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

TL;DR

This study reveals that sediment saltation on Mars occurs at lower wind speeds than previously predicted, due to collective grain effects, challenging existing models of aeolian transport under extraterrestrial conditions.

Contribution

The paper demonstrates experimentally that Martian-like sediment transport thresholds are lower than models predict at high grain-to-air density ratios, highlighting a new saltation regime.

Findings

Saltation threshold is lower than predicted at high density ratios.

Impact ripples form across all tested pressures with consistent wavelength and velocity.

Sediment transport may be dominated by grain inertia effects at low Reynolds numbers.

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 analogues 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 towards 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 $4 \times 10^5$, 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.