The small members of the Kuiper belt – that is, objects with a diameter of two kilometers or less – are obviously rare. Researchers at Kelsi Singer of the Southwest Research Institute in Boulder, Colorado, come to this conclusion. They examined the distributions of the crater of the dwarf planet Pluto and its major moon Charon. They found that impact craters, due to the impact of Kuiper belt objects with diameters less than two kilometers, are much rarer than previously thought.
The planetologists around Singer use the fact that on Pluto and Charon the history of the impact of the last four billion years has been preserved. To this end, they evaluated the images of the New Horizons space probe, which in its rapid flyby in mid-July 2015 was able to photograph about 40% of the surfaces of both celestial bodies with a high spatial resolution. In particular, the surface of Charon is more suitable for crater statistics. Its internal geological activity is much weaker than that of Pluto, which is twice as large, and there is no atmosphere around it. Otherwise, the characteristics of the surface may change due to deposits.
The most relevant information was provided to researchers in the region with the unofficial name Vulcan Planitia, which was captured optimally by New Horizons. In the assessment, Singer and his coauthors took into consideration the geological processes that could erase the craters on Charon. However, it can be seen that impact craters with a diameter of less than 13 kilometers are significantly rarer than assumed by other models of size and frequency distributions for objects in the Kuiper belt. The same frequencies were also found on Pluto.
Researchers then suspect that small objects in the Kuiper belt are much rarer and that objects are mostly left unchanged from the formation of the solar system some 4.6 billion years ago. These ideas are supported by the recent observations of the New Horizons space probe, which explored the small Ultima Thule of Kuipergürtelteljeje at the beginning of 2019. Its surface shows only small pits, where it is still unclear whether it is a crater from impact.