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As sand moves across Earth's landscapes, the shapes of individual grains evolve, and microscopic textures accumulate on their surfaces. Because transport processes vary between environments, the shape and suite of microtextures etched on sand grains provide insights into their transport histories. For example, previous efforts to link microtextures to transport environments have demonstrated that they can provide important information about the depositional environments of rocks with few other indicators. However, such analyses rely on 1) subjective human description of microtextures, which can yield biased, error-prone results; 2) nonstandard lists of microtextures; and 3) relatively large sample sizes (>20 grains) to obtain reliable results, the manual documentation of which is extremely labor intensive. These drawbacks have hindered broad adoption of the technique. We address these limitations by developing a deep neural network model, SandAI, that classifies scanning electron microscope images of modern sand grains by transport environment with high accuracy. The SandAI model was developed using images of sand grains from modern environments around the globe. Training data encompass the four most common terrestrial environments: fluvial, eolian, glacial, and beach. We validate the model on quartz grains from modern sites unknown to it, and Jurassic-Pliocene sandstones of known depositional environments. Next, the model is applied to two samples of the Cryogenian Bråvika Member (of contested origin), yielding insights into periglacial systems associated with Snowball Earth. Our results demonstrate the robustness and versatility of the model in quickly and automatically constraining the transport histories recorded in individual grains of quartz sand. [ABSTRACT FROM AUTHOR]

Dust dynamics influence planetary atmospheres. However, the settling velocity of dust—and thus its residence time in the atmosphere—is often mispredicted. Challenging, indirect experiments involving few ideal particles revealed that dust settling velocity deviates from Stokes' law under rarefied atmospheres. While useful, such experiments are inadequate to simulate more complex scenarios, including variable particles sizes and shapes. Here, we present direct measurements of settling velocity for spherical particles under Earth‐to‐Mars atmospheric pressures using time‐resolved particle image velocimetry (TR‐PIV), and validate their robustness with existing models. Our results demonstrate that TR‐PIV provides a relatively simple approach to quantifying dust settling velocity from direct observations of over 10,000 particles, enabling systematic investigations of dust settling under realistic scenarios. Such experiments will have significant implications for our understanding of Mars' past, present, and future ‐ from providing a tool to decipher its sedimentary record to enhancing predictive capabilities of atmospheric models. Plain Language Summary: Airborne dust strongly affects the environments and atmospheres of Earth and Mars. Knowing the speed at which dust settles is critical because it controls the time dust stays in the atmosphere. However, it is a difficult quantity to measure, and as a result, it is often poorly predicted by existing models. These models were built from experiments that did not match real‐world conditions and typically only considered few, idealized particles. Here, we propose a relatively simple approach to make direct measurements of dust settling under Earth‐to‐Mars atmospheric pressures. This technique, time‐resolved particle image velocimetry, allows us to directly measure the speed of over 10,000 particles. Using ideal particles, we show that our measurements align well with results from previously published models, validating the robustness of our procedure. The same experimental setup can be used to investigate dust settling under more realistic conditions, for example, varying the size distribution of dust particles, their shape, or dust concentration. Such measurements are a stepping stone toward accurate interpretations and predictions of dust dynamics on Earth and Mars. Key Points: Dust settling velocity controls residence time of dust in the atmosphere but is challenging to measure under low atmospheric densityWe measure dust settling velocity under Earth‐to‐Mars‐like conditions using time‐resolved particle image velocimetry (TR‐PIV)TR‐PIV enables systematic studies of dust settling on Mars and other bodies using realistic particles and varying dust concentrations [ABSTRACT FROM AUTHOR]

