Megaripple mechanics: bimodal transport ingrained in bimodal sands

Tholen, Katharina; Pähtz, Thomas; Yizhaq, Hezi; Katra, Itzhak; Kroy, Klaus

doi:10.1038/s41467-021-26985-3

Cited by 17 publications

(21 citation statements)

References 83 publications

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“…On Earth, such ripples can attain heights of decimeters under ideal conditions and are known by many terms (megaripples, coarse‐grained ripples, granule ripples, aeolian ridges) partly because of the diversity such bedforms exhibit depending on the grain size‐frequency of local sediment available for transport and locally prevailing wind strengths (e.g., Bagnold, 1941, pp. 144–160; Fryberger et al., 1992; Jerolmack et al., 2006; Lämmell et al., 2018; Sharp, 1963; Tholen et al., 2022). Despite this nomenclature confusion, what all of these bedforms have in common is that the coarsest grains covering their crests are too large to saltate, so are mobilized in creep‐like movements by high‐speed impacts of finer saltating sand.…”

Section: Analysis Of Aeolian Observations and Climate Model Predictionsmentioning

confidence: 99%

The Aeolian Environment in Glen Torridon, Gale Crater, Mars

et al. 2022

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The Mars Science Laboratory (MSL) rover spent a full martian year exploring the phyllosilicate‐bearing Glen Torridon trough on the flank of Aeolis Mons in Gale crater, enabling in‐depth assessment of aeolian processes. MSL encountered erosional and depositional features recording a long aeolian history. The trough has served as a long‐term conduit for sand transport, probably involving many cycles of sand accumulation and deflation. Rock abrasion textures indicate sand‐driving winds blowing W‐SW (opposite of abrasion textures on the Greenheugh Pediment above the trough floor). Indurated megaripple surfaces with 2–5 mm grains contrast with seasonally active ripples having finer maximum grain sizes, indicating more vigorous saltation in the past. Active ripples display a broad continuum of wavelengths, as well as coarser grains at crests than troughs, consistent with origins as impact ripples. Orientations of a wind streak extending from a large ripple field, and sandy wind tails behind obstacles, indicate sand is driven W‐SW in the current era, approximately along the trough axis. Erosion of drill tailings piles was strongly seasonal, enhanced during late spring and early summer (perihelion). Climate modeling suggests W‐SW sand transport can be attributed to seasonal enhancement of nighttime regional winds entering Gale crater from the N, combined with local katabatic winds flowing down the slopes of Aeolis Mons. However, it is unclear whether sand transport at Glen Torridon is primarily from these wind components combining and acting simultaneously, or occurring in serial at different times of night; field evidence supports both possibilities.

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Section: Analysis Of Aeolian Observations and Climate Model Predictionsmentioning

confidence: 99%

The Aeolian Environment in Glen Torridon, Gale Crater, Mars

et al. 2022

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“…1). On Earth, meter-scale ripples only form in the presence of wide or bimodal grain-size distributions, with two grain-size populations moving through two distinct transport modes-saltation and creep 3 . However, many large ripples on Mars lack a concentration of coarser grains that would be transported in creep near their crests [4][5][6][7][8] and thus cannot be explained by bimodal transport 3 .…”

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confidence: 99%

A distinct ripple-formation regime on Mars revealed by the morphometrics of barchan dunes

et al. 2022

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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.

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“…The more recent work by Tholen et al. (2022) on the formation of bedforms composed of bimodal (finer and coarser) sands, as is often the case for coarse‐grained ripples, predicts that on Mars the maximum size of the coarser fraction should be 4.5 times size of the maximum size of the fraction transported in saltation. This would require that the saltating grains on MP were larger than 0.22 mm—mainly in the medium sand fraction—the fraction that is missing on the plains.…”

Section: Discussionmentioning

confidence: 97%

Sands on Meridiani Planum, Mars

Kozakiewicz,

Kania,

Salata

et al. 2023

JGR Planets

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The analyses of 184 sediment targets and more than 70,000 individual grains revealed that along the Opportunity rover traverse there are four distinct fractions of deposits related to different geomorphological settings: (a) dust mixed with very fine sand is common behind topographical obstacles, (b) fine sands are deposited in depressions, (c) very coarse sands occur in coarse‐grained ripple fields, and (d) gravel dominates at the rims of craters and on bedrock as lag deposits. Medium size sands were not observed on the plains, but they can be trapped in relatively large craters, where they form dunes and are within coarse‐grained ripples and transverse aeolian ridges (TARs). The fine sands show no regional variations in chemical composition and granulometry, as these sands are easily transported by wind. The very coarse sands vary in composition and shape between the plains and the Endeavour crater rim as their sources are local and their transport distances are short. On the plains, the gravel and the coarse sands are enriched in iron and characterized by higher roundness than the grains from the Endeavour crater rim. The source of iron‐rich, rounded grains on the plains are hematite spherules that are eroded out of Burns formation rocks. The smallest, the best sorted, and the least rounded coarse sand samples are found on coarse‐grained ripple crests. They are mainly composed of spherule fragments and their low roundness indicates shorter transport path lengths than those of grains transported in the past when coarse‐grained ripples migrated.

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Megaripple mechanics: bimodal transport ingrained in bimodal sands

Cited by 17 publications

References 83 publications

The Aeolian Environment in Glen Torridon, Gale Crater, Mars

The Aeolian Environment in Glen Torridon, Gale Crater, Mars

A distinct ripple-formation regime on Mars revealed by the morphometrics of barchan dunes

Sands on Meridiani Planum, Mars

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