances by scaling the spectra of 1-D global synthetic seismograms. We use the SEIS noise floors to estimate seismic detectability of impact(s) across azimuth and distance. Our experiments reveal that impact clusters have a higher peak corner frequency resulting from energy contributed by smaller craters to the power spectrum. We also find that the time separation between individual impacts in a cluster is small (< 10-15 milliseconds) and a require a seismometer closely situated to the source (< 10 km) and a high sampling rate (> 100 Hz) to resolve individual impacts within the cluster. Two of the clusters in our database (> 20 m effective diameter) would have been detectable by InSight, with the assumptions that the martian background noise and background attenuation are both low. Joint detection of surface changes from newly formed crater(s) in images and by SEIS will provide precise source locations that are crucial for constraining the internal structure of Mars.

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) spacecraft landed successfully on Mars and imaged the surface to characterize the surficial geology. Here we report on the geology and subsurface structure of the landing site to aid in situ geophysical investigations. InSight landed in a degraded impact crater in Elysium Planitia on a smooth sandy, granule- and pebble-rich surface with few rocks. Superposed impact craters are common and eolian bedforms are sparse. During landing, pulsed retrorockets modified the surface to reveal a near surface stratigraphy of surficial dust, over thin unconsolidated sand, underlain by a variable thickness duricrust, with poorly sorted, unconsolidated sand with rocks beneath. Impact, eolian, and mass wasting processes have dominantly modified the surface. Surface observations are consistent with expectations made from remote sensing data prior to landing indicating a surface composed of an impact-fragmented regolith overlying basaltic lava flows., The InSight spacecraft landed on Mars on November 2018. Here, the authors characterize the surficial geology of the landing site and compare with observations and models derived from remote sensing data prior to landing and from ongoing in situ geophysical investigations of the subsurface.

Previous studies have estimated the ages of lunar lobate scarps, some of the youngest tectonic landforms on the Moon, based on the estimated life-times of their fresh morphologies and associated small graben, using crater degradation ages, or via buffered and traditional crater size-frequency distribution (CSFD) measurements. Here, we reexamine five scarps previously dated by Binder and Gunga (1985) with crater degradation ages to benchmark the evaluation of both the buffered and traditional CSFD approaches for determination of absolute model ages (AMAs) at scarps. Both CSFD methods yield similar ages for each individual scarp, indicating that fault activity not only can be measured on the scarp itself, but also in the surrounding terrain – an indication that tectonic activity causes surface renewal both adjacent to and even kilometers distant from scarps. Size-frequency variations in the regions surrounding the scarps are thus useful for studying the extent and severity of the ground motion caused by coseismic slip events during scarp formation. All age determination approaches continue to indicate that lunar lobate scarps were active in the late Copernican, with some scarps possibly experiencing activity within the last 100 Ma.

Aeolian processes have likely been the predominant geomorphic agent for most of Mars' history and have the potential to produce relatively young exposure ages for geologic units. Thus, identifying local evidence for aeolian erosion is highly relevant to the selection of landing sites for future missions, such as the Mars 2020 Rover mission that aims to explore astrobiologically relevant ancient environments. Here we investigate wind-driven activity at eight Mars 2020 candidate-landing sites to constrain erosion potential at these locations. To demonstrate our methods, we found that contemporary dune-derived abrasion rates were in agreement with rover-derived exhumation rates at Gale crater and could be employed elsewhere. The Holden crater candidate site was interpreted to have low contemporary erosion rates, based on the presence of a thick sand coverage of static ripples. Active ripples at the Eberswalde and southwest Melas sites may account for local erosion and the dearth of small craters. Moderate-flux regional dunes near Mawrth Vallis were deemed unrepresentative of the candidate site, which is interpreted to currently be experiencing low levels of erosion. The Nili Fossae site displayed the most unambiguous evidence for local sand transport and erosion, likely yielding relatively young exposure ages. The downselected Jezero crater and northeast Syrtis sites had high-flux neighboring dunes and exhibited substantial evidence for sediment pathways across their ellipses. Both sites had relatively high estimated abrasion rates, which would yield young exposure ages. The downselected Columbia Hills site lacked evidence for sand movement, and contemporary local erosion rates are estimated to be relatively low.

