GM11.2

Dust devils are atmospheric vortices driven by daytime dry convective circulations and are visible because of the dust entrained from the ground. They are common in deserts on Earth and globally on Mars. They appear as tubular or conical light-coloured clouds of dust that cast a dark shadow which is particularly distinctive in orbital images. They can reach much larger sizes on Mars (several km in height), compared to Earth, perhaps because their size is limited by the depth of planetary boundary layer. Here, we perform a global survey for dust devil vortices by using a neural network to search through the database of Context Camera (CTX) images aboard NASA’s Mars Reconnaissance Orbiter spanning Mars Years 28-35.

We used an off-the-shelf convolutional neural network (CNN) architecture (RetinaNet) as used successfully for previous planetary studies. After training and testing (average precision AP ~0.7) we processed the whole database of CTX images (n=111,547 images) for dust devil detections using the JMARS-served CTX images. Every detection with a CNN confidence level (CT) greater than 0.5 (n=57,051) was verified by a human operator. The effective diameter of the dust devil was estimated from the bounding box size by measuring the diameter of the “cloud” in a sample of 33 dust devils to generate a linear scaling relationship.

3,747 images were found that contained validated dust devils at CT >0.5, comprising 11,201 individual detections. The images spanned MY 28 starting at Ls 114° through to MY 35 at Ls 114°. Trends in frequency and size of dust devils with season agree with previous studies, where higher densities and larger sizes of dust devils are found in local summer for each hemisphere and low levels of activity occur in local winter. Valles Marineris and Elysium Planitia (InSight, MSL) are notable areas lacking dust devils despite good temporal image coverage. We confirm the hotspots of Chryse and Hellas Planitiae noted in some, but not all previous studies. We find two notable hemispherical asymmetries in the data: (a) The peak in size/density occurs just after the solstice in the southern hemisphere, but at the solstice in the northern hemisphere. (b) Excluding known hotspots in Amazonis and Arcadia Planitiae we find that two broad latitudinal zones seem to exhibit both higher frequency and size: 55-70°N at Ls 120-150° and 50-70°S at Ls 300-330°, agreeing with observations of dust devil tracks. We attribute the hemispherical asymmetries to the dominance of the southern summer Hadley circulation and are investigating this further using data from the OpenMARS climate database.

Acknowledgments: we thank the JMars team at ASU for hosting map projected CTX image products used in this work. SJC acknowledges the French Space Agency CNES for supporting her Mars work.

How to cite: Conway, S. J., Bickel, V. T., Patel, M. R., Fenton, L., and Carson, H.: Globally Tracking Dust Devil Vortices on Mars Using Neural Networks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4026, https://doi.org/10.5194/egusphere-egu22-4026, 2022.

The mechanism/s responsible for sediment entrainment by wind and bedform migration on Mars are a matter of debate [1]. Martian large ripples (LRs) migrate under present-day low pressure conditions and have been interpreted has fluid/wind drag ripples [2] or as bedforms formed by aeolian saltation [3]. An important constraint to this debate is the relation between bedform wavelength and atmospheric density (as a function of elevation). This dataset was later complemented by the measurement of bedform wavelengths in other 11 areas [2]. Lapotre el al. [2] proposed that the fluid drag theory fits the measured wavelength vs. atmospheric density relation, a view not shared by Lorenz [1, Fig. 2].

To try to address this divergence, we will present a new method that allows the automatic mapping and morphometric characterization of bedforms (LRs to TARs) using HiRISE imagery. It consists in a windowed multiscale spectral approach, followed by a supervised classification stage using neural networks. This method can accurately identify the bedforms (overall accuracy of 94%) and provide precise wavelength measurements within a ±12% confidence interval. The surveyed bedforms have crests spaced between 1 and 100 m, and include large ripples, megaripples and TARs.

We will review and compare previous datasets and studies with our measurements. The main objective is to re-evaluate how well the wind drag hypothesis can predict bedforms’ spacing on Mars, and for this purpose we employ an improved measurement approach that allows the mapping of entire dune fields. Furthermore, we significantly increased the number of mapped areas and extended the range of sampled elevations.

Preliminary results of this ongoing effort will be presented at the conference.

