Ripples formed in low-pressure wind tunnels suggest Mars’s large windblown ripples are not impact ripples

Domain: earth_science
Published: 2025-03-26
Editorial outcome: major_revision

Abstract
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.

Introduction
Upon its arrival at Mars, the Mars Reconnaissance Orbiter’s High Resolution Imaging Science Experiment (HiRISE)1 revealed an abundance of dark-toned ripples with meter-scale wavelengths throughout the planet’s sandy deserts2. In turn, NASA’s Mars Exploration Rovers and Curiosity rover showed that those large ripples were covered in smaller ripples, with decimeter wavelengths3–5, similar to those we observe on Earth (Fig. 1a). Although meter-scale windblown ripples do not form on Earth in well-sorted sand (Fig. 1b), it was assumed that both scales of martian ripples were analogous to Earth’s impact ripples, i.e., that their formation was controlled by a bed instability governed by grain collisions and reptation (i.e., the splashing of grains sitting on the bed by saltating ones)6–8. Under this scenario, impact ripples would form at an initial, decimeter-scale wavelength, and grow progressively as small ripples coalesce into bigger ones3.

At odds with the impact-ripple hypothesis, no bedforms with intermediate wavelengths (~20–80 cm) have been observed in fine, well-sorted sand4,9–11. Furthermore, large ripples often develop sinuous crestlines and steep downwind faces (or slip faces, with grain flow and grainfall deposits)4,5,9, display relatively low lateral continuity12, and can record more than one wind direction13. These attributes are common in windblown sand dunes14,15 and subaqueous bedforms16,17, but are not observed in impact ripples on Earth18,19. Finally, it was demonstrated that the size of large ripples decreases with increasing atmospheric density (ρa)4,20, following a trend that is quantitatively consistent with that of subaqueous ripples forming in water currents4,10,21. Based on these observations, it was proposed that Mars’s large ripples may instead form from an aerodynamic instability4, analogously to current ripples22 and windblown sand dunes23–25.

The formation mechanism of Mars’s large ripples has since been thoroughly debated. Two models have been proposed: (1) large martian ripples are impact ripples, which unlike on Earth could grow by coalescence of smaller impact ripples due to the lower dynamic pressure of martian winds11,26–28; (2) large martian ripples are windblown drag ripples (or aerodynamic ripples), which can form on Mars due to the low density, and thus high kinematic viscosity, of the atmosphere4,9,10,29,30. Both models have drastically different consequences for our understanding of Mars’s modern surface environments7, its sedimentary record (notably, of early atmospheric density4,31), and for the surface evolution of bodies with tenuous or ephemeral atmospheres across the Solar System32–34.

Despite continuous efforts, reaching a consensus on the origin of large martian ripples has remained difficult10,35, notably because of the challenges associated with direct observations of ripple formation on Mars. On the one hand, HiRISE images cannot resolve bedforms with wavelengths <~75 cm1. On the other hand, observing ripple formation with a rover would require it to remain idle for months36,37. An alternative to direct ground observations on Mars is to reproduce large ripples in low-pressure wind tunnels on Earth38–40.

Large ripples in natural sand have been observed in low-pressure wind tunnel experiments before38, but these experiments long predate the discovery of large martian ripples, and collected data do not permit discrimination between the two ripple-formation hypotheses. More recently, ref. 39 formed impact ripples in a low-pressure wind tunnel, though only one scale of ripples formed at Mars-like pressures39. In contrast, ref. 40 observed two populations of ripples at intermediate low pressures in the same wind tunnel as ref. 39 when using very fine monodisperse glass beads (90 μm). The larger bedforms were shown to be of aerodynamic origin, and were attributed to a secondary, subscale modulation of the aeolian transport layer. However, only small ripples could form at their lowest pressures, where drag ripples would typically be expected. Furthermore, the formation of drag ripples at ambient pressure, in direct contradiction with decades of previous wind-tunnel experiments and observations in terrestrial deserts, remains largely unexplained. Here, we report on direct observations of ripple formation from a flat bed under low-pressure conditions in the larger Mars Surface Wind Tunnel (MARSWIT) at the NASA Ames Research Center (Methods) and explain discrepancies among wind-tunnel studies38–40. Our observations demonstrate that: (i) large and impact ripples in the MARSWIT grow from two distinct bed instabilities; (ii) the size of large ripples is well predicted by drag-ripple theory; and (iii) sand fluxes associated with large-ripple migration are greater than those implied by impact-ripple theory. Together, our results strongly suggest that large martian ripples are not impact ripples.

Results
Ripples sizes under Earth-to-Mars pressures
Experiments were conducted to let ripples evolve from an initially flat bed of well-sorted, ~195 μm sand-sized proxy sediment (crushed nutshells, Methods) at four atmospheric pressures (and thus, four atmospheric densities) - 1020 mbar (ρa ≈ 1.25 kg/m3), 500 mbar (ρa ≈ 0.6 kg/m3), 100 mbar (ρa ≈ 0.12 kg/m3), and 50 mbar (ρa ≈ 0.06 kg/m3). During each experiment, sediment was supplied at the upwind end of the wind tunnel to sustain transport at capacity, and the freestream wind velocity was set to exceed the threshold for the onset of saltation by ~20%. The bed was allowed to evolve under constant wind conditions and experiments were stopped well after the spacing of bedforms stabilized (Methods).

A single scale of ripples formed under ambient pressure (Figs. 1d and 2a), consistent with observations from Earth’s sandy deserts (Fig. 2a). These ripples are relatively linear and display high lateral continuity (Fig. 1d), with wavelengths of ~5 cm, overlapping with the sizes of impact ripples in natural sand.

In contrast, two distinct scales of ripples formed at lower pressures (Figs. 1c and 2a, b). This observation is consistent with the aerodynamic-ripple model, under which large ripples are only predicted to form over aerodynamically smooth beds (i.e., when the characteristic height of roughness elements, r, is small relative to the thickness of the laminar sublayer; for irregular sand grains, r ≈ n d50, where d50 is the median grain size, and n = 2–341). Such conditions are met when the roughness-scale Reynolds number, Rer = u*r/ν (with u* the wind shear velocity and ν the kinematic viscosity of the atmosphere), is less than ~1015,29. Under our experimental conditions, the transition from aerodynamically rough to smooth conditions occurs between 1020 and 500 mbar (Fig. 2a).

The wavelength of the small-ripple population did not vary significantly or consistently over the range of investigated atmospheric pressures, consistent with previously published experiments in low-pressure wind tunnels38,39 and observed small-ripple sizes on Mars (Fig. 2a). Small variations in small-ripple sizes are attributed to variable wind velocities across experiments42 and experimental noise (Fig. 2a). In turn, the wavelength of large ripples increased with decreasing pressure, from ~29 cm at 500 mbar to ~83 cm at 50 mbar (Fig. 2a). The wavelength of large ripples formed in the wind tunnel are well predicted by the scaling relationship for aerodynamic ripples10 (Equation (10) in Methods) for shear velocity to threshold shear velocity ratios, u*/u*t ≳ 1 (Fig. 2a).

Drag-ripple theory can fully explain the differences between the stability of large ripples in these and previous experiments. First, the thickness of the boundary layer increases with decreasing pressure, limiting the maximum size of large ripples (λ) that could form due to flow confinement15. In the MARSWIT, this occurs at pressures < 50 mbar38 (ρa < 0.06 kg/m3; Fig. 2a). In the smaller wind tunnel used by refs. 39,40, confinement occurs at higher pressure, explaining why drag ripples either did not form at all39 or only formed at intermediate low pressures40. Differences between the experiments of refs. 39,40 result from their use of different sediment types (125 μm quartz grains in ref. 39; 90 μm monodisperse glass beads in ref. 40). At such lower pressures, Mars provides a full-scale experiment, in which large ripples may form without confinement. The wavelength of large dark-toned ripples on Mars4 are also well predicted by the same scaling relationship10 for u*/u*t ≳ 1 (Fig. 2a), consistent with wind speeds measured by rovers43.

Second, the formation of drag ripples at ambient pressure in the experiments of ref. 40 can be explained by the use of fine monodisperse glass beads. Under their experimental conditions, r ≈ d50 = 45 μm, such that Rer ≈ 0.5 < 10, and wind flow was aerodynamically smooth even at Earth pressure.

Third, differences in bedform sizes between this study and those of refs. 38 and 40 are fully expected (Fig. 2b), as bedform size is affected by the transport saturation length22,29, which in turn is affected by sediment properties (95 μm natural quartz in ref. 38; 90 μm monodisperse glass beads in ref. 40; 195 μm crushed nutshells in this study).

