James R. Zimbelman

Plains covered by gravel-dominated desert pavement in the Puna of Argentina have an aerodynamic roughness height (or length) z ₀ of ∼1 cm, likely representing a skimming flow regime above the closely spaced gravel particles. Aerodynamic roughness height locally may transition from that of skimming flow over the gravels to a z ₀ that includes the effects of obstacles considerably larger than those of the gravel particles alone. Among large (>60 cm tall) megaripples, z ₀ is elevated beyond that of the gravels alone to values of 2–4 cm. These results represent an analog for an improved understanding of the aerodynamics of gravel-dominated desert pavement and megaripples documented by multiple rovers on Mars.

Rover data can provide important constraints on the possible origin of wind‐related martian features that have been termed “Transverse Aeolian Ridges” (TARs). The large aeolian bedform traversed by Curiosity at Dingo Gap has a planform and albedo similar to members of nearby TAR fields, so that the crossing at Dingo Gap revealed the surface characteristics of a small TAR. Mars Hand Lens Imager images documented 1–2 mm diameter, well‐rounded particles on the Dingo Gap bedform, along with numerous images obtained from the Mastcam, Hazcam, and Navcam science and engineering cameras on the rover. These images confirm that this feature (and, by extension, all small TARs) can be considered a megaripple whose surface is coated by rounded particles that are too large to be set into motion through saltation, but they may be induced to roll (creep) along the surface by the impact of saltating sand grains (a characteristic of megaripples on Earth). The images also confirm that a bimodal wind was present here, a condition that could have contributed to the symmetrical profile of the bedform. The bearing strength at Dingo Gap was better than that of the Purgatory megaripple that trapped Opportunity for several weeks, so that all megaripples should not be automatically presumed to represent a trafficability hazard for a rover.

Basaltic lava flows are common on the surface of the Earth and other terrestrial bodies. However, inflation—including a combination of initially rapid molten core thickening and gradual crustal growth—must be accounted for to enable accurate reconstructions of eruption parameters from observed lava flow morphologies. The shape of an inflated lava flow can change significantly over time. Therefore, incorrectly attributing the flow's final thickness to its dimensions in an initially fully molten state will yield excessively high flow rates, erroneous rheological properties, and unreasonably short eruption durations. To develop improved criteria for identifying inflated lava flows, we examined the McCartys lava flow field in New Mexico, USA. This locality provides an example of how pāhoehoe‐like lava lobes can coalesce and coinflate to form interconnected lava‐rise plateaus with internal inflation pits. These structures were examined using a combination of field observations, low‐altitude kite‐based imaging, and quantitative geomorphology using high‐resolution (1.47 cm/pixel) orthomosaics and stereo‐derived digital terrain models. These observations were used to identify characteristics and diagnostics of inflation, thereby facilitating the interpretation of comparable landforms on other planetary surfaces. Lava‐cooling models were also used to estimate the lava emplacement duration of the ~20‐m‐thick flows by demonstrating that the ~8‐m‐thick upper crust exposed within inflation clefts in the southern part of the McCartys lava flow field would have required 1.2–2.5 years of continuous lava supply to form. This places a minimum bound on the total eruption duration, and implies that comparably thick inflated flows on Mars required years to form.

The transition from sand ripples to megaripples encodes information about the physics of how both sand-sized (moved via saltation) and coarsegrained (moved via impact creep) particles interact under Martian conditions. Previous studies have focused on the aeolian mobility of sand on Mars; here we examine how mobile sand interacts with larger particles moved by creep. HiRISE images of small dunes (lacking well developed slip faces) on Mars reveal a transition of aeolian bedform scale with increasing distance from the dune. Here we document the particles in a similar transition on Earth. High Resolution Imaging Science Experiment (HiRISE) images have documented that sand is moving at many locations around Mars under current conditions. Unlike the active sand deposits, enigmatic “Transverse Aeolian Ridges” (TARs; the non-genetic term for linear to curvilinear aeolian bedforms resulting from either dune- or ripple-forming processes) are found at locations widely distributed across Mars. Recently bright TARs were documented to have moved in HiRISE images taken many Earth years apart at three widely separated locations. Great Sand Dunes National Park and Preserve (GSDNPP) in Colorado has a bimodal particle size distribution along with a seasonal bimodal wind regime, providing the setting to examine the transition from sand ripples (<1 cm in height) to megaripples (typically ~25 cm in height). A Smithsonian Scholarly Studies Award for FY19 funded trips to GSDNPP during May and September of 2019 to collect thousands of digital photographs of ripple-megaripple transitions that were later processed using Multiview Stereo Photogrammetry software to produce detailed Digital Terrain Models (DTMs). The digital images were obtained using a Nikon camera that was motor-driven along a track above the study area. The track was then manually advanced following each photo traverse. Photos were obtained using both a 35 mm lens and a 85 mm Macro lens. The DTMs clearly resolve individual coarse (1-2-mm diameter) particles on the bedforms, providing a detailed record of the surface distribution of coarse grains across both sand ripple and megaripple bedforms, including cases where the crests were continuous between sand ripples and megaripples.

Windblown sand produces distinctive bedforms at scales ranging from normal sand ripples to large reversing sand dunes. We explore how aeolian bedforms evolve at both extremes of this range. An investigation of the transition from sand ripples (100 m height). Surveys of the crests of reversing dunes at GSDNPP reveal a northeastward migration of individual dunes along the southern margin of the main dune mass, consistent with dominant local winds, yet the symmetric reversing dune profile is maintained during the translation. Surveys of the crests of large reversing dunes at BDSP reveal variable adjustments of the crests that may be affected by wind flow altered by the bulk of the dunes themselves, sheltering the southern end of the dunes from one of the seasonal bimodal winds. Results to date indicate that the deformable shape of aeolian bedforms affect wind flow at all spatial scales, influencing the evolution of the features over diverse time scales.

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 bedform mobility 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 of H₂O‐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.

The Planetary Geology Field Conference on Aeolian Processes was held in Southern California on 10–14 January 1978. This meeting brought researchers involved in the study of wind‐related landforms on Mars together with experts in field and laboratory studies of aeolian features on Earth. This seminal field conference established a pattern that has recently been continued through a series of International Planetary Dunes Workshops.