About 5000 microcraters on seven lunar rocks recovered during the Apollo 12 mission have been systematically studied using a stereomicroscope. Based on comparisons with laboratory cratering experiments, at least 95 percent of all millimeter-sized craters observed were formed by impacts in which the impact velocity exceeded 10 km/s. The dynamics of particle motion near the Moon and the distribution of microcraters on the rocks require an extralunar origin for these impacting particles.
The microcrater population on at least one side of all rocks studied was in equilibrium for millimeter-sized craters; i.e., statistically, craters a few millimeters in diameter and smaller were being removed by the superposition of new craters at the same rate new craters were being formed. Selected surfaces of some rocks, particularly those with glass coatings, are not in equilibrium. For every particle incident upon these “production” surfaces, there remains for observation a corresponding crater; thus the population of craters on such a surface is directly related to the total population of particles impacting that surface.
Crater size-distribution data from production surfaces, together with an experimentally determined relationship between the crater size and the physical parameters of the impacting particle, yield the mass distribution of the interplanetary dust at 1 AU. Based on assumptions corresponding to an impact velocity of about 20 km/s and a particle density of 3 g/cm3, the cumulative particle flux versus mass distribution relationship is
where N is the number of particles of mass m in grams, and larger, and C depends on the time-area product, which is, for the present, unknown. For particles smaller than 10-8 g, our observations indicate a sharper decrease in the absolute value of the slope of the flux versus mass curve than is indicated by satellite-borne-experiment data. This result may be due to a genuine relative decrease in the number or kinetic energy of smaller particles, or it may be due to our inability to observe quantitatively the smallest microcraters. For particles larger than 10-6 g, the slope of the flux versus mass curve increases smoothly to an absolute value greater than one.