Two distinct bed instabilities
Distinguishing between the two competing hypotheses for the formation of large ripples requires to determine whether they arise from the coalescence of smaller ripples or whether they form through their own, distinct bed instability. Incipient bedforms took longer to develop as pressure decreased, but incipient small and large ripples first became visible around the same time at any given pressure, and no ripples of intermediate wavelengths were ever observed (see inset in Fig. 3a; Supplementary Movies 1–4).

As a quantitative test, we calculated the time it would take for small ripples to merge into one large ripple (Tmerg) based on observed bedform volumes and migration rates (Methods), and compared that timescale with the time at which large-ripples reached their equilibrium size (Tobs) in each low-pressure experiment. Large ripples formed much faster than the time it would take for small ripples to coalesce into a large one (Fig. 3a). At all experimental pressure conditions under which large ripples could form, Tobs/Tmerg ≲ 20%. This ratio did not vary appreciably with u*/u*t. These observations demonstrate that small and large ripples arise from distinct bed instabilities, consistent with the aerodynamic-ripple hypothesis29 and in direct contradiction with the impact-ripple hypothesis11.

A large-ripple flux greater than the reptation flux
The total windblown sand flux can be conceptualized as the sum of a saltation (grains with ballistic trajectories) and a reptation (splashed grains) components44. Because saltation trajectories are much longer than the wavelength of impact ripples, the migration of impact ripples only reflects the reptation flux, i.e., the motion of splashed grains along much shorter, collision-driven hops44–46. Conversely, the migration of whole dunes is often used (combined with dune volume) to determine the total sand flux (saltation + reptation)7.

Characterizing sand fluxes on Mars is critical, not just to constrain models of windblown sand and dust transport8, but also to understand the spatiotemporal evolution of sediment deposition on the planet47,48 and assess any potential hazard to future human explorers49. To quantify windblown sand fluxes, many studies have assumed that large martian ripples were impact ripples, and thus that their migration rate can be used to calculate the reptation flux7,8,50,51.

Bedform-specific fluxes (ϕ) observed in our low-pressure wind tunnel experiments provide a direct test to the assumption that reptation alone is responsible for large-ripple migration. The flux associated with the migration of small ripples relative to the total sand flux (Qt) remains approximately constant (ϕ/Qt ≈ 10%) for investigated u*/u*t values regardless of pressure (Fig. 3b), consistent with the theoretical models of refs. 44,52,53 and existing field and experimental measurements of the reptation flux54. In contrast, the large-ripple flux constitutes a much larger fraction of the total sand flux (ϕ/Qt ≳ 40%), and that fraction decreases with increasing u*/u*t (Fig. 3b).

The observed decrease in the large-ripple specific ϕ/Qt with increasing u*/u*t reflects the decreased contribution of saltation to the large-ripple flux. As atmospheric density decreases, the typical length of saltation trajectories develops a longer tail as u* increases55, i.e., a larger fraction of all saltating grains experiences long trajectories (relative to the mean) under high u*. In parallel, the wavelength of aerodynamic ripples is expected to slightly decrease with increasing u*10 (Equation (10)). Therefore, as u*/u*t increases, more saltating grains experience longer trajectories than one large-ripple wavelength, and thus can bypass individual large ripples. These grains thus do not contribute to the migration of that large ripple, resulting in an overall large-ripple flux that is lower than the total sediment flux (saltation + reptation). Like saltating grains, splashed grains also display a range of trajectories56, though even the longest reptation lengths on Mars are not thought to exceed ~1 m53.

Implications
Impact ripples and large ripples arise from distinct bed instabilities, and the sediment flux associated with the migration of large ripples is greater than the reptation flux. Thus, large ripples cannot be impact ripples. Instead, their first appearance in low-pressure wind-tunnel experiments for Rer < ~ 10 is consistent with the theory for aerodynamic ripples29. Furthermore, their wavelength is well predicted by theory developed for subaqueous current ripples22, adjusted for sand transport in saltation10 (Equation (10)). Experimental results thus point towards large martian ripples being aerodynamic ripples, analogous to current (or drag) ripples and windblown sand dunes4,15,29,30.

An aerodynamic origin of large martian ripples has fundamental implications for interpretations of planetary sedimentary records, palaeoatmospheric reconstructions, and the characterization of modern surface environments across planetary bodies.

First, it underscores the utility of large ripples and their stratification, as powerful indicators of atmospheric density. Notably, the observation of decimeter-thick cross-sets formed by meter-scale ripples in fine-grained aeolian sandstones across Mars indicates deposition under a thin, modern-like atmosphere as early as 3.8 billion years ago4,48. Such conditions may indicate an early permanent loss of Mars’ atmosphere, or possibly, that the ripples formed during an episode of atmospheric collapse. The latter scenario is consistent with evidence for repeated wet-to-dry environmental transitions57,58, and notably, with meter-scale windblown ripples bounded by subaqueous sedimentary rocks in Gale crater31. Drag ripples may also serve as precursors or templates for the formation and growth of coarse-grained ripples and transverse eolian ridges (TARs)50,59,60. Thus, some coarse-grained ripples and TARs might also have preserved information of Mars’ atmospheric history.

Second, the wavelength of large ripples in the modern martian environment can be used, provided robust estimates of atmospheric density and grain size, to infer u*10. In places where wind-speed measurements exist, this allows for the determination of u*—one of the most critical yet poorly constrained parameters needed to model windblown sand and dust dynamics on modern Mars37,39,61–63.

Third, our results challenge the use of large-ripple migration as a proxy for the reptation flux, as is currently standard practice in Mars sand-flux studies. Whereas large-ripple migration rate offers a robust upper bound for the reptation flux, it may significantly overestimate it. Instead, coarse-grained ripples50,60 might emerge as more reliable targets for reptation-flux estimates, particularly in regions where grain-sized distributions are well-characterized.

Finally, aerodynamic ripples are expected to form on any planetary body where (possibly episodic or transient) atmospheric flows trigger the transport of loose regolith under aerodynamically smooth conditions15 such as comets34, perhaps Pluto64, and, possibly, Earth40. These previously unknown aerodynamic windblown ripples are thus the key to decipher records of past and modern environmental conditions across the Solar System.

Methods
Experimental conditions
Experiments were conducted in the Mars Surface Wind Tunnel (MARSWIT) of the Planetary Aeolian Laboratory at the NASA Ames Research Center, Mountain View, California. The MARSWIT is hosted in a pressure chamber that can be operated under Earth-to-Mars pressure conditions (~1 bar to ~5 mbar). The MARSWIT is a 13-m long, open-circuit, atmospheric boundary layer wind tunnel. It can generate maximum wind speeds of 100 m/s at 5 mbar using air injectors within the tunnel. Operations of the MARSWIT are performed using the LabView data acquisition software, which measures and provides chamber temperature, air density, pressure, freestream and (vertical) differential pressures, pitot tube vertical position, percent relative humidity, and corresponding wind speeds (freestream and at pitot tube height).

Building on decades of sediment-transport experiments in wind tunnels62,65–67, we use crushed nutshells (ρs = 1300 kg/m3). Nutshell grains are irregular, with shapes akin to those of subangular basaltic or quartz grains67. They thus provide a better proxy to natural sand than spherical glass beads owing to their more realistic frictional properties.

Nutshells were not used to mimic the effect of Mars’ lower acceleration of gravity (g = 3.71 m/s2) on saltation trajectories68 but to decrease the time required to generate bedforms under our lower experimental pressure conditions (50-100 mbar). Approximating ripple formation time as t = λ/(2Q) (using the scaling relationship of ref. 10 for drag ripples and assuming a triangular ripple cross-section with h ≈ 0.05λ), where Q = Qs (Equation (6)) is volumetric sediment flux per unit width, we find that t increases with ρa. Regardless of the assumed flux law and bedform geometry, this qualitative result holds because Q decreases with increased threshold shear velocity, whereas the influence of sediment density on drag-ripple size is comparatively weak. Thus, using nutshells results in decreased ripple-formation times relative to, e.g., using quartz sand.

Values of pressure, air density, and temperature were measured during each experiment, and a dynamic viscosity of 1.81 × 10−5 Pa·s was assumed for air. Grain-size distributions were measured using a Retsch Camsizer grain-size analyzer (d16 = 150, d50 = 195, d84 = 230 μm).

The sediment bed was approximately 6.5 m long, 1.3 m wide, and located downwind of a gravel-roughness plate to trip the boundary layer. An additional downwind area (2.5 m long and 1.3 m wide) of the test section was also covered by sediments (Supplementary Fig. S1). Sediment thickness was initially 3 cm across the wind tunnel, and a sediment berm was placed at the upwind end of the bed to act as a continuous sediment source. Bed and berm evolution was monitored during the entirety of experiments using high-resolution Infrared (IR) cameras. Experiments were paused, and the berm was resupplied well before it ran out of sediment to ensure continuous sediment supply to the test section, such that saltation was at capacity throughout the duration of each experiment.