Candidate glaciovolcanic landforms have been identified across Mars, suggesting that volcano‐ice interactions may have been relatively widespread in areas that once contained extensive surface and near‐surface ice deposits. To better constrain the detection of glaciovolcanism in Mars' geological record, this study has investigated and characterized the petrology, geochemistry, and mineralogy of three intraglacial volcanoes and an interglacial volcano in the Þórisjökull area of southwest Iceland. Our results show that glaciovolcanism creates abundant, variably altered hyaloclastite and hyalotuff that is sufficiently geochemically and mineralogically distinctive from subaerially erupted lava for identification using instruments available on Mars rovers and landers. Due to the lower gravity and atmospheric pressure at the surface of Mars, hyaloclastite and hyalotuff are also more likely to form in greater abundance in Martian glaciovolcanoes. Our results support that magmatism following deglaciation likely triggers decompression melting of the shallow mantle beneath Iceland, creating systematic changes in geochemistry and mineralogy. Glaciation can also suppress magmatism at its peak, encouraging the formation of shallow fractionated magma chambers. As such, it is possible for the crustal loading of an ice cap to enhance igneous diversity on a planet without plate tectonism, creating glass‐rich, altered, and mineralogically diverse deposits such as those discovered in Gale crater by the Curiosity rover. However, as the eroded products of glaciovolcanism are similar to those formed through hydrovolcanism, the presence of a glaciovolcanic landform at the source is required to confirm whether volcano‐ice interactions occurred at the sediment source. Plain Language Summary: The ancient Martian climate has been long debated with scientists hypothesizing that the river channels and lake beds preserved on the surface today formed in either a warm and wet environment or a cold and icy environment, with heating events driving large‐scale melting. One such heating event includes volcanism. This study investigated the chemical and mineralogical impact of volcano‐ice interactions in Iceland using techniques available to Mars rovers and landers to improve the detection of these deposits on Mars. Our results show that deposits rich in glass and the early products of low temperature alteration (palagonite, clays, zeolites) are likely to be more prevalent on Mars compared to the Earth due to differences in gravity between the two planets. Furthermore, the presence of a thick ice cap on Mars may encourage a diversity of lava compositions to form in shallow magma chambers as the weight of a thick ice cap can suppress volcanism. These types of deposits are identifiable using Mars rover and lander techniques, but due to their similarities to other volcanic units that may have erupted in the presence of water, a candidate glaciovolcanic landform is needed to confirm that these deposits are from volcano‐ice interactions. Key Points: Glaciovolcanism creates large deposits of altered fragmented volcanic glass distinct from subaerial volcanismGlaciation can suppress magmatism, driving geochemical and mineralogical diversity in an area once deglaciation startsGlaciovolcanism may have occurred on Mars with explosive phreatomagmatic units more likely under Mars' planetary conditions [ABSTRACT FROM AUTHOR]

The recognition of meandering rivers in Earth's prevegetation stratigraphic record is hindered by an absence of depositional models specifically tailored to unvegetated single‐channel rivers. As a result, their abundance on the early Earth is currently unknown. Here, sedimentological studies of two modern meandering river reaches with unvegetated banks were conducted. Both reaches are located in arid basins of Nevada, USA. Stratigraphy was analysed in conjunction with satellite imagery and high‐resolution topographic data to understand how channels and floodplains record past flow conditions. Analyses of point‐bar and channel‐fill deposits showed that lateral accretion sets are not heterolithic but, rather, composed of mixed sand and sand‐sized mud aggregates (clumps of clay and silt) interpreted to have formed through flocculation. Levées are present but subdued. Notably, downstream‐migrating outer‐bank‐attached bars are prominent architectural features that may be common in unvegetated meandering rivers in arid landscapes elsewhere. The identification of such depositional elements in the stratigraphic record may enhance recognition of ancient unvegetated meandering rivers and improve reconstructions of palaeohydrology on early Earth and Mars. [ABSTRACT FROM AUTHOR]

Boulders form from a variety of geological processes, which their size, shape, and orientation may help us better understand. Furthermore, they represent potential hazards to spacecraft landing that need to be characterized. However, mapping individual boulders across vast areas is extremely labor‐intensive, often limiting the extent over which they are characterized and the statistical robustness of obtained boulder morphometrics. To automate boulder characterization, we use an instance segmentation neural network, Mask R‐CNN, to detect and outline boulders in high‐resolution satellite and aerial images. Our neural network, BoulderNet, was trained from a data set of >33,000 boulders in >750 image tiles from Earth, the Moon, and Mars. BoulderNet not only correctly detects the majority of boulders in images but also identifies the outline of boulders with high fidelity, achieving average precision and recall values of 72% and 64% relative to manually digitized boulders from the test data set, when only detections with intersection‐over‐union ratios >50% are considered valid. These values are similar to those obtained from human mappers. On Earth, equivalent boulder diameters, aspect ratios, and orientations extracted from predictions were benchmarked against ground measurements and yield values within ±15%, ±0.20, and ±20° of their ground‐truth values, respectively. BoulderNet achieves better boulder detection and characterization performance relative to existing methods, providing a versatile open‐source tool to characterize entire boulder fields on planetary surfaces. Plain Language Summary: Boulders are one of the most abundant features on the surfaces of solid planetary bodies. Measuring their size, shape, and orientation can tell us about how they formed as well as help select landing sites that minimize hazard to spacecraft. However, mapping boulders across large areas is a labor‐intensive task that often limits the scope and robustness of boulder studies. To overcome this challenge, we trained a machine‐learning algorithm to automatically outline boulders on a variety of planetary surfaces using a database of over 30,000 boulders manually mapped from aerial or satellite images of Earth, the Moon, and Mars. Our algorithm, BoulderNet, performs as well as human mappers and outperforms existing automated tools. BoulderNet is made available to the community. Key Points: We describe BoulderNet, a Mask R‐CNN instance segmentation model trained to characterize boulders on the surfaces of solid planetary bodiesBoulderNet achieves robust performance on Earth, the Moon, and Mars, outlining boulders with high fidelityBoulderNet achieves better boulder detection and morphometric characterization performances than other existing methods [ABSTRACT FROM AUTHOR]