[1] Lorenz, R.D. (2020). Martian Ripples Making a Splash. J. Geophys. Res. Planets 125, 12–15.

[2] Lapotre, M.G.A., Ewing, R.C., Lamb, M.P., Fischer, W.W., Grotzinger, J.P., Rubin, D.M., Lewis, K.W., Ballard, M.J., Day, M., Gupta, S., et al. (2016). Large wind ripples on Mars: A record of atmospheric evolution. Science (80). 353, 55–58.

[3] Sullivan, R., Kok, J.F., Katra, I., and Yizhaq, H. (2020). A Broad Continuum of Aeolian Impact Ripple Morphologies on Mars is Enabled by Low Wind Dynamic Pressures. J. Geophys. Res. Planets 125, 1–39.

[4] Lorenz, R.D., Bridges, N.T., Rosenthal, A.A., and Donkor, E. (2014). Elevation dependence of bedform wavelength on Tharsis Montes, Mars: Atmospheric density as a controlling parameter. Icarus 230, 77–80.

How to cite: Vaz, D. A., Silvestro, S., Chojnacki, M., and Silva, D. C. A.: Global wavelength survey of Martian bedforms: methods and preliminary results, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6697, https://doi.org/10.5194/egusphere-egu22-6697, 2022.

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.

How to cite: Sherman, D., Zhang, P., Butler, R., and Bae, J.: Comparison of Morphological Characteristics of Terrestrial and Martian Barchans, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12992, https://doi.org/10.5194/egusphere-egu22-12992, 2022.

Aeolian processes are at the origin of a large number of bedforms, which are topographic patterns that are spatially organised in a periodic manner and that can be observed both on Earth and on other planetary bodies. Two main categories of bedforms can be distinguished: (i) "loose" bedforms, generated on a bed of mobilisable grains by erosion, transport and deposition and (ii)  "solid" bedforms, not induced by grain transport but by mass transfers such as ice sublimation or condensation under turbulent winds. Although the mechanisms involved in the growth of some solid bedforms have been studied (penitents, sublimation ripples, …), the subject remains largely less treated to date than loose bedforms, partly because of the lack of terrestrial environments favourable to sublimation. Comparison with other planetary environments has opened up new horizons for understanding these objects and the aeolian environments in which they develop.

Among these bedforms, sublimation waves are transverse linear waveforms: regular and parallel ridges oriented perpendicular to the main direction of the turbulent flow interacting with the ice surface. The height of the flow is greater than their wavelength. The emergence of the bedforms is due to a hydrodynamic instability mechanism of the band-pass type which allows their growth. Our theoretical linear stability study shows that this instability appears in the laminar-turbulent transition regime, based on the near-wall Reynolds number, only if the modulation of the viscous sublayer by an effective longitudinal pressure gradient is taken into account in the turbulence model enabling to reproduce the feedback of the topography on the flow.

These sublimation waves have been observed in different environments [Bordiec et al, 2020], by sublimation and diffusion of (a) water ice in air, in Antarctica or Ice caves, (b) water ice in CO2 atmosphere, on some areas of the northern polar cap of Mars, (c) and experimentally with CO2 ice in air. They are also observed on a Martian H₂O glacier near the northern polar cap of Mars [Collet et al, in prep.], however, in the latter case, these sublimation waves are observed on larger icy waves. How can this difference in scale between two wavelengths be explained? What is their size selection process? To answer these questions, we investigate in our theoretical study the dependence on environmental conditions through (i) the fluid properties (wind speed, fluid viscosity) (ii) the direction of the transfer (sublimation or condensation) and (iii) the height of the flow in front of the wavelength (infinite or finite).

How to cite: Carpy, S., Bordiec, M., Collet, A., Massé, M., and Bourgeois, O.: Aeolian processes on planetary icy solid substrates submitted to phase transition: relation between bedforms scales and environmental conditions., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5998, https://doi.org/10.5194/egusphere-egu22-5998, 2022.

