Small Comet Abundance and Solar System Location

We present geological, historical, and observational evidence which implies that the proportion of small comets is related to where in the solar system the population is counted. A detailed count of craters in the 1-5 km size range on Jupiter's moon Europa yields a slope of -1.45 for the cumulative size-frequency crater distribution. This is strongly depleted in small objects relative to the far outer solar system. We compare the Europan distribution to crater counts on Neptune's moon Triton and extrapolated observational evidence of Kuiper Belt Objects. We find that small comets appear to be rarer at Triton than in the Kuiper Belt, and that small comets are five times rarer at Europa than they appear to be at Triton. We have extended our study into the inner solar system by considering the discovery histories of long and short period comets. We find that, while large comets have been discovered at ever-increasing rates over the last few centuries, small comets have not paralleled this increase. In one example, the known number of small Jupiter-family comets in Earth-crossing orbits has not changed significantly in the two and a half centuries since comet detection has been intensely pursued. In another example, we focus on the sungrazing comets as found by the SOHO satellite and otherwise. Many of the sungrazers are undoubtedly small comets. But 94 of the 95 known sungrazers follow the same orbit - i.e., they are fragments of one great comet disrupted by close passage to the Sun long ago. Thus SOHO could see unique small comets, if they were present, and so the number of unique small objects in the inner solar system is few to none. Small comets are therefore found relatively frequently in the outer solar system, but their numbers decrease by the orbit of Jupiter, and they are very rare in the inner solar system. One explanation for this decrease is the preferential sublimation and coupled sublimation-enhancing disaggregation of small icy bodies as they approach the inner solar system. Smaller objects have a greater surface area to volume ratio than larger objects, and thus are more subject to this process.