Source: Journal of Geophysical Research: Planets
Even more extremely than water vapor on Earth, airborne dust alters the weather and climate on Mars. From small dust devils to planet-scale storms, dust is raised by the winds and incorporated into atmospheric circulations, where it interacts with solar and infrared radiation and water ice particles before falling back onto the surface. Amongst these processes, wind-driven mobilization of surface dust is the least understood, precluding predictive capabilities in models for when a dust storm may occur (Newman et al., 2022).
In a new study, Lemmon et al.  use videos of six Ingenuity helicopter flights to study dust lifting during takeoff, traverse, and landing. By creating a mean frame in the video images and removing it from individual frames, the authors unveiled the amount and distribution of lifted dust, which was then colorized to enhance its visibility (see figure below). Finally, the authors used a helicopter ‘brownout’ model (akin to a ‘whiteout’ in snowy conditions) fed with simultaneous meteorological measurements to determine the winds associated with the observed lifted dust (Rabinovitch et al., 2021).
The authors obtained a threshold velocity at 0.4-0.6 meters per second (m/s) for undisturbed terrain during landing, when dust mobilization initiated between 2.3 and 3.6 meters altitude. These values are in line with model predictions of sand mobilization, which has been proposed as a prerequisite to dust mobilization when easier-to-move saltating sand-sized grains, about 100 micrometers (μm) diameter, disturb dust-sized particles (typically less than 5-μm diameter). Interestingly, dust was lifted at estimated threshold velocities of about 0.3 m/s during traverses at cruising altitudes greater than 5 meters. This ‘low’ value cannot be explained with sand mobilization, suggesting non-saltation mechanisms such as mobilization and destruction of dust aggregates.
With sample recovery helicopters planned as part of the Mars Sample Return mission, these results offer a unique opportunity to test the validity of brownout models beyond Earth conditions.
Lemmon, M. T., Lorenz, R. D., Rabinovitch, J., Newman, C. E., Williams, N. R., Sullivan, R., et al. (2022). Lifting and transport of Martian dust by the Ingenuity helicopter rotor downwash as observed by high-speed imaging from the Perseverance rover. Journal of Geophysical Research: Planets, 127, e2022JE007605. https://doi.org/10.1029/2022JE007605
— German Martinez, Associate Editor, JGR: Planets