Large linear dunes are found in great abundance across the equatorial regions of Saturn’s moon Titan. They are similar in width and spacing to the large dunes of the Saharan, Arabian and Namibian deserts, indicating atmospheric conditions, sand sizes and winds are comparable to those on Earth. An examination of their geomorphometric properties, such as length, width, spacing and distribution can reveal aspects of their relationship with wind strength and direction and controls by underlying topography. We traced long axes of about 70% of all measurable dunes, which involved over 20,000 measurements. We mapped all of the dunes in Shangri-La, Fensal, Aztlan, and half of the Belet Sand Sea. In addition, we measured 90,000 dune widths across Titan at 500 m intervals and fit a nonstationary statistical model with a Gaussian spatial process to determine correlations of dune spacings. Dune long axes are dominantly oriented E-W, a proxy for the sand flux and wind directions. Dunes range to over 400 km in length, with an average length of 40 km. The average length may reflect a rough spacing of obstacles, large-scale topographic variations, or the availability of sand. Dunes are directed slightly NE in the Belet Sand Sea, where dunes are especially abundant and wider. The longest dunes are also found here. Belet may thus represent a fully mature sand sea, where dunes are free to grow as long and large as possible. To the east is the Shangri-La sand sea, which is the location of the Dragonfly landing site. Shangri-La hosts dunes directed dramatically southward, especially near the Xanadu region margin. Dunes here are narrower and interdunes are clearly visible near the elevated rim of the Selk impact crater and other topographic obstacles. Sand collects most densely along the eastern boundary, at the margin of Xanadu, and at the downwind margins of all sand seas. This perhaps indicates that sand is transported until major boundaries are encountered that preclude sand movement. Dune width values can be divided into about 5 major (20 minor) regions globally within the sand seas, with widest groupings at the sand sea centers and isolated, narrower groupings at higher latitudes. The narrowest dunes appear to have the most obstacles or topographic control or be at the highest latitudes. However, within each cluster, dunes of any size within the 1-3 km width range can exist. These studies reveal that while local controls are impactful, dunes will ultimately grow to the extent possible under the conditions present, which on Titan are highly favorable for large linear dunes. Further examination of dune parameters can reveal details about the landscape, basement bedrock conditions, sand transport history and regional wind effects on the dunes of Titan.

How to cite: Radebaugh, J., Rose, D., Wright, M., Lake, B., Tass, S., Christiansen, E., Rodriguez, S., and Turtle, E.: Dune length, width and orientation in the sand seas of Titan reveal regional properties, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8961, https://doi.org/10.5194/egusphere-egu22-8961, 2022.

Seasonal changes in wind regime have driven the formation and emergence of reversing dunes and crest reversal in the inland arid and coastal areas of Asia, but due to the strong prevailing winds, the reversing dunes or reversing crest can be flipped. Therefore, the transient reversing dunes or crest reversal will be ignored and unobserved. To investigate dune morphology and sedimentology concerning seasonal alternation of the wind regime, we reconstructed dune topographies using aerial drone photos and analyzed the grain-size parameters and internal sedimentary structures of dunes. Morphological results show that wind-blown sands from the lee side are transported and deposited on the upper stoss side because of the reversing winds. Then, the dune crestal area is flattened, surface sand compositions were reorganized from fining to coarsening at the dune crest. Combining these field surveys with numerical simulation results, we found that the internal sedimentary structures are composed of high-angle cross-strata and low-angle bounding surfaces. The dip angles of the bounding surfaces gradually decrease from the bottom to the top because of the reversing wind erosion on the lee side. The increase in sand flux on the lee side plays a critical role in shaping the dip angle of the bounding surfaces due to the speed-up effect.

How to cite: Zhang, D., Chen, J., Yang, X., Lehmkuhl, F., and Jiang, W.: The effects of seasonal wind regimes on the evolution of reversing dunes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13070, https://doi.org/10.5194/egusphere-egu22-13070, 2022.