Windblown dunes are common features in our solar system, forming on planetary surfaces that span wide ranges in gravity and both atmospheric and sediment properties. The patterns formed by their crests, which are readily visible from orbital images, can record information about recent changes in boundary conditions, such as shifts in wind regime or varying sediment availability. Here, we demonstrate that the density of dune interactions (where neighboring crestlines are close to each other) within a dune field is an indicator of such changes. Using orbiter-based images of 46 dune fields on Earth and Mars, we compiled a database of pattern parameters including dune spacing, crestline orientation, and interaction density. Combined with sediment fluxes derived from ERA5-Land data and a martian global circulation model, we also compiled dune turnover time scales (the time it takes for a dune to migrate one dune length) for each investigated dune field. First, we show that dune fields undergoing changes in boundary conditions display higher than expected dimensionless interaction indices. Second, dune fields with longer turnover times display a wider range in interaction indices on both Earth and Mars because they are more likely to be observed while still adjusting to recent changes in boundary conditions. Thus, a dune field's interaction index offers a novel tool to detect and possibly quantify recent environmental change on planetary surfaces. [ABSTRACT FROM AUTHOR]

The lateral migration of a river meander is driven by erosion on the outer bank and deposition on the inner bank, both of which are affected by shear stress (and therefore channel slope) through complex morphodynamic feedbacks. To test the sensitivity of lateral migration to channel slope, we quantify slope change induced by glacial isostatic adjustment along the Red River (North Dakota, USA and Manitoba, Canada) and two of its tributaries over the past 8.5 ka. We demonstrate a statistically significant, positive relationship between normalized cutoff count, which we interpret as a proxy for channel lateral migration rate, and slope change. We interpret this relationship as the signature of slope change modulating the magnitude of shear stress on riverbanks, suggesting that slope changes that occur over thousands of years are recorded in river floodplain morphology. Plain Language Summary: Rivers move through the landscape by eroding river bank material on their outer bank and depositing sediment on their inner bank, a process that forms meander bends. Understanding what factors drive river meandering is important for interpreting how rivers interact with landscapes. One factor that could impact river meandering is river slope. To understand the impact of slope on river meandering we quantify how slope has changed along the Red River (North Dakota, USA and Manitoba, Canada) over the past 8.5 Kyr. Over this time, vertical land movement substantially reduced the slope of the river, through the ongoing solid Earth response to the retreat of massive North American ice sheets in a process known as glacial isostatic adjustment (GIA). We find that change in slope, induced by GIA, positively correlates with river migration rate along the Red River, suggesting that slope plays an important role in determining the pace of river meandering. Key Points: Glacial isostatic adjustment (GIA) is the primary control on slope change for the Red River (ND, USA and MB, Canada) since it began to flow 8.5 kaSlope change caused by GIA significantly correlates with river cutoff frequency, a proxy for lateral migration rateWe infer that slope change modulates the magnitude of shear stress on the riverbank, driving changes in lateral migration rate [ABSTRACT FROM AUTHOR]