Two series of experiments were conducted. First, the threshold freestream and shear (u*) velocities were determined for the specific sediments used here over a flat bed at 50, 100, 500, and 1020 mbar (see below and Supplementary Fig. S2). Second, ripples were allowed to evolve from initially flat beds at 50, 100, 500, and 1020 mbar, under freestream wind speeds that exceed the flat-bed threshold (freestream) velocity, as determined from the first series of experiments (Supplementary Fig. S2). To complete the latter series, several runs (and thus pumpdowns) were sometimes required for a given atmospheric pressure condition to resupply the berm and ensure beds remained depositional while bedforms equilibrated. A single pumpdown was required for each of the 1020 and 500 mbar experiments, two at 100 mbar, and three at 50 mbar.

Shear velocity (u*) and threshold shear velocity (u*t)
The threshold shear velocity, u*t, was defined as the onset of continuous transport of grains throughout the wind tunnel62.

In both flat and (equilibrium) rippled bed configurations and under each investigated pressure condition, vertical wind-velocity (u(z)) profiles were measured using a variable-height pitot tube at a frequency of 0.3 Hz. Wind velocity data were collected at 4-5 different vertical positions (z) for 45–60 seconds at each position. These measurements were used to calculate the threshold shear velocity (flat-bed series) or shear velocity (with equilibrated ripples) using the law of the wall
u(z) = (u*/κ) ln(z/z0), (1)
where z0 is the aerodynamic roughness length and κ ≈ 0.41 is the von Kármán constant. Values (and uncertainties) of u*t, u*, and z0 are reported in Table S1 (Supplementary Information) for each pressure condition.

Estimation of boundary layer thickness
The thickness of the boundary layer, δ, was estimated from the wind profile, with u(z=δ) = 0.99 U∞, where u is wind velocity, U∞ is free stream wind velocity (measured about 50 cm above the bed), and z is height above the bed.

Bedform dimensions
High-resolution infrared cameras recorded the evolution of bedforms continuously through a top-down view of the test section, with each test concluding after the bedforms ceased growing (Supplementary Movies 1–4). These recordings were used to measure the wavelength of incipient ripples, their celerity (or migration rate; Supplementary Fig. S3), and their final (stabilized) states. At the end of each experiment and after winds stopped, the pressure chamber was reset to ambient pressure, allowing further documentation of the final bed state through three-dimensional (3D) scans and high-resolution color imaging (Fig. 1c, d; Supplementary Fig. S4). 3D scans were acquired with an iPad Pro with Pix4Dcatch software, and data were processed using MeshLab.

3D scans were used to document equilibrium bedforms and measure their dimensions along downwind-oriented topographic profiles (crest-to-crest wavelength and trough-to-peak height) under consistent lighting conditions. Scanning resolution was not sufficient to resolve the height of small ripples, so they were calculated as hi = αλi, where α ≈ 5%18 and λi is the measured wavelength of small ripples. Calculated small-ripple heights span ~1–3 mm, consistent with ripple-height estimates of ~1.6–1.8 mm from final bed states at 50 mbar using shadow lengths and lighting geometry (Supplementary Fig. S5). The transient evolution of bedforms was documented through measurements of bedform wavelength in individual frames from IR camera recordings, at regular time intervals. Bedform dimensions are reported in Table S1 (Supplementary Information) for each pressure condition.

Bedform migration rates
High-resolution IR videos were used to calculate bedform migration rate, or celerity, c, as
c = λ / Δtλ, (2)
where Δtλ is the time it takes for a bedform crest to migrate a distance equivalent to one bedform wavelength. The migration rates of all bedform scales were determined using this approach under all pressure conditions, with the exception of small ripples at 500 mbar, for which the quality of the IR recording did not permit tracking individual small ripples through time. Instead, the migration rate of small ripples (ci) at 500 mbar was calculated following ref. 39 as
ci = 0.098 u* (ρa/ρs) (u*/u*t)2 (1 - u*t/u*), (3)
where ρs is sediment density (Supplementary Fig. S3). We verified the robustness of Equation (3) by comparing its predictions at 50, 100, and 1020 mbar with measured small-ripple migration rates. Predictions match observations within ~17% (Supplementary Fig. S6); an uncertainty of ~17% was thus applied to the calculated value of ci at 500 mbar. Bedform migration rates are reported in Table S1 (Supplementary Information) for each pressure condition.

Theoretical timescale of large-ripple formation through coalescence (Tmerg)
The volume (per unit width) of small and large ripples are estimated as λ2h (triangular cross-sections), such that the number, N, of small ripples that need to merge to form a large ripple is given by
N = (λl2 hl) / (λi2 hi), (4)
where subscripts l and i denote large and impact (small) ripples, respectively. Assuming that the migration rate of large ripples is negligible relative to that of small ripples, the time required to form a large ripple from the coalescence of small ripples (Tmerg) is given by
Tmerg = N (λi / ci), (5)
where ci is the migration rate of small ripples. Equation (5) (Methods) neglects the migration of transient large ripples and thus provides a conservative lower bound on Tmerg.

Total and bedform-specific sand fluxes
The total sand flux per unit width, Qt, is calculated following ref. 69 as
Qt = C (ρa/g) u*t (u*2 - u*t2), (6)
with C = 5. A sensitivity analysis (not shown here) revealed that sand flux values, Q, provided by two alternative mass-flux formulations (65,70) fall within the uncertainty range of values calculated with the formulation of ref. 69.

For both small and larger ripples, bedform-specific fluxes per unit width are calculated as
ϕ = c h / 2, (7)
where c and h are the migration rate and trough-to-crest height of either small or large ripples.

Uncertainty in Qt was quantified using ± one standard deviation in u* and u*t. Uncertainty in c was quantified using the 68% confidence intervals of measured bedform-specific λ (from Eq. (2), Methods).

Scaling relationship for the wavelength of aerodynamic ripples
The equilibrium wavelength of drag ripples (e.g., subaqueous current ripples or aerodynamic aeolian ripples) is controlled by the sediment-transport saturation length, lsat, and the viscous length scale, ν/u*15,22,29. In dimensionless form, it was shown that
λ* / lsat = f(ν / (u* lsat)), (8)
where λ* = λ ν / u*4,10,22. Following ref. 29, we assume that
lsat / d ≈ (ρs/ρa) (u*2 / (Rg)), (9)
for transport in saltation. Combining Eqs. (8) and (9) (Methods), ref. 10 showed that the wavelength of aerodynamic ripples can be estimated as
λ ≈ 600 (ρs/ρa) (ν2 / (Rg)) (u*t / u*)2, (10)
where R = (ρs - ρa)/ρa is the specific submerged density of the sediment. For Mars conditions, values of ρs = 2900 kg/m3, g = 3.71 m/s2, and μ = ρaν = 1.3 × 10−5 Pa·s were used.

Data availability
The 3D scanning data generated in this study are available in the Zenodo database under accession code https://doi.org/10.5281/zenodo.14567698. The experimental parameters and derived measurements generated in this study are provided in the Supplementary Information file. Source data are provided with this paper.

