The small‐comet hypothesis: An upper limit to the current impact rate on the moon

Frank et al. [1986b] and Frank and Sigwarth [1993] hypothesized the intense bombardment of the terrestrial atmosphere by small comets. Their model requires that the Moon is impacted by small comets (10 7 ‐10 8 g) at a rate of almost one per minute. We calculate that an object of this mass, even with an exceedingly low density and relatively low velocity, will nevertheless produce a crater at least 50 m in diameter. These craters will excavate immature lunar soil and produce a very bright spot with a diameter of at least 150 m. If low‐density comets exist that might not create deep craters [O'Keefe and Ahrens, 1982], they will nevertheless disturb the regolith sufficiently to create detectable bright spots. If the small‐comet hypothesis is correct then the near‐global lunar imaging returned by Clementine in 1994 should reveal ∼10 7 bright spots in locations where craters are not present in images acquired in the 1960's and early 1970's. We find no new bright spots in a carefully‐studied area of 5.2×10 4 km², so an upper limit to the current cratering rate by small comets is 33/yr, ∼10 4 below that expected if the small‐comet hypothesis were valid.