Titan is a sedimentary world, with lakes, rivers, canyons, fans, dissected plateaux, and sand dunes. Sediments on Saturn's moon are thought to largely consist of mechanically weak organic grains, prone to rapid abrasion into dust. Yet, Titan's equatorial dunes have likely been active for 10s–100s kyr. Sustaining Titan's dunes over geologic timescales requires a mechanism that produces sand‐sized particles at equatorial latitudes. We explore the hypothesis that a combination of abrasion, when grains are transported by winds or methane rivers, and sintering, when they are at rest, could produce sand grains that maintain an equilibrium size. Our model demonstrates that seasonal sediment transport may produce sand under Titan's surface conditions and could explain the latitudinal zonation of Titan's landscapes. Our findings support the hypothesis of global, source‐to‐sink sedimentary pathways on Titan, driven by seasons, and mediated by episodic abrasion and sintering of organic sand by rivers and winds. Plain Language Summary: Like Earth, Saturn's moon Titan hosts lakes, rivers, canyons, fans, eroded plateaux, and sand dunes. On Titan, loose solid particles (or sediments) are likely made of soft hydrocarbon grains, prone to rapid breakdown into dust. Yet, Titan's equatorial dunes have been active for up to several hundreds of thousands of years, suggesting that some mechanism must produce sand‐sized particles at these latitudes. We explore the hypothesis that a combination of abrasion, when grains are transported by winds or methane rivers, and sintering, when they are at rest, could produce sand grains that maintain an equilibrium size. Our model demonstrates that seasonal sediment transport could produce sand on Titan and could explain the distribution of Titan's landscapes. Altogether, our findings support the hypothesis of global sedimentary pathways on Titan, driven by seasons, and mediated by episodic abrasion and sintering of organic sand by rivers and winds. Key Points: Long‐lived active dune fields are in apparent contradiction with the predicted rapid abrasion of windblown organic sediment on TitanEpisodic abrasion and sintering of organic sediment, driven by seasons, could generate sand with equilibrium sizes on TitanTitan's undifferentiated plains and labyrinth terrains could result from prolonged sintering and diagenesis where transport is infrequent [ABSTRACT FROM AUTHOR]

Two scales of ripples form in fine sand on Mars. The larger ripples were proposed to have an equilibrium size set by an aerodynamic process, making them larger under thinner atmospheres and distinct from smaller impact ripples. Sullivan et al. (2020, https://doi.org/10.1029/2020JE006485) show that large ripples can develop in a numerical model due to Mars' low atmospheric pressure. Although their proposed growth‐limiting mechanism is consistent with an aerodynamic process, they argue that the ripples in their model are simply large versions of impact ripples, not a separate class of ripples. Here, we explore this debate by synthesizing recent advances in large‐ripple formation (including initiation and subsequent evolution to equilibrium). Although significant knowledge gaps remain, it is clear that large Martian ripples in well sorted sand are larger under thinner atmospheres, and thus remain a powerful paleoclimate indicator. Plain Language Summary: Earth's sandy deserts host small ripples and large dunes, but Mars' dune fields also host a third type of sedimentary pattern—large ripples with meter‐scale spacings. The large ripples were previously proposed to be distinct from small Earth‐like ripples, and their size was suggested to result from the low atmospheric pressure on Mars. In contrast, Sullivan et al. (2020, https://doi.org/10.1029/2020JE006485) show that, in a numerical model, small ripples can grow large on Mars. Here, we explore this debate by synthesizing recent advances in large‐ripple formation. While significant knowledge gaps remain, it is clear that large Martian ripples in well sorted sand are larger under thinner atmospheres, and thus remain a powerful tool to probe Mars' ancient climate. Key Points: A new model predicts small windblown ripples should grow to be meter‐scale on MarsThat model cannot explain some observations that appear to be more consistent with an aerodynamic origin for large Martian ripplesAnswering remaining questions on large‐ripple formation will require new observations, both in situ and from wind‐tunnel experiments [ABSTRACT FROM AUTHOR]