Code availability
Code to reproduce the data shown in Figs. 2 and 3 is available in the Zenodo database under accession code https://doi.org/10.5281/zenodo.14567698.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "See the also attached files because not all the symbols appear correctly below.\nReview of manuscript NCOMMS-24-51001-T\nOn Earth, ordinary impact ripple wavelengths are typically below 30 cm, corresponding to crest heights less than 1 cm, while\nmegaripples can be considerably larger but are armored with coarse grains. However, on Mars, ripple-like bedforms can be\nsmall or large, both in wavelength and height, while their crests lack very coarse grains. It was argued that these two types of\nsmall and large ripples correspond to distinct size distributions, since intermediate transitional bedforms, roughly in the 20–\n80 cm range, seem to be lacking. Which could be indicative of two distinct formation mechanisms. According to this theory,\nthe large ripples are fluid (or wind) drag ripples, which are similar in their morphology to subaqueous ripples. An interesting\ncontinuous transition between these enigmatic large ripples and megaripples has been observed on Mars by analyzing\nHiRISE images (Zimbelman, 2019), adding a new perspective to the ongoing debate on their formative mechanism (Duran\nVinent et al., 2019; Sullivan et al., 2020; Lorenz, 2020; Yizhaq et al., 2021; Lapotre et al., 2021). It also remains unexplained\nwhy small ripples are found to be superimposed on the larger meter-scale ripples. Thus, our understanding of the formation\nof aerodynamic ripples is not complete and also it is not known why they are very rare on Earth.\nThe current manuscript \"Mars’ large windblown ripples are not impact ripples\" by Dr Alvarez and colleagues report results of\nlow-pressure large wind tunnel experiments that show another experimental evidence that the large martian ripples are\naerodynamic ripples and not impact ripples as shown recently in Yizhaq et al., 2024 (published in Nature Geoscience). I\nthink that the work is an important contribution for the understanding of the large ripples, but I have many concerns which I\nlisted below. My recommendation is major revision.\n1. The experiments have been done with walnut shell with d50 =195 micron and density 1300 kg/m3. It is written in lines\n233-235 that the reason to use the walnut shell is not to mimic the low gravity of Mars but to decrease the time for ripple\ndevelopment. This is not clear, are any experiments with regular sand have been done to confirm this claim? How this low\ndensity can explain the large martian ripples formation where the grain density is 3000 kg/m3 and the grain size is about 100\nmicron? What is the shape of this walnut shell? How these low density grains behave in the wind tunnel?\nI think that is very important to confirm the development large ripples with regular sand or glass beads.\n2. The recent paper published in Nature Geoscience (Yizhaq et al., 2024) is systematically overlooked in this work in several\nplaces. Mainly, this already published paper clearly showed that the large martain ripples are aerodynamic ripples and not\nimpact ripples both in boundary layer and low-pressure wind tunnel (in Aarhus university in Denmark) with 90 micron glass\nbeads. A subscale saturation length has been suggested to explain the hydrodynamic instability. Nothing from these results\nare mentioned in the current work, and it looks like the authors find these two scale ripples at the first time and this is not\ncorrect (e.g. line 78).\nThus the results of Miller et al., 1987 and Yizhaq et al., 2024 should be incorporated into Figure 2 to show the how the\nresults of the current study fit the other two works where aerodynamic ripples were observed.\n3. Line 108-110. Note that in the experiments in the low pressure wind tunnel in Yizhaq et al., 2024 the impact ripple\nwavelength increases for lower pressure (Table S2 in the supplementary information. The experiments done by Andreotti et\nal.,2021 were probably too short to find this trend.\n4. It is not clear why the large ripples did not develop for atmospheric pressure? The authors related it to the roughness-\nscale Reynolds number that should be ~ 10. They have to show the exact calculation. How was r measured? Then I would\ntry to find the exact pressure for the transition from rough regime to smooth regime between 1020 and 500 mbar. Such\nprocedure will be a more convincing proof for their hypothesis regarding this transition.\n5. Did the impact ripples stable for all wind velocities in Yizhaq et al., 2024 they disappear for high wind velocity?\n6. In lines 50-52 it is written that large ripples often develop sinuous crestlines and steep downwind faces or slip faces. Is\nthere any data for the large ripple sinuosity or documentation of slip faces during the experiments?\n7. Line 118- It is not clear how the thickness of the boundary layer was estimated?\n8. In Figure 3, the dashed line is based on only three points so the fit is not convincing.\n8. Why Eq. 6 used to calculate the sand flux? There are many other formulas that are used in aeolian studies. Here more\nexplanation is needed.\n9. What is the l_sat according to Eq. 9? and how it is related to the formation of the hydrodynamic ripples? Also Eq. 10\nshould be calculated for the wind tunnel experiments and not for martian conditions as the exact shear velocities are not\nexactly known.\nMinor comments:\n1. Indicate the wind direction in the panels of Fig. 1\n2. In the caption of Fig. 2 define the meaning of and\n3. In the caption of Fig.3 define .\n4. The videos are not so clear (why they are green?).\n5. More minor comments to the supplementary information in the annotated pdf.", "reviewer_2": "This is a very well-written, concise, and important contribution to the field. The data and theory are sound, and are presented\nwell. The arguments are clear, the data are persuasive, and the conclusions are conservative yet significant. I am struggling\nto find flaw with anything but the most minor writing points of the paper.\nPlease see the attached document with comments and the annotated manuscript.", "reviewer_3": "Overview of the paper\nIn this manuscript, the authors address the highly debated question of the nature of Martian large meter-scale ripples, whose\ngrowth mechanism was so far unclear. To this aim, they conducted a series of wind-tunnel experiments with various\npressure conditions. Starting from a flat bed under low-pressure conditions, they were able to observe incipient ripples of two\ndistinct wavelengths. From the estimation of sand fluxes and bedform velocities, they infer that the large ripples do not form\nfrom the coalescence of smaller impact ripples, as initially presumed. Instead, they result from a flat bed instability, much like\nsubaqueous ripples and transverse aeolian dunes. Therefore, their wavelength is expected to be controlled by the saturation\nlength for saltation and by the viscous length.\nGeneral comments\nThe overall presentation is clear and pleasant to read. Most importantly, the demonstration and the materials provided are\nconvincing and should greatly contribute to resolve the current controversy.\nDetailed comments\nThere are a few minor concerns, mainly in the method part, that could be addressed for consistency. However, this should\nhave no impact on the main results.\n• P. 6, lines 171, 185 : Please remove the reference to “Equation 10” or make clear that it is not in [10].\n• P. 8 lines 215-218 : The description of the online content is missing.\n• P. 9, lines 222-223 : The given density (1300 kg/m3) seems unrealistic for wallnut shells (when compared to water), but it\ncould match that of macadamia. If the authors are unsure, they could be less specific and simply replace every occurrence of\n“wallnut shells” by “nutshells”.\n• P. 11, line 322 : The given reference [33] seems irrelevant here.\n• P. 12, eq. (9) : According to the given references [22] and [29], this relationship is likely not an equality but rather some\nrough proportionality like in equation (8). Since there is no theoretical consensus yet, I would suggest that the authors be\nmore cautious in this part. One could further check that the expression on the right-hand side gives an unrealistic value for\nthe saturation length of typical sand saltation at Earth atmospheric pressure."}, "author_response": "Response to Reviewers for\n“Mars’ large windblown ripples are not impact ripples”\nby Alvarez et al.\nUnless otherwise noted, all cited line numbers refer to the revised manuscript (without changes\ntracked). All reviewer comments were addressed point by point and in full, as detailed in our\nresponses below.\n---------------------------------------------------------------------------------------------------------------------\nReviewer #1: Anonymous\n---------------------------------------------------------------------------------------------------------------------\nOn Earth, ordinary impact ripple wavelengths are typically below 30 cm, corresponding to crest\nheights less than 1 cm, while megaripples can be considerably larger but are armored with coarse\ngrains. However, on Mars, ripple-like bedforms can be small or large, both in wavelength and\nheight, while their crests lack very coarse grains. It was argued that these two types of\nsmall and large ripples correspond to distinct size distributions, since intermediate transitional bedforms,\nroughly in the 20–80 cm range, seem to be lacking. Which could be indicative of two distinct\nformation mechanisms. According to this theory, the large ripples are fluid (or wind) drag ripples,\nwhich are similar in their morphology to subaqueous ripples. An interesting continuous transition\nbetween these enigmatic large ripples and megaripples has been observed on Mars by analyzing\nHiRISE images (Zimbelman, 2019), adding a new perspective to the ongoing debate on their\nformative mechanism (Duran Vinent et al., 2019; Sullivan et al., 2020; Lorenz, 2020; Yizhaq et\nal., 2021; Lapotre et al., 2021). It also remains unexplained why small ripples are found to be\nsuperimposed on the larger meter-scale ripples. Thus, our understanding of the formation of\naerodynamic ripples is not complete and also it is not known why they are very rare on Earth.