In the dimly lit spaces of our solar system beyond the orbit of Neptune is the Kuiper Belt. This is a region 35-50 times farther from the Sun than the Earth, populated by icy bodies so little distributed that they never had the chance to collide and merge into objects the size of a planet.
Pluto is the biggest we know, but it's fair. And over the past two decades, telescope research has found a couple of thousands of people ranging in size to just a few tens of miles in diameter. The problem is that most objects of this size or smaller are too weak to be seen by telescopes. So it will be difficult to figure out how many small but invisible bodies are in the Kuiper belt. Now a new article, published in Science, used an ingenious method to help us find out.
This is important, because scientists believe that the objects of the Kuiper Belt are survivors of the birth of the solar system, developed from a primordial cloud of dust and gas. This means that its size distribution may have a lot to tell us about how the material from where the planets grew was initially assembled.
Instead of counting the small objects of the Kuiper belt directly, the researchers behind the new study told of craters made by the random sample of objects that impacted the surfaces of Pluto and its larger moon, Charon. There, craters 13 km in diameter would have been made by objects only 1 km to 2 km in size. This is already well below the telescopic detection limit for Kuiper belt objects, but the images of the flyby of NASA's New Horizons mission in 2015 allow craters of up to 1.4 km to be mapped. Those must have been made by impacts of Kuiper Belt objects not much larger than 100 meters in size.
The researchers' analysis shows that, for craters of 13 km or more in Pluto and Charon, the frequency of impacts of various sizes seems to coincide with what would be expected from the known size distribution of Kuiper belt objects. However, for smaller craters the abundance drops dramatically, and by implication owes the abundance of Kuiper belt objects capable of making these craters. The same does not happen with well-documented asteroids that collide with bodies in the region of Jupiter, Mars and Earth, nor is it consistent with theoretical models.
The interpretation of crater-filled terrains led researchers to dismiss that small craters were erased by geological resurfacing, such as cryovulcanic activity (ice-cold fluid outbreaks) over the last four billion years. This reinforces the conclusion that smaller craters were never made in the expected numbers, so there must be a mysterious corresponding deficit of Kuiper belt objects less than 1-2km in size.
Blorping and flomping
When researchers, led by Kelsi Singer of the Southwest Research Institute (Boulder, Colorado), wrote their articles, no one had seen a small Kuiper Belt object in detail. However, New Horizons recently passed a 30 km object known as 2014 MU₆₉ (more controversially dubbed "Ultima Thule") on January 1, and now has probably conveyed the best images we are going to get.
Sometimes described as "snowman shaped," it is a "contact torque" of two wolves, almost certainly formed by a fusion of two round objects that happen so slowly and smoothly that none of the components has been deformed in the process. But what happened before that? If you look at the larger of the two wolves in particular, you can distinguish what appear to be traces of component parts that mingled vigorously enough to come together in an approximate sphere but with insufficient violence to crush each other.
These ideas inspired new, peculiar terms. "Blorping" refers to the collisional fusion of material to mount each of the lobes, and "flomping" describes joining when two lobes meet without causing any deformation. More importantly, this could provide insight into the processes that stole the Kuiper belt from smaller objects that would otherwise have impacted to form small craters on Pluto and Charon.
The relative lack of small objects in the Kuiper Belt may be because instead of dividing each other in collisions, they tend to merge by blorping – eventually growing into objects like 2014 MU₆₉. If that is correct, when we try to count them, we will see a growth record instead of collisional fragmentation.
Orbital velocities are slower as farther away from the sun, so we expect the collisions to be less violent in the Kuiper belt than in the inner solar system. But even so, a "blorp" event to merge two pieces together, instead of breaking them, probably requires that the ice that makes up most of its substance is much less fragile and softer than we might expect. This is crucial information, since these pieces are made of the raw material from which the solar system has formed, throwing important light on its evolution.
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