A steady-state model for the lunar soil
Grain-size analysis of lunar soils returned by the Apollo missions suggests that the textural parameters of the lunar soil evolve in a complex interaction with the meteoroid flux at the lunar surface. The soil evolves as a result of a balance (or lack of balance) between destructive processes (comminution) and constructive processes (agglutination) induced by meteoroid impact. The evolution occurs in three stages: (1) a comminution-dominated stage when the mean grain size of the soil is too large for agglutination to be an effective balancing mechanism, (2) an agglutination-dominated stage when constructional processes begin to balance comminution as the mean grain size of the soil approaches the mean agglutinate size (2.5 ϕ or 177 µm), and (3) a steady-state stage where the effects of agglutination and comminution come into balance on a long-term basis. On a short-term basis, large impacts destroy agglutinates and cause a cycling of the grain-size parameters within a narrow range as agglutination brings the soil back to equilibrium. The kinetic energy released by the meteoroid flux is partitioned in a complex manner, depending on the mass of the impacting particle and on the average properties of the particles forming the soil. Less than 1.2 percent of the kinetic energy of the flux is effective in eroding the bedrock substrate. Most of the energy is absorbed in reworking the soil by means of agglutination, comminution, and ejection. Possibly as much as 67 percent of the flux mass is involved in agglutination alone. The flux of newly eroded sediment at the lunar surface is 1.97 × 10 −7 g cm −2 yr −1 . The current flux of terrestrial materials entering the oceans is 2.5 × 10 4 times larger than the lunar-sediment flux on the basis of a unit surface area.