\nThe current manuscript \"Mars’ large windblown ripples are not impact ripples\" by Dr Alvarez and\ncolleagues report results of low-pressure large wind tunnel experiments that show another\nexperimental evidence that the large martian ripples are aerodynamic ripples and not impact ripples\nas shown recently in Yizhaq et al., 2024 (published in Nature Geoscience). I think that the work is\nan important contribution for the understanding of the large ripples, but I have many concerns\nwhich I listed below. My recommendation is major revision.\n>> We thank the Reviewer for recognizing the importance of our work and for their detailed review\nof our manuscript. The points raised by the Reviewer helped us to not only enhance the impact of\nour work but also to give a better account of previous work on the subject. Our responses to each\ncomment and suggestion are detailed below.\nThe experiments have been done with walnut shell with d50 =195 micron and density 1300 kg/m3.\nIt is written in lines 233-235 that the reason to use the walnut shell is not to mimic the low gravity\nof Mars but to decrease the time for ripple development. This is not clear, are any experiments\nwith regular sand have been done to confirm this claim?\n>> We thank the Reviewer for identifying this shortcut in the original manuscript. Experiments\nwith natural sand were in fact conducted by Miller et al. (1987), though they unfortunately did not\n1\nreport ripple formation times. When designing the experiments, we instead calculated formation\ntimes from first principles.\nWe first calculated the expected ripple size (ripple wavelength, 𝜆, and height, h) using the scaling\nrelationship of Lapôtre et al. (2021) and assuming a typical bedform aspect ratio of h/𝜆\n~0.05. Second, we estimated the volumetric sand flux, 𝑄, using the formulation of Martin & Kok\n(2017). The bedform restitution time, defined as bedform volume divided by volumetric sand flux,\nwas used as a proxy for bedform formation time and approximated as 𝑡 = ℎ𝜆 /2𝑄 (assuming a\ntriangular ripple cross-section).\nWe note that the details of this calculation (e.g., flux law or assumed bedform geometry) do not\nalter the qualitative argument as long as they remain the same regardless of sediment density – a\nreasonable assumption within the relatively small range of investigated densities. Denser grains\nresult in reduced volumetric flux (notably through increased threshold wind shear velocity),\nwhereas bedform size only weakly varies with sediment density. As a result, stable ripples are\nexpected to take a longer time to develop in natural, denser sand than in nutshells.\nSuch considerations are critical in designing MARSWIT experiments because, whereas this was\nnot the case at the time Ron Greeley and others used the facility, pump downs can only last a\nlimited amount of time (~30 mins) nowadays, only two pump downs can reasonably be conducted\nin a week, and each costs $1,500. [A pump down refers to the operation of pumping the air out of\nthe entire facility to reduce the pressure from Earth’s to its desired value.]\nOur rationale for using nutshells is now better justified at l. 264-271: “Approximating ripple\n!\" \nformation time as 𝑡 = (using the scaling relationship of Ref. [10] for drag ripples and assuming\n#$ \na triangular ripple cross-section with\n\" \n≈ 0.05), where 𝑄 = \n$! \n(Equation. 6) is volumetric\n! %\" \nsediment flux per unit width, we find that 𝑡 increases with 𝜌 . Regardless of the assumed flux law\n& \nand bedform geometry, this qualitative result holds because 𝑄 decreases with increased threshold\nshear velocity, whereas the influence of sediment density on drag-ripple size is comparatively\nweak. Thus, using nutshells results in decreased ripple-formation times relative to, e.g., using\nquartz sand.”\nHow this low density can explain the large martian ripples formation where the grain density is\n3000 kg/m3 and the grain size is about 100 micron?\n>> Our experiments do not seek to reproduce Mars-like conditions, but rather, to investigate the\nphysics of bedform formation through controlled experiments. In fact, our lowest experimental\npressure was 50 mbar, or 5-10 times higher than typical surface pressures on Mars. Again, the\nnature of our chosen sediments was purely selected on logistical grounds. We note, however, that\nnutshells are arguably a better proxy to natural sand than spherical glass beads owing to the more\nirregular shape of the grains, which produce more realistic solid frictional (and bed roughness)\nproperties (e.g., Leach et al., 1989).\n>> They are irregular grains with shapes that are “very much like that of slightly worn basalt or\nquartz particle[s]” (Leach et al., 1989). We now discuss this point at l. 257-260.\nHow these low density grains behave in the wind tunnel?\n>> We are unsure about the specific aspects of grain behavior referred to by the Reviewer, but\nnutshell grains follow the same laws as any other solid grains, and have been extensively used in\nprevious flume and wind tunnel studies to investigate aspects of sand transport and bedform\nformation, including at the MARSWIT (e.g., White, 1979; Greeley et al., 1980; Leach et al., 1989,\nSwann et al., 2020). This point is now made explicitly at l. 256-257.\nWhereas they follow the same physics, saltation height and length in nutshell are expected to differ\nfrom those in basaltic sand, but that is not a problem unless one’s goal is to reproduce Mars-like\nconditions.\nI think that is very important to confirm the development large ripples with regular sand or glass\nbeads.\n>> We agree with the Reviewer that the influence of grain properties is an interesting question in\nitself, though we contend that the development of large ripples in regular sand or glass beads does\nnot need to be confirmed anymore at this stage. They are observed in natural sand on Mars, and\nwere observed on Earth in wind tunnels, both in quartz (Miller et al., 1987) and in glass beads\n(Yizhaq et al., 2024).\nWhereas Miller et al. (1987) were able to use natural grains, they again had complete access to and\ncontrol of the MARSWIT, and thus were able to maintain low-pressure conditions for extended\nperiods. Furthermore, bed roughness, form drag, and collisional dynamics in glass beads differ\nsignificantly from those involving irregular grains (Loth, 2008; Staudt et al., 2019). Nutshells thus\noffer an optimal and practical alternative, and their use does not put into question the repeatability\nof our qualitative observations in natural sand.\n2. The recent paper published in Nature Geoscience (Yizhaq et al., 2024) is systematically\noverlooked in this work in several places. Mainly, this already published paper clearly showed that\nthe large martain ripples are aerodynamic ripples and not impact ripples both in boundary layer\nand low-pressure wind tunnel (in Aarhus university in Denmark) with 90 micron glass beads.\nA subscale saturation length has been suggested to explain the hydrodynamic instability. Nothing\nfrom these results are mentioned in the current work, and it looks like the authors find these two\nscale ripples at the first time and this is not correct (e.g. line 78)\n>> We are grateful to the Reviewer for pointing out this shortcoming. It was never our intention\nto overlook the important findings of Yizhaq et al. (2024) or to imply our experiments were the\nfirst ones to produce two scales of ripples. We apologize that it came across that way. As a\ntestament to our good faith, we’d like to point out that our initial manuscript did cite Yizhaq et al.\n3\n(2024), and that it was explicitly stated that Miller et al. (1987) had generated large ripples in the\nMARSWIT before: “Large ripples have been observed in low-pressure wind tunnel experiments\nbefore [38], but these experiments long predate the discovery of large martian ripples, and the\ncollected data do not permit discrimination between the two ripple-formation hypotheses.”\nThe Yizhaq et al. (2024) is now cited and discussed at more length at lines [76, 82-89, 92, 126-\n147, 241]. There are several aspects of the study of Yizhaq et al. (2024), however, that are not fully\nresolved (some of which are directly acknowledge in the paper), which together may cast doubt\non the fact that Yizhaq et al. (2024) “clearly showed that the large Martian ripples are aerodynamic\nripples and not impact ripples.”\nFirst, they observed two sizes of ripples at ambient pressure, which contrasts with decades of wind-\ntunnel studies conducted at ambient pressure and that only reported the formation of small ripples.\n[We believe we understand why, see our response to another point below.] Furthermore, whereas\nit is possible that such aerodynamic ripples may form in natural settings on Earth, they have not\nbeen widely identified or recognized to date.\nSecond, they only observed one population of ripples (small) at their lowest atmospheric pressure\nconditions (100 and 16 mbar). We believe that the height of the wind tunnel at Aarhus, which\naffects the thickness of the boundary layer, is too small at such low pressures to allow for the\ndevelopment of aerodynamic ripples (see our response to another comment below). Instead, the\nmaximum bedform size that could form was capped by the hampered flow thickness (see, e.g.,\nCourrech du Pont et al., 2024). [For the same reason, aerodynamic ripples did not form below 50\nmbar in the Miller et al. 1987 MARSWIT experiments].