By mapping the scars of ancient impacts on the surfaces of Pluto and Charon, researchers have gained a better understanding of the formation and evolution of the Kuiper Belt, a vast halo of orbiting debris lying at the fringes of our Solar System. According to the new study, the four billion-year-old craters observed by New Horizons record the size distribution of small Kuiper belt objects (KBOs) that impacted the surfaces. By studying the sizes of craters, they can probe KBOs too small to be directly observed from Earth. While the Kuiper belt is home to several large objects – dwarf planets like Pluto – little is known about the distribution of Kuiper belt objects (KBOs) smaller than 100 kilometers (km) in size, which are remnants from the formation of the Solar System. Telescopic observations of small KBOs from Earth is difficult with only the largest objects identified. However, for billions of years, Pluto and its satellite, Charon, have collected craters formed by collisions with smaller KBOs. According to the authors, their pockmarked surfaces provide useful records for characterizing the size distribution of objects within the Kuiper belt. Using detailed images collected by the New Horizons spacecraft as it drifted past in 2015, Kelsi Singer and colleagues mapped the craters on Pluto and Charon to determine the sizes of the impacting KBOs. Accounting for recent geological processes, which can erase older impact craters, Singer et al. identified some surfaces on Pluto and Charon which are at least four billion years old. They also report a lack of craters smaller than 13 km in diameter. This means there are fewer KBO’s smaller than two km than previously predicted. These findings are not consistent with collision models that suggest a population of objects in traditional collisional equilibrium, according to the authors. They may instead indicate the primordial nature of the Kuiper belt – with constituent objects less likely to have evolved from collisions, but instead surviving intact into the present from the Solar System’s earliest days.
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