Theoretical analysis of pneumatic mobility and size classification of regolith particles under Martian environmental conditions

Martian atmospheric and gravity conditions are unlike those found on Earth, presenting a unique aerodynamic regime for granular material. The CO₂-dominatedatmosphere at 610 Pa and ~210 K is thin enough that the gas mean free path approaches the size of fine regolith particles, yet it is dense enough to sustain aerodynamic forces that causeparticle mobility.

Aerodynamic behavior of regolith has a great impact on the overall Martian environment. Global and local disruptive events such as dust storms and dust devils are commonplace, triggering constant regolith mixing, transport and size sorting. Furthermore, dust lifting, dune migration, aeolian transport and surface-atmosphere exchange are governed by the aerodynamics of regolith particles. Existing models often neglect the Cunningham slip effect, which becomes prominent at particle sizes below 100 microns. This work presents a theoretical analysis of Martian transport models using regolith-specific metrics and in-situ grain size data from Curiosity (Preston et al. 2024).

Specifically, Ganser drag with Cunningham slip correction using Rader constants are used (Ganser 1993; Rader 1990). As Curiosity measurements highlight, modelling non-spherical particle drag is essential to fully represent Martian conditions. The Ganser drag coefficient is given by,

Where Re is the Reynolds number, and K₁ and K₂ are functions of particle sphericity φ. Specifically,

Martian regolith sphericity has been computed to be 0.74 by calculating the geometric average of grains measured in-situ (Preston et al. 2024). For small particles, the Cunningham slip correction follows (Rader 1990),

With K_n, being the Knudsen number that compares the molecular mean free path with a representative physical scale, in this case the particle diameter, as we are correcting the effects of flow on small particles. Used constants, A, B and C are for CO₂ taken from literature. The threshold friction velocity is modeled as the velocity limit necessary to be lifted using adapted sand transport equations that take into account particle cohesion (Kok et al. 2012). These equations are used in order to simulate the aerodynamic behavior of regolith particles under Martian conditions and in compressed Martian atmosphere applications.

Figure 1: Cunningham slip correction influence in terminal velocity for Martian regolith and environmental conditions. v_gas represents the reference gas speed used in the simulations.

Figure 1 shows how the effect of the Cunningham slip correction is greatest for small particles. Specifically, at 5 microns the correction factor reaches C_C≈4, reducing the effective drag by a factor of four relative to the continuum prediction.

Curiosity sampling locations show substantially different grain size distributions. This implies that aerodynamic mobility (i.e., which grains are lifted, transported or deposited) varies significantly across the Gale crater traverse. Simulation results presented in Figure 2 show how the lifted fine fraction of two distributions varies significantly (yields vary up to 1.7 times). The fine distribution is entrained by the wind field while the coarse fraction represents particles that settle under gravity. Influence of particle shape (φ=0.65-0.83 consistent with Curiosity observations) shifts the cut size, d₅₀ , up to 10 microns. Across the full range φ=0.25-1.0, the shift exceeds 90 microns, meaning that particle mobility is sensitive to particle heterogeneity within a sample. Hence, grain morphology has a direct influence on surface-atmosphere exchange.

Figure 2: Particle size distribution of two Martian in-situ samples (Preston et al. 2024) and the simulated resulting coarse and fine distributions being lifted by v_gas.  Y_f and Y_c and  represent the yield of the fine and coarse distribution.

These aerodynamic characterizations places the Martian atmospheric gases as active agents, collecting, transporting and classifying regolith by particle size. Therefore, this project investigated the possibility of actively utilizing the gas for pneumatic surface collection, pipeline transport and aerodynamic size classification. For each process, the theoretical models were adapted to assess their feasibility and identify the main physical constraints. Collection via suction nozzles is found to be feasible for typical subsystem Martian power budgets; yet, smaller particles require greater pickup velocities due to cohesion forces. Horizontal transport is shown to require gas velocities that remain in a turbulent regime due to the low atmospheric density, making pipe diameter the primary design variable. Aerodynamic size classification via vertical elutriation emerges as the most promising concept, offering a tunable cut size with low power budget. A trade-off analysis across mechanical and pneumatic regolith handling alternatives confirmed that pneumatic classification is the most competitive approach for producing size-sorted regolith feedstock under Martian surface conditions.

Together, these results suggest that a complete description of Martian regolith aerodynamics requires joint characterization of grain size, grain shape, and local atmospheric state. Crucially, the same atmosphere that causes regolith transport can be exploited as the working medium for pneumatic size classification at blower powers below 0.1 W, demonstrating that the Martian environment can enable resource for producing size-sorted regolith feedstock in support of in-situ resource utilization processes.

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