\nIn summary, whereas the results of Yizhaq et al. (2024) are important, thought-provoking, and\nimpactful, their unique experimental conditions promoted the formation of two ripple scales at\nambient pressure, contrary to decades of previous observations, while preventing the formation of\nlarge ripples at lower pressures, where they are expected.\nFinally, their proposed subscale saturation length mechanism, whereas extremely interesting,\nremains a hypothesis that has not been validated to date. Unfortunately, our observations do not\npermit to test this hypothesis. We again apologize for omitting to discuss this hypothesis; it is now\nreferenced at l. 84-85.\nThus the results of Miller et al., 1987 and Yizhaq et al., 2024 should be incorporated into Figure 2\nto show the how the results of the current study fit the other two works where aerodynamic ripples\nwere observed.\n>> We fully agree with the Reviewer and modified Fig. 2 to incorporate the results from these two\nstudies into Fig. 2b. Note that data scatter is expected in this parameter space because bedform\nsize is also influenced by the transport saturation length, which differs across the presented cases\n(notably through the use of different particle types). We now discuss Fig. 2b at l. 143-147.\n3. Line 108-110. Note that in the experiments in the low pressure wind tunnel in Yizhaq et al.,\n2024 the impact ripple wavelength increases for lower pressure (Table S2 in the supplementary\n4\ninformation. The experiments done by Andreotti et al.,2021 were probably too short to find this\ntrend\n>> This is indeed an interesting point, though we do not believe that the increase in impact-ripple\nwavelength with decreasing pressure is representative of what would happen in unconfined flow\nconditions, for a couple of reasons. First and foremost, impact ripples have similar wavelengths\non Mars than on Earth (e.g., Lapôtre et al., 2016). Second, the calculated impact-ripple wavelength\nat the lowest two pressures of Yizhaq et al. (2024) may have been affected by the formation of\nunstable aerodynamic ripples that could not grow to their equilibrium size due to capping of the\nboundary layer (i.e., their measurements might reflect a mix of true impact ripples and slightly\nlarger unstable aerodynamic ripples).\n4. It is not clear why the large ripples did not develop for atmospheric pressure?\n>> As stated in the manuscript, drag ripples are not expected to form under aerodynamically\ntransitional or rough flows (Duran et al., 2019), which are typical of winds over sandy beds on\nEarth. It is thus not surprising that large ripples did not form at ambient atmospheric pressure.\nWe’d argue instead that the fact that large ripples formed at ambient pressure in the experiments\nof Yizhaq et al. (2024) is the more surprising result.\nFor drag ripples to form under terrestrial conditions, one needs to reproduce aerodynamically\nsmooth flows, with a roughness-scale Reynolds number, Re = (u r/ν) < ~10. Given the low\nr * \nkinematic viscosity of Earth’s atmosphere, this is only possible if the bed is extremely smooth (i.e.,\nr is very small). We believe that this was achieved by Yizhaq et al. (2024) specifically due to their\nuse of glass beads. For a compacted bed of glass beads, r ~ d/2, or ~ 45 μm in their experiments\n(relative to ~ 180-270 μm in similarly sized natural sand). This point is further substantiated below\nin our response to the next comment.\nThe authors related it to the roughness-scale Reynolds number that should be ~ 10. They have to\nshow the exact calculation. How was r measured? Then I would try to find the exact pressure for\nthe transition from rough regime to smooth regime between 1020 and 500 mbar. Such procedure\nwill be a more convincing proof for their hypothesis regarding this transition.\n>> We added further explanation on how to calculate the characteristic height of the roughness\nelements, 𝑟, in the revised manuscript at l. 112–113. To be clear, what the Reviewer suggested is\nexactly how the location of the shaded grey area in Fig. 2 was determined. Our study shows a\ntransition between aerodynamically transitional and smooth flows between 1020 and 500 mbar.\nTo propagate uncertainty on several parameters (r, 𝑢 , etc.), we opted to represent the transition\n∗ \nwith a shaded range rather than by identifying a single value (Fig. 2a in the revised version of the\nmanuscript).\nIn the case of irregular sand grains, 𝑟 ≈ 𝑛 𝑑 , with n = 2-3 (Kamphuis, 1974), and for spherical\n() \n* \nparticles, 𝑟 ≈ (Nikuradse, 1933), where 𝑑 and 𝑑 represent the median grain size and the sphere\n() \n# \ndiameter, respectively. A flow is considered aerodynamically smooth when the roughness\nelements are fully submerged within the viscous sublayer (the innermost layer of the boundary\nlayer) and 𝑅𝑒 < ~ 10. Conversely, when 𝑅𝑒 > ~ 10, the roughness elements protrude beyond\n+ + \nthe viscous sublayer, and the flow is considered aerodynamically transitional or rough - as is the\n5\ncase of sandy deserts on Earth (Courrech du Pont, et al., 2024; Durán et al., 2019). In Table 1\nbelow, we compare the flow regimes at different atmospheric pressures in our study with those in\nYizhaq et al. (2024).\nTable 1. Parameters used to determine the flow conditions in this study and in the study by Yizhaq\net al. (2024).\npressure 𝑑 (m) 𝑟 (m) 𝜈/𝑢 (m) 𝑅𝑒 flow\n∗ + \n(mbar) condition\n1020 3.71 ×10-5 13.15 transitional\nthis\nstudy 500 5.21 × 10-5 9.36 smooth\n195 × 10-6 4.88 × 10-4\n100 2.07 × 10-4 2.36 smooth\n50 2.98 × 10-4 1.64 smooth\n1020 8.10 ×10-5 0.56 smooth\n500 1.46 × 10-4 0.31 smooth\nYizhaq\net al. 250 90 × 10-6 45.0 × 10-6 2.08 × 10-4 0.22 smooth\n(2024)\n100 3.32 × 10-4 0.14 smooth\n16 8.44 × 10-4 0.05 smooth\n5. Did the impact ripples stable for all wind velocities in Yizhaq et al., 2024 they disappear for\nhigh wind velocity?\n>> Only one velocity was investigated at each pressure in our experiments. The freestream wind\nvelocity was set to exceed the threshold for the onset of saltation by approximately 20% (as stated\nin the manuscript). In turn, the threshold velocity for the onset of saltation was determined for\npressures ranging from ~5 to 1020 mbar (Fig. S2 in Supplementary Information).\nWe could not increase the velocity once rippled had formed because we wanted to preserve the\nbed conditions for conducting both three-dimensional scanning and high-resolution imaging of the\nbed after each experiment (once pressure in the chamber at re-equilibrated with Earth’s\natmosphere). Thus, investigating a second wind velocity would have implied doubling the number\nof pump downs, which was not possible within the scope of funding.\n6. In lines 50-52 it is written that large ripples often develop sinuous crestlines and steep downwind\nfaces or slip faces. Is there any data for the large ripple sinuosity or documentation of slip faces\nduring the experiments?\n6\n>> Whereas sinuosity and slipfaces are not defining characteristics of drag ripples (straight linear\ndrag ripples without slipfaces have been observed under bimodal wind regimes on Mars; e.g.,\nLapôtre et al., 2018), those formed in our experiments did develop sinuous crests and slip faces\n(see Figure 1 below). However, we opted not to elaborate further because other ripple\ncharacteristics, such as ripple height, wavelength, and celerity, are more relevant to the study's\nobjective to test the aerodynamic nature of the large ripples.\n7. Line 118- It is not clear how the thickness of the boundary layer was estimated?\n>> We thank the Reviewer for pointing this out. The thickness of the boundary layer, 𝛿, was\nestimated from the wind profile, with u(𝑧 = δ) = 0.99𝑈 , where u is wind velocity, 𝑈 is the\n, , \nfreestream wind velocity, and 𝑧 is height above the bed. This is now discussed at l. 308-311.\n8. In Figure 3, the dashed line is based on only three points so the fit is not convincing.\n>> The dashed line is not a fit to the data for the very reason highlighted by the Reviewer. Instead,\nthe dashed line is only meant to draw the eye to a possible trend. Because this trend is highly\nuncertain, the line is dashed, and a question mark was added next to “decrease”.\nFigure 1. Oblique photograph of a rippled bed after completing a test at 50 mbar (wind from top\nleft to bottom right). Note the non-straight nature of the large ripples, and the smooth (without\nimpact ripples) short and steeper ripple faces downwind of large ripple crests. The wind tunnel is\n1.3 m wide, and it is located approximately at the center of the photo in the vertical direction.\n7\n9. Why Eq. 6 used to calculate the sand flux? There are many other formulas that are used in\naeolian studies. Here more explanation is needed.\n>> We thank the Reviewer for making this point. We did conduct a sensitivity analysis that\nrevealed that our conclusions remain unchanged regardless of the flux law used for the three flux\nlaws shown in Table 2 below. We opted to use that of Martin & Kok (2017) for illustrative\npurposes (it is similar to that of Durán et al., 2011, also used in Ayoub et al., 2014). Figure 2 below\ncompares calculated 𝑄 using those three formulations – all falling within the range of dispersion\n- \nof our measurements. This is now discussed in lines 363–365.\nAuthor Equation 𝐶\n! \nLettau & Lettau (1978) 𝑄 = 𝐶 #! 𝑢& (𝑢&−𝑢& ) 5.0\n\" ! ∗ ∗ ∗\" \n$ \nWhite (1979) 𝑄 = 𝐶 #! (𝑢 −𝑢 )(𝑢 +𝑢 )& 2.61\n\" ! ∗ ∗\" ∗ ∗\" \n$ \n𝜌 𝑢 \nMartin & Kok (2017) 𝑄 = 𝐶 ' ∗\" (𝑢&− 𝑢& ) 5.0\n\" ! 𝑔 ∗ ∗\" \nTable 2. Formulations for estimating sediment mass flux. White (1979) provided the value of the\nconstant, and for Lettau & Lettau (1978), we used the same value as in Martin & Kok (2017).\nFigure 2. Estimated mass flux as a function of the pressure.\n8\n10. What is the l_sat according to Eq. 9? and how it is related to the formation of the\nhydrodynamic ripples?