Direct Measurements of Dust Settling Velocity Under Low-Density Atmospheres Using Time-Resolved Particle Image Velocimetry

Dust dynamics influence planetary atmospheres. However, the settling velocity of dust—and thus its residence time in the atmosphere—is often mispredicted. Challenging, indirect experiments involving few ideal particles revealed that dust settling velocity deviates from Stokes' law under rarefied atmospheres. While useful, such experiments are inadequate to simulate more complex scenarios, including variable particles sizes and shapes. Here, we present direct measurements of settling velocity for spherical particles under Earth-to-Mars atmospheric pressures using time-resolved particle image velocimetry (TR-PIV), and validate their robustness with existing models. Our results demonstrate that TR-PIV provides a relatively simple approach to quantifying dust settling velocity from direct observations of over 10,000 particles, enabling systematic investigations of dust settling under realistic scenarios. Such experiments will have significant implications for our understanding of Mars' past, present, and future - from providing a tool to decipher its sedimentary record to enhancing predictive capabilities of atmospheric models.