Aeolian sediment transport in saltation shapes erodible surfaces and affects the dust cycles and climates of planetary bodies. For the approximately unidirectional near-surface winds often temporarily prevailing in planetary atmospheres, saltation transport approaches an equilibrium state when given enough fetch to adapt. However, predictions of even this arguably simplest transport state have relied on oversimplified physical models or empirical models derived exclusively from measurements under Earth's atmospheric conditions. Here, we use grain scale-resolved sediment transport simulations to derive general scaling laws for equilibrium planetary saltation transport. The simulations, consistent with terrestrial measurements, cover seven orders of magnitude in the particle-fluid-density ratio s, ranging from water to extremely rarefied air on Pluto. They reveal that the saltation threshold exhibits a parabolic dependency on the grain size, with a pronounced threshold minimum that scales as s^1/3. In contrast, previous studies reported a s^1/2-scaling and substantially larger threshold values for nonequilibrium conditions. Furthermore, the simulations reveal that the saltation mass flux and grain impact energy flux, which is responsible for the emission of soil dust into a planetary body's atmosphere, obey scaling laws resembling the classical law by Ungar and Haff (Sedimentology 34, 289-299, 1987), but with nonconstant scaling coefficients proportional to s^1/3. Our results, summarized in phase diagrams for the cessation threshold, mean mass flux, and dust emission potential, are consistent with several geomorphological observations across Solar System bodies, such as the eastward propagation of Titan's dunes despite predominant westward winds.

How to cite: Pähtz, T., Durán, O., and Comola, F.: Scaling of equilibrium planetary saltation transport, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11113, https://doi.org/10.5194/egusphere-egu22-11113, 2022.

Dune fields are commonly associated with periodic patterns that are among the most recognizable landscapes on Earth and other planetary bodies. In zones of loose sand, this periodicity is associated with the development of the flat bed instability, coupling wind, sediment transport and sand bed evolution. However, in zones of limited sediment supply, where periodic dunes elongate and align in the direction of the resultant sand flux, there has been no attempt to explain the emergence of such a regular pattern. Here, we show, by means of numerical simulations, that the elongation growth mechanism does not produce a pattern with a specific wavelength. Periodic elongating dunes appear to be a juxtaposition of individual structures, the arrangement of which is due to regular landforms at the border of the field acting as boundary conditions. This includes, among others, dune patterns resulting from bed instability, or the crestline reorganization induced by dune migration. The wavelength selection in fields of elongating dunes therefore reflects the interdependence of dune patterns over the course of their evolution.

How to cite: Gadal, C., Narteau, C., Courrech du Pont, S., Rozier, O., and Claudin, P.: Periodicity in fields of elongating dunes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1813, https://doi.org/10.5194/egusphere-egu22-1813, 2022.

Wind-blown sand and dust emissions shape singular landscapes in arid environments and globally impact climate, life and human activities. However, the accurate quantification of aeolian sediment fluxes are still subject to considerable uncertainties. Since extensive measurements are difficult to implement in the field, this quantification rely essentially on remote sensing data and transport laws that integrate a large number of parameters for the airflow and granular bed. However, confronted with all the sources of natural variability (wind regime, air recirculation, grain-size distribution, soil composition, etc.), a complete mass balance of aeolian transport remains challenging. Here we consider long time scales to smooth out such variability and integrate arid landscape dynamics into the source-to-sink assessment of aeolian mass transfers in the Lut Desert (Iran). Taking advantage of new remote sensing imagery and dating techniques, together with more accurate wind data and a deeper understanding of dune dynamics, we analyze major landforms of this desert to provide a comprehensive picture of aeolian transport on time scales from decades to millions of years. We map the modern sandflows, along which we evaluate the volume and chronology associated with the excavation of mega-yardangs upwind and the formation of giant dunes downwind. Sediment discharges deduced from long-term erosion and deposition are of the same order of magnitude (10⁵ to 10⁶ m³yr^-1)  as short and medium-term sand discharges derived from wind data and dune morphodynamics. At the scale of the internal aeolian sediment-routing system of the Lut, we establish an overall sediment budget constrained by the joint development of the erosional and depositional landforms. Our findings thus quantify the geomorphic controls of aeolian processes on arid landscapes at multiple length and time scales, while providing information on mass exchanges between continents and atmosphere.

How to cite: Barrier, L., Chanteloube, C., Derakhshani, R., Gadal, C., Braucher, R., Payet, V., Léanni, L., and Narteau, C.: Aeolian fluxes from arid landscape dynamics in the Lut Desert (Iran), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2806, https://doi.org/10.5194/egusphere-egu22-2806, 2022.