\n>> We are grateful to the Reviewer for asking this question. First, it points at a typo in Eq. 9, where\nthe “equal” sign was supposed to be a “proportional to” sign. We fixed that. Second, we were not\nable to invert for l directly from our experiments because we have fewer data points than\nsat \nunknowns. For that reason, we simply used the relationship of Lapôtre et al. (2021), which assumes\nthe l formulation of Duran et al. (2019) for saltation with constants calibrated using martian\nsat \nripples (yielding a proportionality constant of ~600). The shaded area in Fig. 2a specifically\nreflects this relationship for\n.∗ \n= 1–3.\n.∗! \nAlso Eq. 10 should be calculated for the wind tunnel experiments and not for martian conditions\nas the exact shear velocities are not exactly known.\n>> No wind shear velocity is directly assumed. Rather, we consider a range of\n.∗ \nbetween 1\n.∗! \n(below which no transport could occur) and 3 (which are never observed on Mars; see, e.g.,\nViúdez-Moreiras et al., 2019), and used the appropriate threshold shear velocity (e.g., as measured\nin the wind tunnel for the wind tunnel data).\nMinor comments:\n1. Indicate the wind direction in the panels of Fig. 1\n>> Wind direction is now indicated in the caption to avoid cluttering the figure itself.\n2. In the caption of Fig. 2 define the meaning of and\n>> Done.\n3. In the caption of Fig.3 define .\n>> Done.\n4. The videos are not so clear (why they are green?).\n>> The videos were captured with high-resolution infrared cameras. The quality was enhanced\nframe by frame through a process causing the frames to appear green. This was done to highlight\nthe motion of the small and large ripples. This is explained in the captions for the videos, which\nare included in the Supplementary Information (SI) document.\n5. More minor comments to the supplementary information in the annotated pdf.\n>> All suggestions were adopted.\nSupplementary Materials:\nAt what height the freestream velocity was measured?\n9\n>> The freestream velocity measurements were performed using a pitot tube fixed in the vertical\ndirection and positioned approximately 50 cm above the tunnel’s bottom wall. At this height, the\npitot tube is located outside the perturbed zone, ensuring accurate measurements of the freestream\nwind speed.\nHow the continuous saltation was defined?\n>> The threshold shear velocity, 𝑢 , was defined as the onset of continuous grain transport\n∗- \nthroughout the wind tunnel following the criterion of Swann et al. (2020) under flat-bed conditions\n(l. 297–298). Subsequently, the freestream wind velocity was set to exceed the threshold for the\nonset of saltation by approximately 20% (l. 100–101).\nWhy there is no trend in the small ripple wavelength?\n>> See our response to a previous comment. Importantly, ground observations show that impact\nripples on Mars have similar wavelengths to those observed on Earth, so no such trend is expected.\n10\n---------------------------------------------------------------------------------------------------------------------\nReviewer #2: Anonymous\n---------------------------------------------------------------------------------------------------------------------\nThis is a very well-written, concise, and important contribution to the field. The data and theory\nare sound, and are presented well. The arguments are clear, the data are persuasive, and the\nconclusions are conservative yet significant. I am struggling to find flaw with anything but the\nmost minor writing points of the paper. Please see the attached document with comments and the\nannotated manuscript\n>> We are grateful for the Reviewer’s detailed review and enthusiasm about our results. The points\nraised by the Reviewer allowed us to enhance the significance of our work. Our responses to the\nReviewer’s comments are detailed below.\n1. The drag ripple formation hypothesis has also been proposed for TARs, which I recognize fall\noutside the scope of this paper, but the convergence is heartening.\n>> We too have a strong suspicion that TARs, megaripples, and large ripples are genetically\nrelated. TARs are thought to be polydisperse however (Geissler & Wilgus, 2017) and likely\ninvolve dust deposition, which complicates the story. For this reason, we opted to omit a discussion\nof TARs in the manuscript.\n2. Is this the journal’s standard for referring to another section? Suggest “(see Methods)” or similar.\nAs written, it reads awkwardly after the other capitalized words in the sentence\n>> We checked and this is in keeping with other published papers in the journal.\n3. I suggest either expanding on why walnut shells were used here (rather than making the reader\nwait for the Methods), or simply shortening this to “loose sand proxy material” or similar. On my\nfirst read, the reason walnut shells were chosen was not clear, and it raised distracting questions\nthis early on.\n>> We thank the Reviewer for this suggestion. We incorporated the suggestion at l. 96 and\nextended the explanation for using nutshells at l. 256–260 and 264–271.\n4. This is a very well-written paragraph—thank you\n>> Thank you!\n5. Same question as #2, but here it is less ambiguous.\n>> See above.\n6. Another excellent paragraph—I wish more papers were this clear. The color gradient in Figure\n3. is baffling. If you are going to use color, there are clearer colormaps that are both colorblind-\nfriendly and would better identify your points. If you want something that reproduces well in\ngreyscale ‘viridis’ and ‘magma’ come to mind immediately as options that have more information\nand convert to greyscale see: https://cran.r-project.org/web/packages/viridis/vignettes/intro-to-\n11\nviridis.html#comparison or\nhttps://journals.plos.org/plosone/article?id=10.1371/journal.pone.0199239#.\n>> We thank the Reviewer for their kind words. We updated Fig. 3 using a different color scale.\n7. This is quite a significant conclusion! Feels a bit like burying the lede to have it as number\nthree—but perhaps that’s my own bias showing.\n>> Ordering in the conclusion does not reflect importance but rather mimics the logical\nprogression of the paper. However, we fully agree that this is an important result and that the\noriginal manuscript did not do it justice, so we opted to emphasize the significance of this result at\nl. 232–237.\n8. First, I feel that some discussion of what this study means for aeolian features on Mars more\nbroadly would be welcome here. I understand that the authors likely wish to limit the paper to\nsolely ripples, but dunes and TARs are notably absent in this larger discussion.\n>> We thank the Reviewer for this suggestion. We incorporated the suggestion at l. 223-226\nSecond, the comma after “aerodynamic” separates two adjectives from the third and the noun\nthey’re all referring to, I strongly suggest deleting that comma.\n>> Done!\n9. Is this filler from the paper template?\n>> Correct. We removed this section because it is absent from recent publications of the journal.\n10. I believe you can generate a Zenodo DOI at will, the DOI should be in the paper even if the\nrepositories are still private.\n>> We have now provided DOI information in the revised manuscript.\n11. Looks like your citation manager panicked, likely just need to manually delete these.\n>> Good catch – fixed.\n12\n---------------------------------------------------------------------------------------------------------------------\nReviewer #3: Anonymous\n---------------------------------------------------------------------------------------------------------------------\nOverview of the paper\nIn this manuscript, the authors address the highly debated question of the nature of Martian large\nmeter-scale ripples, whose growth mechanism was so far unclear. To this aim, they conducted\na series of wind-tunnel experiments with various pressure conditions. Starting from a flat bed under\nlow-pressure conditions, they were able to observe incipient ripples of two distinct wavelengths.\nFrom the estimation of sand fluxes and bedform velocities, they infer that the large ripples do not\nform from the coalescence of smaller impact ripples, as initially presumed. Instead, they result\nfrom a flat bed instability, much like subaqueous ripples and transverse aeolian dunes. Therefore,\ntheir wavelength is expected to be controlled by the saturation length for saltation and by the\nviscous length.\nGeneral comments\nThe overall presentation is clear and pleasant to read. Most importantly, the demonstration and the\nmaterials provided are convincing and should greatly contribute to resolve the current controversy.\n>> We thank the Reviewer for their detailed review of our manuscript and their enthusiasm about\nour results. The points raised by the Reviewer helped us to strengthen the impact of our work. Our\nresponses to each of their points are detailed below.\nDetailed comments\nThere are a few minor concerns, mainly in the method part, that could be addressed for consistency.\nHowever, this should have no impact on the main results.\n1. P. 6, lines 171, 185: Please remove the reference to “Equation 10” or make clear that it is not in\n[10].\n>> Ha, it is an unfortunate coincidence that both reference #10 ([10]) and equation #10 are referred\nto here. We believe that following journal formatting conventions consistently, and using\nparentheses and the word “Equation”, should make the distinction clear to the reader.\n2. P. 8 lines 215-218: The description of the online content is missing.\n>> We omitted this section as it appears to be absent in recent publications of the journal.\n3. P. 9, lines 222-223: The given density (1300 kg/m3) seems unrealistic for wallnut shells (when\ncompared to water), but it could match that of macadamia. If the authors are unsure, they could be\nless specific and simply replace every occurrence of “wallnut shells” by “nutshells”.