Primitive asteroids that by no means suffered huge collisions all seem like bigger than 100 km. Why?

Planetary systems are formed from the residual gas and the dust of a primordial star. The material collapses into a protoplanetary disk around the young star, and the clumps that form inside the disk eventually become planets, asteroids, or other bodies. While we understand the big picture of planetary formation, we still need to fully understand the details. That’s because the details are complicated.

Take, for example, the mystery of asteroids and collisions. You would think planets and asteroids were gradually forming. Small clumps collide with others to form larger clumps. As an object gets larger, it is more likely to attract other bodies and thus more likely to experience collisions. But as a recent study shows, this is not always the case with asteroids.

How turbulence plays a role in the formation of asteroids. Photo credit: MPIA / MPIA, Judith Neidel

Many asteroids can be divided into so-called families because they are similar in their chemical composition. It is believed that the smaller family members likely formed when the parent asteroid collided with another large asteroid. Because of this, you can build a family tree of asteroids created by collisions in the early solar system. When the team designed the family tree in 2017, they found 17 asteroids with no families. It seems that these asteroids had never seen a major collision. This is interesting because it means that they are still in the original state in which they formed.

What is more interesting is that these original asteroids are similar in size. The sweet point appears to be about 100 kilometers in diameter. Primordial asteroids are far more likely to be this size than smaller or larger. If asteroids grew gradually in the early solar system, you would expect to find a wide variety of sizes. Why are they almost all the same size?

The answer seems to be turbulence. You probably know turbulence as the chaotic movement of air that can make a flight a little rough, but turbulence is also the swirl of smoke from a candle or the waves of water flowing over stones. In the early solar system, these turbulent eddies would tend to trap dust and debris in a small region, giving the material time to collapse through gravitational attraction. The team’s research shows that turbulent formation rather than simple collisions may explain the consistent size of primordial asteroids. Thus, early asteroids quickly formed, which formed the basis for the formation of larger planetary bodies.

If this model is correct, it could explain why some asteroids are more like lumps of gravel than a solid body. It could also explain why early asteroid collisions were so common.

Reference: Klahr, Hubert and Andreas Schreiber. “Turbulence sets the length scale for planetesimal formation: local 2D simulations of streaming instability and planetesimal formation.” The Astrophysical Journal 901.1 (2020): 54.

Like this:

To like Loading…