Lifting and transport of Martian dust by the Ingenuity helicopter rotor
downwash as observed by high-speed imaging from the Perseverance rover
Abstract
Martian atmospheric dust is a major driver of weather, with feedbacks
between atmospheric dust distribution, circulation changes from
radiative heating and cooling driven by this dust, and winds that
mobilize surface dust and distribute it in the atmosphere. Wind-driven
mobilization of surface dust is a poorly understood process due to
significant uncertainty about minimum wind stress, and whether saltation
of sand particles is required. This study utilizes video of six
Ingenuity helicopter flights to measure dust lifting during helicopter
ascents, traverses, and descents. Dust mobilization persisted on
take-off until the helicopter exceeded 3 m altitude, with dust advecting
at 4-6 m/s. During landing, dust mobilization initiated at 2.3-3.6 m
altitude. Extensive dust mobilization occurred during traverses at
5.1-5.7 m altitude. Dust mobilization threshold friction velocity of
rotor-induced winds during landing are modelled at 0.4-0.6 m/s (factor
of two uncertainty in this estimate), with higher winds required when
the helicopter was over undisturbed terrain. Modeling dust mobilization
from >5 m cruising altitude indicates mobilization by 0.3
m/s winds, suggesting non-saltation mechanisms like mobilization and
destruction of dust aggregates. No dependence on background winds was
seen for the initiation of dust lifting, but one case of takeoff in 7
m/s winds created a track of darkened terrain downwind of the
helicopter, which may have been a saltation cluster. When the helicopter
was cruising at 5-6 m altitude, recirculation was seen in the dust
clouds.