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

Aeolian sediment transport has been seen 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 parameter sets. Here, using sediment transport laboratory experiments performed in a closed-circuit wind tunnel placed in a vacuum chamber, which is operated at extremely low pressures, we show that Martian conditions belong to a previously unexplored saltation regime. The saltation transport wind speed is quantitatively predicted by the state-of-the art models up to a density ratio between grain and air of 4x10⁵, but unexpectedly falls, above this cross-over point, to much lower values than expected. By contrast, impact ripples, whose emergence is continuously observed on the granular bed over the whole pressure range investigated, display characteristic wavelength and propagation velocity essentially independent of the pressure. Testing these findings against 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.

Ref: B. Andreotti, P. Claudin, J.J. Iversen, J.P. Merrison and K.R. Rasmussen, Proc. Natl. Acad. Sci. USA 118, e2012386118 (2021).