Following an approach similar to that used for the Moon, Mercury's surface units were subdivided into five time-stratigraphic systems based on geologic mapping using Mariner 10 images. The absolute time scale originally suggested for the time periods associated with these systems was based on the assumption that the lunar impact flux history applied to Mercury. However, we find that the duration and onset of corresponding periods in the stratigraphic sequences on Mercury and the Moon are not the same. Using high-resolution and multiband image data obtained by the MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) spacecraft, we identify and catalog fresh impact craters interpreted to have formed during Mercury's two most recent periods, the Mansurian and Kuiperian. We use the densities of the inferred Kuiperian- and Mansurian-aged crater populations to estimate new limits for the age boundaries of these time intervals. Results suggest that both the Mansurian and Kuiperian periods began more recently and extended for significantly shorter durations of time than previously suggested. The Kuiperian is estimated to have initiated as recently as approximately 280 plus or minus 60 Ma (million years) and the Mansurian as recently as approximately 1.7 plus or minus 0.2 Ga (billion years).

The Kuiper Belt is a distant region of the Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU69, a Cold Classical Kuiper Belt Object, a class of objects that have never been heated by the Sun and are therefore well preserved since their formation. Here we describe initial results from these encounter observations. MU69 is a bi-lobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color and compositional heterogeneity. No evidence for satellites, ring or dust structures, gas coma, or solar wind interactions was detected. By origin MU69 appears consistent with pebble cloud collapse followed by a low velocity merger of its two lobes., 43 pages, 8 figure

The Martian outflow channels comprise some of the largest known channels in the Solar System. Remote-sensing investigations indicate that cataclysmic floods likely excavated the channels ~3.4 Ga. Previous studies show that, in the southern circum-Chryse region, their flooding pathways include hundreds of kilometers of channel floors with upward gradients. However, the impact of the reversed channel-floor topography on the cataclysmic floods remains uncertain. Here, we show that these channel floors occur within a vast basin, which separates the downstream reaches of numerous outflow channels from the northern plains. Consequently, floods propagating through these channels must have ponded, producing an inland sea, before reaching the northern plains as enormous spillover discharges. The resulting paleohydrological reconstruction reinterprets the 1997 Pathfinder landing site as part of a marine spillway, which connected the inland sea to a hypothesized northern plains ocean. Our flood simulation shows that the presence of the sea would have permitted the propagation of low-depth floods beyond the areas of reversed channel-floor topography. These results explain the formation at the landing site of possible fluvial features indicative of flow depths at least an order of magnitude lower than those apparent from the analyses of orbital remote-sensing observations.

Impact investigations will be an important aspect of the InSight mission. One of the scientific goals of the mission is a measurement of the current impact rate at Mars. Impacts will additionally inform the major goal of investigating the interior structure of Mars. In this paper, we review the current state of knowledge about seismic signals from impacts on the Earth, Moon, and laboratory experiments. We describe the generalized physical models that can be used to explain these signals. A discussion of the appropriate source time function for impacts is presented, along with spectral characteristics including the cutoff frequency and its dependence on impact momentum. Estimates of the seismic efficiency (ratio between seismic and impact energies) vary widely. Our preferred value for the seismic efficiency at Mars is $5 \times 10^{- 4}$ , which we recommend using until we can measure it during the InSight mission, when seismic moments are not used directly. Effects of the material properties at the impact point and at the seismometer location are considered. We also discuss the processes by which airbursts and acoustic waves emanate from bolides, and the feasibility of detecting such signals. We then consider the case of impacts on Mars. A review is given of the current knowledge of present-day cratering on Mars: the current impact rate, characteristics of those impactors such as velocity and directions, and the morphologies of the craters those impactors create. Several methods of scaling crater size to impact energy are presented. The Martian atmosphere, although thin, will cause fragmentation of impactors, with implications for the resulting seismic signals. We also benchmark several different seismic modeling codes to be used in analysis of impact detections, and those codes are used to explore the seismic amplitude of impact-induced signals as a function of distance from the impact site. We predict a measurement of the current impact flux will be possible within the timeframe of the prime mission (one Mars year) with the detection of ∼ a few to several tens of impacts. However, the error bars on these predictions are large. Specific to the InSight mission, we list discriminators of seismic signals from impacts that will be used to distinguish them from marsquakes. We describe the role of the InSight Impacts Science Theme Group during mission operations, including a plan for possible night-time meteor imaging. The impacts detected by these methods during the InSight mission will be used to improve interior structure models, measure the seismic efficiency, and calculate the size frequency distribution of current impacts.

