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Meandering rivers are diagnostic landforms of hydrologically active planets, and their migration regulates the continental component of biogeochemical cycles that stabilize climate and allow for life on Earth. The rise of river meanders on Earth has been linked to riverbank stabilization driven by the Palaeozoic evolution of plant life about 440 million years ago. Here we provide a fundamental test for this hypothesis using a global analysis of active meander migrations that includes previously ignored unvegetated rivers from the arid interiors of modern continents. When normalized by channel size, unvegetated meanders universally migrate an order of magnitude faster than vegetated ones. While providing irrefutable evidence that vegetation is not required for meander formation, we demonstrate how profoundly vegetation transformed the pace of change for Earth’s landscapes, and we at last offer a mechanistic explanation for the radically distinct stratigraphic records of barren and vegetated rivers. We posit that the migration slowdown driven by the rise of land plants dramatically impacted biogeochemical fluxes and rendered Earth’s landscapes even more hospitable to life. Therefore, the tenfold faster migration of unvegetated rivers may be key to deciphering the environments of barren worlds such as early Earth and Mars.

Stabilization of riverbanks by vegetation has long been considered necessary to sustain single-thread meandering rivers. However, observation of active meandering in modern barren landscapes challenges this assumption. Here, we investigate a globally distributed set of modern meandering rivers with varying riparian vegetation densities, using satellite imagery and statistical analyses of meander-form descriptors and migration rates. We show that vegetation enhances the coefficient of proportionality between channel curvature and migration rates at low curvatures, and that this effect wanes in curvier channels irrespective of vegetation density. By stabilizing low-curvature reaches and allowing meanders to gain sinuosity as channels migrate laterally, vegetation quantifiably affects river morphodynamics. Any causality between denser vegetation and higher meander sinuosity, however, cannot be inferred owing to more frequent avulsions in modern non-vegetated environments. By illustrating how vegetation affects channel mobility and floodplain reworking, our findings have implications for assessing carbon stocks and fluxes in river floodplains. Riparian vegetation densities critically mediate the morphodynamics of meandering rivers: plants slow the rate at which channels move laterally and reinforce the key, first-order control that curvature exerts on meander planform evolution.

Sand mobilized by wind forms decimeter-scale impact ripples and decameter-scale or larger dunes on Earth and Mars. In addition to those two bedform scales, orbital and in situ images revealed a third distinct class of larger meter-scale ripples on Mars. Since their discovery, two main hypotheses have been proposed to explain the formation of large martian ripples—that they originate from the growth in wavelength and height of decimeter-scale ripples or that they arise from the same hydrodynamic instability as windblown dunes or subaqueous bedforms instead. Here we provide evidence that large martian ripples form from the same hydrodynamic instability as windblown dunes and subaqueous ripples. Using an artificial neural network, we characterize the morphometrics of over a million isolated barchan dunes on Mars and analyze how their size and shape vary across Mars’ surface. We find that the size of Mars’ smallest dunes decreases with increasing atmospheric density with a power-law exponent predicted by hydrodynamic theory, similarly to meter-size ripples, tightly bounding a forbidden range in bedform sizes. Our results provide key evidence for a unifying model for the formation of subaqueous and windblown bedforms on planetary surfaces, offering a new quantitative tool to decipher Mars’ atmospheric evolution. Dust storms on Mars drive water escape to space. Here, the authors show the impact Martian dust storms have on the abundance of atmospheric hydrogen and oxygen, and how this helps to overall oxidize the Martian atmosphere.

Sand ripples record interactions between planetary surfaces and environmental flows, providing paleoenvironmental archives when preserved into rocks. Two main ripple types form in sand: drag ripples, common in water, and impact ripples, exclusive to windblown surfaces. Enigmatic meter-scale aeolian ripples on Mars have been assumed to be impact ripples, though ground and orbiter-based observations suggest they may be drag ripples instead. Here, we report on low-pressure wind tunnel experiments in which large ripples formed and evolved from a flat bed. Observations demonstrate that impact and large ripples grow from distinct mechanisms. Large-ripple size aligns with predictions from drag-ripple theory, and associated sand fluxes are greater than predicted for impact ripples. These findings are inconsistent with an impact-ripple origin and instead suggest that large martian ripples are drag ripples. Windblown drag ripples constitute an untapped record of atmospheric evolution on planetary bodies with tenuous or ephemeral atmospheres across the Solar System. Low-pressure wind tunnel experiments suggest that large sand ripples on Mars are drag ripples, not impact ripples. Windblown drag ripples constitute an untapped record of atmospheric evolution under tenuous atmospheres across the Solar System.

