What did the earliest bodies in our Solar System look like, and what was their fate? It's difficult to tell, because it's not clear that there are any of them left. Lots of the earliest material was swept up into the planets. Many of the smaller bodies that remained are products of multiple collisions and have perhaps formed and re-formed multiple times—some are little more than rubble piles barely held together by gravity.

Without some knowledge of what these bodies looked like, then, it's difficult to determine whether our models of the physics of the early Solar System are right and whether similar processes are likely to be in play in exosolar systems.

Now, some researchers have found a way to infer the sizes of objects present in the early Solar System: looking at the craters they left behind when they smashed into Pluto and Charon. The results suggest a shortage of objects smaller than 2km in diameter and suggest that much of the material in the Kuiper Belt was quickly swept up into larger objects, which somehow avoided smashing into each other and liberating a new generation of smaller fragments.

Getting belted

It's relatively easy to build models of the behavior of the particles of dust and ice that orbited our then-forming Sun. And a key determinant of their possible accuracy is their ability to form planet-like objects, since we know that was the eventual fate of the Solar System. But understanding the Kuiper Belt is harder, since there are no large planets out there and our telescopes are not good enough to get a strong sense of how many small objects there are.

Models of the Kuiper Belt's formation aren't much help. Depending on the initial assumptions, they sometimes produce lots of small-to-medium sized objects that frequently fragment through further collisions. Change the model's assumptions and you get the rapid growth of large bodies, which lowers the probability of collisions and limits the presence of smaller bodies. Without a sense of the size distribution of present-day Kuiper Belt objects, it's tough to figure out which of these models is closer to right.

One way to get a grip on the size of the objects out there is to look at the marks they leave on other objects. Bigger objects will leave bigger craters behind when they smash into a planet or moon, so checking crater sizes can give us a sense of what was once present in the nearby environment. We've done that with the moons of Jupiter and Saturn, but both of those have likely seen impacts from both Kuiper Belt objects and asteroids. Anything with an obvious surface further out in the Solar System is largely a pixellated blur.

That changed when New Horizons shot past Pluto, providing the first detailed images of it and its moons. Many of these were a high-enough resolution to allow a detailed crater count, so an enormous team of scientists has now had a look at what has hit the dwarf planet and its largest moon, Charon.

Going large

One of the challenges in the project is that Pluto is geologically active. Sputnik Planitia, Pluto's heart-shaped plane, is a mass of slowly churning nitrogen ice; there are essentially no craters there at all. By contrast, Charon's Vulcan Planitia appears to have seen extensive cryovolcanism, but this happened early enough in the body's history that the area is crater-filled. To deal with these sorts of differences, the researchers performed multiple analyses, each for an area with a single type of terrain. If any of these were destroying craters of a specific size, it should stand out.

All of the images had resolutions where every pixel was less than 850 meters across; many had a resolution of less than 200 meters. For context, a one-kilometer diameter object would make a crater about 13 kilometers across. Thus, the team was able to detect some impacts that were caused by objects only 100m across.

Regardless of where they looked, the same general trend was apparent: there weren't enough small craters. Above about 10km across, the rate of cratering is about what you'd expect for a smooth distribution of impactor sizes (meaning you expect fewer big objects). But below 10km, things drop rather dramatically.

The researchers went back and looked at the sort of processes that might selectively erase small craters and came up empty. The freezing out of gases from the atmosphere could fill in smaller craters on Pluto, but Charon doesn't have an atmosphere. One region of Pluto does show fewer small craters and partially filled big ones, but the rest don't. Cryovolcanism seemed to occur early in Charon's history but stopped quickly enough that the volcanic regions have a large collection of craters.

So overall, the dearth of small craters seems to be real. And going from crater to impactor sizes means that there seem to be fewer than expected Kuiper Belt objects below 1km in diameter. A group of bodies that are at "collision equilibrium"—meaning they smash into each other enough to create an even distribution of sizes—would have many more small objects in it. So the crater count seems to suggest that most Kuiper Belt objects are going to be primordial; they've survived unscathed since the dawn of the Solar System. That in turn means that any comet sampling we do is likely to be providing us with a picture of the early Solar System.

The analysis also implies that the processes that built Kuiper Belt objects effectively shuttled mass into building larger objects. There are models that do this by starting with an uneven distribution of starting materials, which gets enhanced by the gravitational forces that they generate. And while these results are specific to the Kuiper Belt, it's likely that similar physics applied to other areas of the Solar System.

Science, 2019. DOI: 10.1126/science.aap8628 (About DOIs).