Dusty streaks on the Moon: fingerprints of multiphase flow instabilities

Abstract

From the crewed Apollo missions to the recent Chinese Chang'e landings, the interaction between spacecraft exhaust plumes and lunar soil produces dusty clouds with high-speed particle ejection. Despite varying landing sites, remarkably stable streak patterns were observed, raising questions about their origin. We solved this puzzle by showing that these patterns were driven by Görtler instability from the curved compressed shear layer of the supersonic but surprisingly laminar jet. This instability creates vortical structures that entrain and eject particles. The number of streaks exhibits an interesting scaling with the jet pressure ratio, which can be modeled with linear instability theory and shows excellent agreement with scaled-down experiments, simulations, and actual observations in landing videos. Our findings provide a fluid physics explanation of extraterrestrial landings, highlighting the role of particle-laden flows and paving the way for future missions to optimize landing strategies and mitigate dust cloud effects on equipment and visibility.

Fig. 1. Images from different lunar landings as the spacecraft approaches the Moon’s surface, generating radial ejecta streaks.

a Apollo 12 (1969), b Apollo 15 (1971), c Chang'e 4 (2019).

Fig. 2. Experimental facility to study the ejecta streaks.

a Experimental schematic of the supersonic jet impinging on a granular bed, b the impinging jet diameter as a function of the jet pressure ratio, c half-view of the front window showing the elevated ejecta streaks, and d schematic of counter-rotating streamwise vortices interacting with a granular bed.

Fig. 3. The number of streaks n as a function of the jet pressure ratio.

The data includes current experiments and simulations, data from the literature, and estimations from the lunar landing images for the Apollo and Chang'e missions.

Fig. 4. Single-phase jet profiles under varying ambient pressures.

a, d Planar laser-induced fluorescence images of a Mach 5.3 jet impinging on a flat plate at η_e = 50 and η_e = 500, respectively. The scale for the color bar represents the image intensity, which qualitatively translates to the gas density. b The measured radius of curvature and e compressed shear layer thickness as a function of downstream distance for η_e = 50. c The radius of curvature and f compressed shear layer thickness as a function of the jet pressure ratio. The scale bar is 1.38 cm. Vertical bars in (b) and (e) show the standard deviation from analysis of various single-phase images at similar conditions. Vertical bars in (c) and (f) show the variation of $\mathcal{R}$ and δ in the range of z/D_e used.