  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.
