Physical Characteristics of the Lunar Regolith Determined From Surveyor Television Observations


  • E. M. Shoemaker,

  • E. C. Morris


The new data on the physical characteristics of the lunar surface derived from the Surveyor pictures can be fitted to a simple ballistic model for the origin and development of the lunar regolith. At a given locality, the size-frequency distributions of craters on the lunar surface can be represented by two functions: Small craters follow a steady-state distribution of the form F = ΦCμ, where F is the cumulative number of craters with a diameter ≥ c, c is the diameter of the craters, Φ and μ have the steady-state values Φ = 1010·9, and μ = −2.00 at all five Surveyor landing sites. Larger craters are represented by the function F = χcλ, where λ < μ, and χ varies from one landing site to another. The solution for c at the intersection of F = χcλ with F = Φcμ, designated as cs, is the upper limiting crater diameter for the steady-state distribution. The value of cs is a function of the age of the surface on which the regolith has formed. The thickness of the lunar regolith may be estimated from a variety of observational data. The estimated thickness of the regolith at a given Surveyor landing site is bracketed by the original depths of (1) the smallest blocky-rimmed craters that cut through the regolith and excavate coherent material beneath, and (2) the largest, sharp, raised-rim craters without blocks that have been excavated wholly within the slightly cohesive material that forms the regolith. Other direct estimates of the thickness of the regolith are the inferred original depth of the largest craters believed to have been formed by drainage of the regolith material into subregolith fissures and, at the Surveyor-7 site, the depth at which the surface sampler instrument encountered coherent material. The thickest regolith was found at the Surveyor-6 site, where it is estimated to be more than 10 meters thick, and the thinnest was found at the Surveyor-7 site, where it is estimated to be 2 to 15 cm thick. Particle counts from sample areas at each of the Surveyor landing sites show an approximately linear relationship between the log of the cumulative particle counts and the log of the particle size. A power function of the form N = KDλ (where N is the cumulative number of particles with diameter equal to or larger than D, and D is the diameter of particles) can be fitted to the data at each site. The size-frequency distribution of resolvable fragments at the Surveyor 3, 5, and 6 landing sites was found to be the same, within errors of estimation, but at the Surveyor 1 and 7 sites coarse fragments are more numerous. Considering all five sites, we found a strong inverse correlation between the abundance of coarse blocks and the thickness of the regolith. The coarsest fragments are most abundant at the sites with the thinnest regolith.