Morphological characteristics were measured for barchan dunes on Earth (2,686 dunes in 30 barchan fields) and Mars (720 dunes in 10 barchan fields) using satellite images. The data were used to (a) develop a new barchan classification system; (b) compare characteristics of barchans on Earth and Mars; and (c) assess whether barchans, in bulk, display allometric or scale‐invariant characteristics. Dimensional metrics were obtained for the width and length of barchan bodies, the width and length of barchans including the horns, and the length of each horn. Dimensionless metrics were derived for the ratios of the body width to the width between the tips of the horns (width ratio), the length of the entire barchan to the length of the body (length ratio), and the length of the longer horn to the shorter horn (symmetry ratio). The width, length, and symmetry ratios were used to classify barchans into eight types and compare the characteristics of their distributions on the two planets. From this analysis, it was established that, statistically, barchans on Earth are distinctive from those on Mars based on the morphometrics, with terrestrial barchans being, on average, of smaller size and more often symmetrical, while Martian barchans more often have convergent horns that are short relative to the central dune body and are more often asymmetrical. The analysis further reveals that barchan planform morphology can be considered scale‐invariant, and we argue that body width is the most appropriate measure representing barchan size.

Wind-blown sand concentrations decay rapidly and in an orderly manner with height above the surface. The saltation flux profiles are of interest to understand wind and sand interactions and for fundamental measurement and modeling of associated transport rates. This study compares methods to measure and represent aeolian sand flux profiles. We measured vertical flux profiles and used quality-controlled data to test power, logarithmic, and exponential functions to reproduce the profiles. These results are used in a pragmatic assessment of the efficiency of reproducing flux profiles from vertically discontinuous arrays of traps or sensors compared to profiles obtained from continuous vertical arrays of segmented traps. Our analysis corroborates previous findings demonstrating that exponential decay functions are statistically the best method to approximate flux profiles. The results are used in a novel application to compare flux profiles reproduced from vertically discontinuous arrays of devices with those obtained from continuous vertical arrays comprising nine mesh-style traps. The results indicate that discontinuous arrays of 3, 4, 5, or 6 devices deployed less than 200 mm from the surface will effectively reproduce results from the continuous array, with average errors less than 3%. Errors increase when devices are at greater heights or as the number of devices decreases. Discontinuous arrays typically do not capture creep transport which would contribute to error in our comparisons. Therefore, creep must comprise less than 3% of total aeolian sand flux, contradicting typical assumptions of 25%.

Acoustic distance sensors have a long history of use to detect subaqueous bedforms. There have been few comparable applications for aeolian bedforms such as ripples. To address this, we developed a simple and reliable apparatus comprising a pair of distance sensors, a bracket upon which they are mounted, and a base upon which the bracket can slide. Our system relies on two Senix Corporation (Hinesburg, VT, USA), ToughSonic^® model 14-TSPC-30S1-232 acoustic distance sensors: one to measure surface elevation changes (in this case, ripple morphology) and a second to measure horizontal location. The ToughSonic^® vertical resolution was 0.22 mm and the horizontal scan distance was about 0.60 m with a locational accuracy of 0.22 mm. The measurement rate was 20 Hz, but we over-sampled at 1 KHz. Signal processing involves converting volts to meters, detrending the data, and removing noise. Analysis produces ripple morphologies and migration rates that conform with independent measurements. The advantages of this system relative to terrestrial laser scanning or structure from motion are described.

Barchans represent a common dune type found on Earth and Mars. Their morphological characteristics are singular and easily recognized. Their formation is favored on relatively immobile substrates with near-unidirectional winds that sculpt the distinctive crescentic, aerodynamic morphology. Barchans often occur isolated from one another, although they can occur in organized sets or barchanoid dune fields.  Long and Sharp (1964) and Bourke and Goudie (2009) measured attributes of barchan morphology and identified four archetypal shapes based on the ratio of the length of the stoss slope to the distance between the ends of the horns.            In this study, we report findings based on measurements of 3,406 barchans: 2,686 from 20 terrestrial dune fields and 720 from 10 Martian dune fields. Barchan morphology was characterized by six metrics: body length (L1), measured from the upwind nose of the barchan to the nearest base of the slipface; total length (L2) measured to the (average) ends of the horns; body width (W1), measured on a line perpendicular to L1 and intersecting at the base of the slipface; horn-to-horn width (W2), measured perpendicular to L2 and parallel to W1; and horn lengths (H1 and H2) measured perpendicular to W1. The morphometric data were used to develop three new shape metrics as a basis for barchan shape characterization: 1) a width ratio (WR: W1/W2); 2) a length ratio (LR: L1/L2); and 3) a symmetry ratio (SR: longer horn length/shorter horn length). The barchan stereotype (Type 1) was defined as meeting three criteria: SR between 1.0–1.2, WR between 0.95-1.58 (mean value +/- one standard deviation) and LR between 0.52–0.76. Cluster analysis was used to define three additional characteristic shapes. Type 2 barchans are moderately symmetrical ( =1.47), uniform in width (  = 1.01), and elongated (  = 0.53). Type 3 barchans are moderately symmetrical (  = 1.4), with converging horns (  = 1.56), and compact (  = 0.74). Type 4 barchans are asymmetric (  = 3.46) uniform in width (  = 1.15) and average elongation (  = 0.64).            We found that, on average, terrestrial barchans are shorter, proportionately wider, and more symmetric than those on Mars. Most barchans are Type 1, 2, or 3 (26%, 32%, and 35%, respectively), and relatively few are Type 4 (8%). The distributions of types, however, is quite different for the two planets. On Earth, most barchans are Type 2 (38%) and Type 1, stereotypical barchans comprise 30% of our samples. Type 4 barchans are least common (6%). On Mars, most barchans are Type 3 (64%). The distributions of Types 1, 2, and 4 are similar. Type 1, stereotypical barchans, the least common on Mars, comprise 11% of our samples, and Types 2 and 4 each represent 13% of our samples. These results indicate that most barchans do not conform to our idealized morphological image on either Earth or Mars. In our sample, Martian barchans are larger than terrestrial, with shapes characterized largely by asymmetric, converging horns.

