Regolith bulk density–depth models. (A) Athy exponential compaction model predicting a rapid decrease in porosity of regolith with depth. Model assumes an initial porosity ϕ_o = 0.52%. (B) Three density–depth model plots: hyperbolic (D. Carrier et al. 1991), power-law (D. Carrier et al. 1991), and Athy exponential (T. R. Watters et al. 2024b).