Aeolian sand transport frequently occurs on Mars despite wind speeds rarely exceeding predicted thresholds for motion. This dissonance is problematic for understanding aeolian processes and the resulting geomorphologic responses in contemporary and ancient atmospheres. To address the apparent discrepancy between speeds required to initiate sand motion and transport observed from orbital and in situ observations, we conducted a series of wind tunnel experiments at Martian atmospheric pressures and used buoyancy as the similitude parameter to simulate the force required to initiate particle motion on Mars. Shear velocities were derived from velocity profiles corresponding to the onset of sporadic transport, which can induce downwind cascading motion. Here, we find that threshold shear velocities are slower than previously thought by a factor of 1.6 to 2.5. Measured wind speeds on Mars exceed our observed thresholds, thus offering one mechanism behind the dissonance between observations of transport below previous thresholds of motion.

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.

The development of distributaries in large river deltas plays an important role in the geology, hydrology, and ecology of the coastal ocean, as large rivers are a dominant mechanism by which particulate, suspended, and dissolved material is delivered from the continents to the global ocean. And yet, there is relatively little, near-real time observational data on the development of distributaries in large river deltas -- in part because the development of modern observation coincides with an era when rivers have been controlled by large engineering projects (i.e., the 20th and 21^st centuries). This article reports on Neptune Pass, the largest new distributary to form in the Mississippi River in nearly a century. It developed between 2019 and 2021 when a small canal rapidly expanded by at least an order of magnitude. The system now carries about 15–17% of the flow of the Mississippi River, > 3,000 m³ s^-1 when the Mississippi River is at moderately high flows. This is comparable to the 10th largest river in North America and the 100th largest river on Earth. Neptune Pass is building a delta, and this study sought to examine whether this delta is comprised largely of material eroded from the Neptune Pass (redistributed sediment hypothesis), or includes material recently derived from the Mississippi River (new sediment hypothesis). These hypotheses were tested using a combination of marine-geophysical surveys, remote sensing techniques, and sediment core collections. Results indicate that the delta in Quarantine Bay was 56–79% larger than the material excavated from Neptune Pass, corroborating the new sediment hypothesis, and indicating that it is a net land building system. These findings provide key insights that are critical to the restoration and safe management of the Mississippi River and its delta, the largest system of its kind in North America.

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.