Flume experiments and field observations show that bank vegetation promotes the formation of narrow and deep single‐thread channels by strengthening riverbanks. Consistent with this idea, the pre‐Silurian fluvial record generally consists of wide monotonous sand bodies often interpreted as deposits of shallow braided rivers, whereas single‐thread rivers with muddy floodplains become more recognizable in Silurian and younger rocks. This shift in the architecture of fluvial deposits has been interpreted as reflecting the rise of single‐thread rivers enabled by plant life. The deposits of some single‐thread rivers, however, have been recognized in pre‐Silurian rocks, and recent field studies have identified meandering rivers in modern unvegetated environments. Furthermore, single‐thread‐river deposits have been identified on Mars, where macroscopic plants most likely never evolved. Here we seek to understand the formation of those rarely recognized and poorly characterized single‐thread rivers in unvegetated landscapes. Specifically, we quantitatively explore the hypothesis that cohesive muddy banks alone may enable the formation of single‐thread rivers in the absence of plants. We combine open‐channel hydraulics and a physics‐based erosion model applicable to a variety of bank sediments to predict the formation of unvegetated single‐thread rivers. Consistent with recent flume experiments and field observations, results indicate that single‐thread rivers may form readily within muddy banks. Our model has direct implications for the quantification of riverbank strengthening by vegetation, understanding the hydraulic geometry of modern and ancient unvegetated rivers, interpreting pre‐Silurian fluvial deposits, and unraveling the hydrologic and climate history of Mars. Plain Language Summary: Plants tend to strengthen riverbanks, favoring the formation of single‐thread rivers (i.e., relatively deep flows within a single channel) over that of braided rivers (i.e., relatively shallow flows distributed among several interlaced channels). In parallel, geologists have observed a shift in the structure of river deposits coincident with the evolution of land plants, commonly interpreted as the signature of the rise of single‐thread rivers, sparked by plant life. However, recent studies have identified single‐thread‐river deposits in both modern unvegetated environments and in rocks that predate the greening of the continents, and deposits of ancient single‐thread rivers have also been identified on Mars, where large plants realistically never evolved. Thus, single‐thread rivers can clearly form in vegetation‐free environments; here we seek to understand how. Employing a conceptual model based on the mechanics of river flow and bank erosion, we show that sticky mud may strengthen riverbanks enough to resist erosion and prevent river braiding, suggesting that mud alone could have enabled the formation of single‐thread rivers on ancient Earth and Mars. The model may help to quantify plant‐driven riverbank strength, understand river geometry in barren landscapes, interpret ancient river deposits on Earth, and, possibly, decipher the climate history of Mars. Key Points: A simple mechanistic model is developed to explore the formation of single‐thread rivers in a broad range of unvegetated substratesResults show single‐thread rivers may form in barren muddy banks, consistent with experiments, ancient deposits, and modern riversThe model offers a new framework to interpret the ancient record of single‐thread rivers on Earth and Mars [ABSTRACT FROM AUTHOR]

Biotic forcing on river meandering is a highly debated topic in sedimentology. Vegetation is assumed to hold a vital role on channel stability and sinuosity, for example through bank stabilization and pedogenic production of cohesive clays. However, statistically solid and causal relationships between vegetation density and river sinuosity remain largely untested in natural systems. This study investigates physical and biotic forcings on channel sinuosity in the Bonneville Basin of Utah (USA), an endorheic depression flanked by active fluvial networks ('washes') that display diverse vegetation density and channel‐planform style. By means of remote sensing and ground‐data collection, 58 washes are considered, 0·1 to 90 km2 in surface area and drained by trunk channels <45 m wide and <1·2 m deep. Each wash is composed of a catchment basin connected downstream to an aggradational and distributive channel network. Statistically solid regressions highlight the primary roles played by base level and catchment size on fluvial morphogenesis. In contrast, no correlation is found between vegetation density and other parameters such as trunk‐channel width or surface area of the largest meander in a wash. Similarly, no statistical correlation exists between vegetation density and meander size or sinuosity index. Rather, larger and more sinuous meanders are invariably associated with lower vegetation density. These results are corroborated by field evidence showing that sparse vegetation promotes flow disturbance, channel branching and bar braiding instead of stabilizing sediment surfaces. Thus, river meandering is attributed to cohesion offered by mud retention within the endorheic basin, as well as discharge and stream‐power modulation along bifurcating and low‐gradient channel reaches. Hence, this work demonstrates how meandering‐channel patterns may arise from entirely physical forcings in the absence of vegetation. [ABSTRACT FROM AUTHOR]

Loose sand, blown away from source regions by winds, is transported across Mars's surface into sand sheets and dunes and accumulates within topographic sinks. In the absence of plate tectonics, impact craters constitute a dominant sink for windblown sediments on Mars today. We analyzed the volume of all mapped eolian sands in martian craters >1 km in diameter to reveal spatiotemporal patterns of sediment accumulation on the planet's surface. We combined our results with global climate simulations, maps of depth to the ice table and dust cover, as well as lithologic and age information of the underlying geologic units, to better understand the controls on intracrater sand accumulation rates. We find that crater age, latitude, and lithology influence the accumulation rate of windblown sand and, notably, that it is enhanced in mechanically weaker substrates, high-latitude craters (suggesting that modern cryogenic processes may enhance sand production), and in Late Noachian and Early Hesperian craters (possibly hinting at increased erosion rates at that time). [ABSTRACT FROM AUTHOR]