The planet Mercury has contracted over its history. The identification of small thrust fault scarps suggests the occurrence of tectonic activity on Mercury within the past 50 million years and thus a slow-cooling planetary interior. Large tectonic landforms on the surface of Mercury, consistent with significant contraction of the planet, were revealed by the flybys of Mariner 10 in the mid-1970s1. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission confirmed that the planet’s past 4 billion years of tectonic history have been dominated by contraction expressed by lobate fault scarps that are hundreds of kilometres long2,3,4,5. Here we report the discovery of small thrust fault scarps in images from the low-altitude campaign at the end of the MESSENGER mission that are orders of magnitude smaller than the large-scale lobate scarps. These small scarps have tens of metres of relief, are only kilometres in length and are comparable in scale to small young scarps on the Moon6,7,8. Their small-scale, pristine appearance, crosscutting of impact craters and association with small graben all indicate an age of less than 50 Myr. We propose that these scarps are the smallest members of a continuum in scale of thrust fault scarps on Mercury. The young age of the small scarps, along with evidence for recent activity on large-scale scarps, suggests that Mercury is tectonically active today and implies a prolonged slow cooling of the planet’s interior.

Crater size–frequency analyses have shown that the largest volcanic plains deposits on Mercury were emplaced around 3.7 Ga, as determined with recent model production function chronologies for impact crater formation on that planet. To test the hypothesis that all major smooth plains on Mercury were emplaced by about that time, we determined crater size–frequency distributions for the nine next-largest deposits, which we interpret also as volcanic. Our crater density measurements are consistent with those of the largest areas of smooth plains on the planet. Model ages based on recent crater production rate estimates for Mercury imply that the main phase of plains volcanism on Mercury had ended by ~3.5 Ga, with only small-scale volcanism enduring beyond that time. Cessation of widespread effusive volcanism is attributable to interior cooling and contraction of the innermost planet.

At the time before ∼3.5 Ga that life originated and began to spread on Earth, Mars was a wetter and more geologically dynamic planet than it is today. The Argyre basin, in the southern cratered highlands of Mars, formed from a giant impact at ∼3.93 Ga, which generated an enormous basin approximately 1800 km in diameter. The early post-impact environment of the Argyre basin possibly contained many of the ingredients that are thought to be necessary for life: abundant and long-lived liquid water, biogenic elements, and energy sources, all of which would have supported a regional environment favorable for the origin and the persistence of life. We discuss the astrobiological significance of some landscape features and terrain types in the Argyre region that are promising and accessible sites for astrobiological exploration. These include (i) deposits related to the hydrothermal activity associated with the Argyre impact event, subsequent impacts, and those associated with the migration of heated water along Argyre-induced basement structures; (ii) constructs along the floor of the basin that could mark venting of volatiles, possibly related to the development of mud volcanoes; (iii) features interpreted as ice-cored mounds (open-system pingos), whose origin and development could be the result of deeply seated groundwater upwelling to the surface; (iv) sedimentary deposits related to the formation of glaciers along the basin's margins, such as evidenced by the ridges interpreted to be eskers on the basin floor; (v) sedimentary deposits related to the formation of lakes in both the primary Argyre basin and other smaller impact-derived basins along the margin, including those in the highly degraded rim materials; and (vi) crater-wall gullies, whose morphology points to a structural origin and discharge of (wet) flows.