Wind-blown sand dunes emerge due the linear instability of a flat sedimentary bed. This instability has been studied in experiments and numerical models but rarely in the field, because of the large time and length scales involved. Here we examine dune formation at the upwind margin of the White Sands Dune Field in New Mexico (USA), using 4 years of lidar topographic data to follow the spatial and temporal development of incipient dunes. Data quantify dune wavelength, growth rate, and propagation velocity and also the characteristic length scale associated with the growth process. We show that all these measurements are in quantitative agreement with predictions from linear stability analysis. This validation makes it possible to use the theory to reliably interpret dune-pattern characteristics and provide quantitative constraints on associated wind regimes and sediment properties, where direct local measurements are not available or feasible.

Reference: Gadal et al., Geophys. Res. Lett. 47, e2020GL088919 (2020).

How to cite: Claudin, P., Gadal, C., Narteau, C., Ewing, R. C., Gunn, A., Jerolmack, D., and Andreotti, B.: Spatial and temporal development of the dune instability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2376, https://doi.org/10.5194/egusphere-egu22-2376, 2022.

We designed a landscape-scale experiment at the edge of the Gobi desert, China, to quantify the development of incipient dunes under the natural action of winds (Lü et al., 2021). High-resolution topographic data documenting 42 months of bedform dynamics are examined to provide a spectral analysis of dune pattern formation. We identified two successive phases in the process of dune growth, from the initial flat sand bed to a meter-high periodic pattern. We focus on the initial phase, when the linear regime of dune instability applies, and measure the growth rate of dunes of different wavelengths. We identify the existence of a maximum growth rate, which readily explains the mechanism by which dunes select their size, leading to the prevalence of a 15 m-wavelength pattern. We quantitatively compare our experimental results to the prediction of the dune instability theory using transport and flow parameters independently measured in the field. The remarkable agreement between theory and observations demonstrates that the linear regime of dune growth is permanently expressed on low-amplitude bed topography, before larger regular patterns and slip faces eventually emerge. Our experiment underpin existing theoretical models for the early development of eolian dunes, which can now be used to provide reliable insights into atmospheric and surface processes on Earth and other planetary bodies.

Bibliography:

Lü P., C. Narteau, Z. Dong, P. Claudin, S. Rodriguez, Z. An, L. Fernandez-Cascales, C. Gadal, S. Courrech du Pont, Direct validation of dune instability theory, Proceedings of the National Academy of Sciences, 118, 17 (2021).

How to cite: Narteau, C., Lü, P., Claudin, P., Dong, Z., Rodriguez, S., An, Z., Fernandez-Cascales, L., Gadal, C., and Courrech du Pont, S.: Direct validation of dune instability theory, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2079, https://doi.org/10.5194/egusphere-egu22-2079, 2022.

Dune fields are recognized both by the occurrence of periodic bedforms and isolated dunes of different shapes and orientations. Nevertheless, there are still no field examples of whether this apparent duality results from synchronous dune growth, and on what timescales. Here, by leveling neighboring parcels of a dune field, we develop landscape-scale experiments with controlled initial and boundary conditions to test the influence of sand availability on dune formation. Starting from a flat sand bed, we observe the emergence of periodic dunes and measure for more than 3 years how they grow as they interact with each other. Over the same time period, by regularly feeding sand heaps deposited nearby on a non-erodible bed. we observe how dune shape changes, eventually leading to the elongation of isolated dunes with a different orientation. These experiments are unique by their size and duration. Under natural conditions, they show that the same wind regime can be associated with two dune growth mechanisms according to sand availability. The coexistence of these two dune growth mechanisms provides a basis for examining the diversity of dune shapes on Earth or other planetary bodies depending on local climatic conditions.

How to cite: Narteau, C., Lü, P., Claudin, P., Dong, Z., Rodriguez, S., An, Z., Gadal, C., and Courrech du Pont, S.: Coexistence of two dune growth mechanisms in a landscape-scale experiment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13080, https://doi.org/10.5194/egusphere-egu22-13080, 2022.