We present field data demonstrating relationships associated with the development of aeolian mass flux profiles. Trajectories of saltating grains increase in height and length as their coefficient of restitution increases, thus do median saltation heights, hs50. The coefficient depends, in part, on the momentum of a saltating grain relative to the inertia of the bed material that it impinges upon. This manifests in the smaller grains in a population bouncing higher off of a given surface than the coarser grains in that same population, as first described by Bagnold (1937), and more recently by Dong et al. (2004) and Namikas (2006), among others. More controversial has been the relationship between median saltation height and shear velocity, u*, with some arguing that hs50 increases with u* (e.g., Bagnold, 1941; Owen, 1964; Willetts and Rice, 1985; Rice et al., 1995; Dong et al., 2012). Field investigations, however, have shown that hs50 is insensitive to u* (e.g., Creyssels et al., 2009; Ho et al., 2012; Martin and Kok, 2017; Delorme et al., 2023). These, and related studies, motivate this research.Measurements were made at three sites in the Mojave/Sonoran Deserts of Southern California (CA sites) and three sites in the Jericoacoara National Park in Ceará, Brazil (BR sites). Sites were chosen to represent a range of grain sizes. At each site, the data-gathering protocols were similar. Flux profile data were obtained using vertical stacks of mesh traps, grain size statistics were obtained from the trapped sand, and shear velocities were estimated using 3-D ultrasonic anemometer measurements. Mean grain sizes, d50, at CA1, CA2, and CA3 were 0.17, 0.18, and 0.16 mm. The respective values at BR1, BR2, and BR3 were 0.22, 0.39, and 0.39 mm. A total of 63 flux profiles and related data were evaluated.Regression analysis was used to test for a dependence of hs50 on a dimensionless shear velocity (u* /u*t, where the latter is the threshold shear velocity, estimated with d50) and indicated a strong, inverse relationship (R2 = 62%, P < 0.0001). When hs50 is normalized by dividing by d50, and the regression repeated, a statistically significant relationship was found. Finally, we tested for a relationship between the simple variables of median saltation height and mean grain size, finding that hs50 increases with d50 (R2 = 53%, P < 0.0001). In summary, median saltation height is not dependent on shear velocity but does depend on mean grain size.

The transport of sand by wind has been the subject of extensive research. Despite numerous efforts, however, substantial discrepancies persist among theoretical models, wind tunnel experiments, and field observations concerning the magnitude of the critical threshold necessary to induce sand movement over heterogeneous surfaces.In this study, we conducted field experiments to quantify surface shear velocities and wind velocities across seven heterogeneous surfaces characterized by a broad range of grain sizes. Surface shear velocities were measured using flush-mounted Irwin sensors (IS), while wind velocities were measured using a thermal anemometer (TA) positioned 30 mm above the surface and a 3-D ultrasonic anemometer (UA) 0.6 m above the surface. A high-resolution video camera monitored the surface to document sand motion. IS and TA sensors were emplaced within the view field of the camera. Reptating sand grains were captured using adhesive-coated plastic sticks. A total of 46, quality-controlled, threshold event data sets were acquired from the IS and TA measurements and 39 from the UA measurements.For each event, surface shear velocities and wind velocities were block-averaged at 1-second intervals. Maximum and minimum values of shear (IS) and wind velocities (TA) were determined for each threshold event. Maxima are defined as the fastest shear and wind velocities without sand movement in the 5 seconds before an event, and minima are defined as the slowest velocities with movement in the 10 seconds before an event. Shear velocities in the constant stress layer (UA) were estimated with 2 minute-averaged Reynolds stress derivations. Median grain sizes for each event were estimated via photosieving. This approach enabled the estimation of threshold conditions corresponding to specific grain sizes, bracketed by their respective maximum and minimum values.Our results from surface and near-surface data indicate no statistically-significant relationship between threshold shear or wind velocities and median grain sizes ranging from 131 to 475 µm. The mean of the maximum and minimum surface shear velocities for the 1-second block averages is 0.17 m/s (standard deviation, 0.02 m/s). The respective wind velocity threshold is 5.28 m/s (standard deviation, 1.64 m/s). Testing longer averaging intervals (1.5, 2, and 3 seconds) yielded consistent results, with a reduction in the differences between maximum and minimum values as the interval increased, but yielding no significant relationships. The shear velocity estimates from the ultrasonic anemometer data indicate a statistically significant, Bagnold-type relationship for the threshold, but with an “A” coefficient of 0.14 (standard deviation, 0.05) for the fluid threshold.These findings support the concept of equal mobility, whereby a single threshold shear or wind velocity can initiate sand motion for a range of grain sizes on heterogeneous surfaces. The findings also indicate a decoupling of threshold conditions as measured at or near the surface and those as measured typically in the constant stress layer.