\n>> In the revised manuscript, we changed walnut shells to nutshells throughout.\n13\n4. P. 11, line 322: The given reference [33] seems irrelevant here.\n>> Reference [33] was removed from this sentence.\n5. P. 12, eq. (9): According to the given references [22] and [29], this relationship is likely not an\nequality but rather some rough proportionality like in equation (8). Since there is no theoretical\nconsensus yet, I would suggest that the authors be more cautious in this part. One could further\ncheck that the expression on the right-hand side gives an unrealistic value for the saturation length\nof typical sand saltation at Earth atmospheric pressure.\n>> We are grateful to the Reviewer for catching this typo. Equation 9 was updated.\n14"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I read the rebuttal and the revised manuscript and I think that the authors have done a very good\nwork in answering all my comments in the revised manuscript.\nI still have the following comments:\n1. The title of the manuscript is a little bit problematic because the wind tunnel experiments have\nnot been done with basaltic grains and that the gravity was not the martian gravity (and also not\nthe pressure). Thus, the wind tunnel experiment results are not a decisive proof that the large\nripples cannot form from merging of small ripples even if it will take more time for\ndevelopment. Thus, I suggest to change the title to:\n\"Mars’ large windblown ripples are probably not impact ripples\" I know that this is a less\nattractive title but since that nobody has seen the formation of the large martian ripples a cation\nis needed.\n2. Line 84 – add …glass beads (90 micron).\n3. Line 89- In Yizhaq et al., 2024 there is a detailed explanation why these ripples can be\nobserved in the wind tunnel in very fine sand.\n4. Fig. 1 I think that the dominant wind direction can be also added to panel (a).\n5. Line 121\nNote that impact ripple wavelength is known to increase with wind velocity\nSee:\nHuo, X., Herrmann, H. J., Zhang, J., & Huang, N. (2024). Mid-air collisions control the\nwavelength of aeolian sand ripples. Journal of Geophysical Research: Earth Surface, 129,\ne2023JF007365. https://doi.org/10.1029/2023JF007365\n6. Line 143. Note that even using 200 micron glass beads drag ripple can be observed under\nambient pressure.\n7. Line 187 –Is phi is the same in the two above ratios? or one is for the impact ripples and the\nother for the large ripples?\n8. Line 226 -Note also the interesting transition between large ripples and megaripples in\nZimbelman, J. R. 2019. The transition between sand ripples and megaripples on Mars, Icarus,\nhttps://doi.org/10.1016/j.icarus.2019.05.017.\n9. Line 379- The question is what saturation length dictates the wavelength of the drag ripples. Is\nit the regular saturation length (Eq. 9) or the small subscale saturation length suggested by\nYizhaq et al., 2024. This is a critical point in our understanding of the formation of the large\nripples. I suggest here to add a table with the calculated wavelengths of the aerodynamic ripples\naccording to Eq. 10 and the measured wavelengths in the wind tunnel experiments.\n10. Table S1. I would also add to the table the kinematic viscosity and the roughness-scale\nReynolds number to give the reader the full information about the experiments."}, "author_response": "Second Response to Reviewers for\n“Mars’ large windblown ripples are not impact ripples”\nby Alvarez et al.\nUnless otherwise noted, all cited line numbers refer to the revised manuscript (without changes\ntracked). All reviewer comments were addressed point by point and in full, as detailed in our\nresponses below.\n---------------------------------------------------------------------------------------------------------------------\nReviewer #1: Anonymous\n---------------------------------------------------------------------------------------------------------------------\nI read the rebuttal and the revised manuscript and I think that the authors have done a very good\nwork in answering all my comments in the revised manuscript.\n>> We are grateful for the Reviewer’s comments and their enthusiasm about the revised version\nof our manuscript. Our responses to the Reviewer’s comments are detailed below.\nI still have the following comments:\n1. The title of the manuscript is a little bit problematic because the wind tunnel experiments have\nnot been done with basaltic grains and that the gravity was not the martian gravity (and also not\nthe pressure). Thus, the wind tunnel experiment results are not a decisive proof that the large\nripples cannot form from merging of small ripples even if it will take more time for\ndevelopment. Thus, I suggest to change the title to:\n\"Mars’ large windblown ripples are probably not impact ripples\" I know that this is a less\nattractive title but since that nobody has seen the formation of the large martian ripples a cation\nis needed.\n>> The title was changed to: “Ripples formed in low-pressure wind tunnels suggest Mars’ large\nripples are not impact ripples”.\n2. Line 84 – add …glass beads (90 micron).\n>> Done.\n3. Line 89- In Yizhaq et al., 2024 there is a detailed explanation why these ripples can be\nobserved in the wind tunnel in very fine sand.\n>> The explanation of Yizhaq et al. (2024) is centered around the subscale saturation length, which\nalthough extremely interesting, remains a hypothesis that has not yet been validated (and\nunfortunately our observations do not allow us to test it).\n4. Fig. 1 I think that the dominant wind direction can be also added to panel (a).\n>> Done.\n5. Line 121\nNote that impact ripple wavelength is known to increase with wind velocity\nSee:\nHuo, X., Herrmann, H. J., Zhang, J., & Huang, N. (2024). Mid-air collisions control the\nwavelength of aeolian sand ripples. Journal of Geophysical Research: Earth Surface, 129,\ne2023JF007365. https://doi.org/10.1029/2023JF007365\n>> We added a sentence noting that the ripple wavelengths may vary with wind speed and cited\nthis paper at l. [122-123].\n6. Line 143. Note that even using 200 micron glass beads drag ripple can be observed under\nambient pressure.\n>> Neat! This matches our expectations because the roughness-scale Reynolds number for 200\nmicron glass beads at ambient pressure still indicates that flow is aerodynamically smooth (Re =\nr \nu r/ν ~1.2).\n* \n7. Line 187 –Is phi is the same in the two above ratios? or one is for the impact ripples and the\nother for the large ripples?\n>> Yes, we use phi as a generic symbol for bedform-specific flux (either impact or large ripples).\nThis is now clarified in the caption of Fig. 3.\n8. Line 226 -Note also the interesting transition between large ripples and megaripples in\nZimbelman, J. R. 2019. The transition between sand ripples and megaripples on Mars, Icarus,\nhttps://doi.org/10.1016/j.icarus.2019.05.017.\n>> This paper is now cited at l 228.\n9. Line 379- The question is what saturation length dictates the wavelength of the drag ripples. Is\nit the regular saturation length (Eq. 9) or the small subscale saturation length suggested by\nYizhaq et al., 2024. This is a critical point in our understanding of the formation of the large\nripples. I suggest here to add a table with the calculated wavelengths of the aerodynamic ripples\naccording to Eq. 10 and the measured wavelengths in the wind tunnel experiments.\n>> Done.\n10. Table S1. I would also add to the table the kinematic viscosity and the roughness-scale\nReynolds number to give the reader the full information about the experiments.\n>> Done.\n1. The drag ripple formation hypothesis has also been proposed for TARs, which I recognize fall\noutside the scope of this paper, but the convergence is heartening.\n2. Is this the journal’s standard for referring to another section? Suggest “(see Methods)” or similar.\nAs written, it reads awkwardly after the other capitalized words in the sentence.\n3. I suggest either expanding on why walnut shells were used here (rather than making the reader\nwait for the Methods), or simply shortening this to “loose sand proxy material” or similar. On my\nfirst read, the reason walnut shells were chosen was not clear, and it raised distracting questions\nthis early on.\n4. This is a very well-written paragraph—thank you!\n5. Same question as #2, but here it is less ambiguous.\n6. Another excellent paragraph—I wish more papers were this clear.\nThe color gradient in Figure 3. is baffling. If you are going to use color, there are clearer\ncolormaps that are both colorblind-friendly and would better identify your points. If you want\nsomething that reproduces well in greyscale ‘viridis’ and ‘magma’ come to mind immediately as\noptions that have more information and convert to greyscale see: https://cran.r-\nproject.org/web/packages/viridis/vignettes/intro-to-viridis.html#comparison or\nhttps://journals.plos.org/plosone/article?id=10.1371/journal.pone.0199239#.\n7. This is quite a significant conclusion! Feels a bit like burying the lede to have it as number\nthree—but perhaps that’s my own bias showing.\n8. First, I feel that some discussion of what this study means for aeolian features on Mars more\nbroadly would be welcome here. I understand that the authors likely wish to limit the paper to\nsolely ripples, but dunes and TARs are notably absent in this larger discussion. Second, the\ncomma after “aerodynamic” separates two adjectives from the third and the noun\nthey’re all referring to, I strongly suggest deleting that comma.\n9. Is this filler from the paper template?\n10. I believe you can generate a Zenodo DOI at will, the DOI should be in the paper even if the\nrepositories are still private.\n11. Looks like your citation manager panicked, likely just need to manually delete these."}]

Decision letter