Extensive dune fields nearly encircle the equatorial regions of Titan, Saturn's largest moon. Dunes evolve in response to environmental change, offering a record of recent geologic and climate history. A global analysis reveals that Titan's dunes become more narrowly spaced and increasingly more regular along a continuous eastward transport path, starting east of the Xanadu region, around the equator, and terminating abruptly at Xanadu's western margin. Xanadu is a rugged, tectonically active, water‐ice‐rich region with low topography and a thin layer of atmospherically deposited organic‐rich material. Our results demonstrate that windblown grains must withstand long transport distances. Furthermore, environmental conditions along the eastern margin of Xanadu set a template over which dunes evolve, only gradually modified as sediment supply or availability increases downwind. Together, these results highlight the oversized impact that Xanadu has on Titan's dune fields, which in turn play a critical role in regulating Titan's sedimentary and carbon cycles. Plain Language Summary Saturn's moon, Titan, is a sedimentary world like Earth and Mars. Observations from orbit reveal a geologically active surface with extensive fields of sand dunes. These dunes nearly encircle the equatorial regions, only interrupted by a mysterious terrain, Xanadu. Because dunes evolve in response to atmospheric and surface conditions, they offer a window into Titan's recent geologic and climate history. We conduct a global analysis of the patterns formed by dune crestlines. We find that dune spacing decreases continuously and their patterns become better organized along a pathway beginning at Xanadu's eastern margin, around the equator, terminating abruptly at Xanadu's western margin. These findings demonstrate that Titan's dune fields constitute a linked sedimentary system and that sand grains, previously hypothesized to be mechanically weak, must in fact withstand long travel distances. Key Points Titan's dune fields constitute a continuous sediment transport pathway rather than disconnected sediment sinks Sand grains must withstand long transport distances, even though theory and experimentation predict mechanically weak grains Xanadu exerts a primary control on the evolution of Titan's dunes as a source and sink of sediment and a physical barrier to transport

As winds blow over sand, grains are mobilized and reorganized into bedforms such as ripples and dunes. In turn, sand transport and bedforms affect the winds themselves. These complex interactions between winds and sediment render modeling of windswept landscapes challenging. A critical parameter in such models is the aerodynamic roughness length, z 0 , defined as the height above the bed at which wind velocity predicted from the log law drops to zero. In aeolian environments, z 0 can variably be controlled by the laminar viscous sublayer, grain roughness, form drag from bedforms, or the saltation layer. Estimates of z 0 are used on Mars, notably, to predict wind speeds, sand fluxes, and global circulation patterns; yet, no robust measurements of z 0 have been performed over rippled sand on Mars to date. Here, we measure z 0 over equilibrated rippled sand beds with active saltation under atmospheric pressures intermediate between those of Earth and Mars. Extrapolated to Mars, our results suggest that z 0 over rippled beds and under active saltation may be dominated by form drag across a plausible range of wind velocities, reaching values up to 1 cm—two orders of magnitude larger than typically assumed for flat beds under similar sediment transport conditions. Low-pressure wind tunnel experiments suggest that the aerodynamic roughness length on Mars, over rippled beds and under active saltation, may be dominated by form drag, reaching values up to two orders of magnitude larger than typically assumed.

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 density We 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

Arctic regions are disproportionately affected by atmospheric warming, with cascading effects on multiple surface processes. Atmospheric warming is destabilizing permafrost, which could weaken riverbanks and in turn increase the lateral mobility of their channels. Here, using timelapse analysis of satellite imagery, we show that the lateral migration of large Arctic sinuous rivers has decreased by about 20% over the last half-century, at a mean rate of 3.7‰ per year. Through a comparison with rivers in non-permafrost regions, we hypothesize that the observed migration slowdown is rooted in a series of indirect effects driven by atmospheric warming, such as bank shrubification and decline in overland flow and seepage discharge along channel banks, linked in turn to permafrost thaw. As lower migration rates directly impact the residence timescales of sediment and organic matter in floodplains, these surprising results may lead to important ramifications for watershed-scale carbon budgets and climate feedbacks.Climate warming affects permafrost regions, with strong impacts on the environment such as the greening of river plains. Here the authors use satellite data to show that these changes have stabilized large Arctic sinuous rivers by slowing their lateral migration by about 20% over the past half-century.

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 ka Slope change caused by GIA significantly correlates with river cutoff frequency, a proxy for lateral migration rate We infer that slope change modulates the magnitude of shear stress on the riverbank, driving changes in lateral migration rate

As natural access points to the subsurface, lava tubes and other caves have become premier targets of planetary missions for astrobiological analyses. Few existing robotic paradigms, however, are able to explore such challenging environments. ReachBot is a robot that enables navigation in planetary caves by using extendable and retractable limbs to locomote. This paper outlines the potential science return and mission operations for a notional mission that deploys ReachBot to a martian lava tube. In this work, the motivating science goals and science traceability matrix are provided to guide payload selection. A Concept of Operations (ConOps) is also developed for ReachBot, providing a framework for deployment and activities on Mars, analyzing mission risks, and developing mitigation strategies