A lower-than-expected saltation threshold at Martian pressure and below

Abstract

Aeolian sediment transport is observed to occur on Mars as well as other extraterrestrial environments, generating ripples and dunes as on Earth. The search for terrestrial analogs of planetary bedforms, as well as environmental simulation experiments able to reproduce their formation in planetary conditions, are powerful ways to question our understanding of geomorphological processes toward unusual environmental conditions. Here, we perform sediment transport laboratory experiments in a closed-circuit wind tunnel placed in a vacuum chamber and operated at extremely low pressures to show that Martian conditions belong to a previously unexplored saltation regime. The threshold wind speed required to initiate saltation is only quantitatively predicted by state-of-the art models up to a density ratio between grain and air of 4×105 but unexpectedly falls to much lower values for higher density ratios. In contrast, impact ripples, whose emergence is continuously observed on the granular bed over the whole pressure range investigated, display a characteristic wavelength and propagation velocity essentially independent of pressure. A comparison of these findings with existing models suggests that sediment transport at low Reynolds number but high grain-to-fluid density ratio may be dominated by collective effects associated with grain inertia in the granular collisional layer.

Unified Model of Sediment Transport Threshold and Rate Across Weak and Intense Subaqueous Bedload, Windblown Sand, and Windblown Snow

“…An empirical scaling relation that is consistent with experimental data is where c e is a proportionality constant close to unity (Beladjine et al, 2007). Figure 8e shows model predictions of Θ cont , using Equations 26, 27a, and 27b with c e = 0.8, and compares them with measurements of Θ cont for aeolian NST (Andreotti et al, 2021;Carneiro et al, 2015;Ho, 2012;Martin & Kok, 2018). Note that Andreotti et al (2021) carried out their experiments in a wind tunnel with varying air pressure and defined threshold conditions as the transition between saltation of groups of particles (bursts) to intermittent saltation of single particles (at high pressure) or no transport (at low pressure).…”

“…Figure 8e shows model predictions of Θ cont , using Equations 26, 27a, and 27b with c e = 0.8, and compares them with measurements of Θ cont for aeolian NST (Andreotti et al, 2021;Carneiro et al, 2015;Ho, 2012;Martin & Kok, 2018). Note that Andreotti et al (2021) carried out their experiments in a wind tunnel with varying air pressure and defined threshold conditions as the transition between saltation of groups of particles (bursts) to intermittent saltation of single particles (at high pressure) or no transport (at low pressure). These authors interpreted their measurements as measurements of the transport threshold Θ t , that is, they assumed Θ cont = Θ t .…”

“…Figure 8e shows model predictions of Θ cont , using Equations 26, 27a, and 27b with c e = 0.8, and compares them with measurements of PÄHTZ ET AL. (Andreotti et al, 2021) or ρ f = 1.2 kg/m 3 and varying d (Carneiro et al, 2015;Ho, 2012;Martin & Kok, 2018). The symbol color indicates the value of the density ratio s. Lines in (e) correspond to model predictions of Θ cont and the transport threshold Θ t .…”

“…(e) Θ cont versus Galileo number Ga. Symbols in (e) correspond to windblown sand measurements of Θ cont for either d = 125 μm and varying ρ f(Andreotti et al, 2021) or ρ f = 1.2 kg/m 3 and varying d(Carneiro et al, 2015;Ho, 2012;Martin & Kok, 2018). The symbol color indicates the value of the density ratio s. Lines in (e) correspond to model predictions of Θ cont and the transport threshold Θ t .…”

An Evolving Understanding of Enigmatic Large Ripples on Mars

“…(1987) that showed that a population of small ripples had a stable size under a wide range of atmospheric pressures, without significant changes in wavelength, whereas a second population of ripples grew in size with decreasing atmospheric pressure. Similarly, recent low‐pressure wind‐tunnel experiments also have demonstrated the characteristic wavelength of small impact ripples do not change substantially as atmospheric pressure is changed from terrestrial to Martian values (Andreotti et al., 2021; Claudin et al., 2020).…”