How is asteroidal rock formed?

Question

So when planets form, dust from the protoplanetary nebula gets collected by gravity and then heated and reformed under pressure until it forms dense masses of stuff which we call rock.

However, asteroids don't have nearly enough gravity to do this, but pictures of asteroids like Bennu still show sizeable boulders on their surface. I'm aware that some asteroids are formed from fragments knocked off actual planets, but isn't that a tiny proportion of the whole? I'd expect asteroids to be made out of fluffy dust lightly sintered together from vacuum welding, but that's not what we see. Why?

(Osiris-Rex's photo of Bennu's surface)

Answer 1

Another major theory regarding the early solar system is that there was a relative abundance of short-lived radioactive isotopes at the time the solar system formed. These short-lived isotopes would have contributed greatly to the heating of stony materials, thereby making it possible for rocky planetesimal-sized (~ 1km diameter) objects to form in conditions that only existed for an astronomically short period of time. Aluminum-26 (²⁶Al), with a half-life of 717 thousand years, is the key suspect with regard to this heating.

There are lots of signs that ²⁶Al did exist in the early stages of the solar system and did contribute significant heating to planetesimal-sized objects. For a while it was thought that these signs of ²⁶Al meant that a nearby nova / supernova must have occurred right about at the time that our solar system started to form. Recent research indicates that a protostar might be able to produce ²⁶Al by itself.

Once that period of early heating via short-lived radioactive isotopes ended, the resulting planetesimal-sized objects would have been subjected to collisions, and those collisions would have resulted in either larger or smaller objects, depending on collisional energy. The asteroid belt, being subject to perturbations from Jupiter, would have been subject to greater collisional energy than the inner solar system and thus a greater chance of objects breaking apart.

References:

Typhoon Lee, D. A. Papanastassiou, and G. J. Wasserburg. "Aluminum-26 in the early solar system-Fossil or fuel" The Astrophysical Journal 211 (1977): L107-L110

Using Aluminum-26 as a Clock for Early Solar System Events

Brandt AL Gaches, Stefanie Walch, Stella SR Offner, and Carsten Münker. "Aluminum-26 enrichment in the surface of protostellar disks due to protostellar cosmic rays" The Astrophysical Journal 898, no. 1 (2020): 79

Answer 2

One major theory for the evolution of the asteroid belt is that planetesimals (early precursors to planets) formed early in the development of the Solar System in Solar orbits around the asteroid belt. These planetesimals had enough internal gravity and heat to form rocks. From wikipedia:

Planetesimals within the region that would become the asteroid belt were too strongly perturbed by Jupiter's gravity to form a planet. Instead, they continued to orbit the Sun as before, occasionally colliding.[37] In regions where the average velocity of the collisions was too high, the shattering of planetesimals tended to dominate over accretion,[38] preventing the formation of planet-sized bodies.

Here on Earth, we get boulders from rock layers being broken up by geological processes like glaciation, earthquakes, erosion, etc... In the asteroid belt, the boulders and smaller debris (according to this theory) was formed from collisions breaking up planetesimals. Collisions increased the number of fragments in these orbits, causing a runaway process which filled the region with debris of various sizes. After sufficient material was ejected from these orbits, collisions would begin to decline, allowing boulder piles to come back together to form loose asteroids.

As an interesting aside, NASA scientist Don Kessler hypothesized that something similar may happen with human artificial satellites in Earth orbits. From his seminal paper abstract Kessler and Cour-Palais 1978:

As the number of artificial satellites in earth orbit increases, the probability of collisions between satellites also increases. Satellite collisions would produce orbiting fragments, each of which would increase the probability of further collisions, leading to the growth of a belt of debris around the earth. This process parallels certain theories concerning the growth of the asteroid belt.

This process is now known as the Kessler syndrome, or collisional cascading.

Answer 3

You imagine a cloud of molecules that is very gently pulled together by mutual gravity. This is not how things happened.

In stead you have many proto-asteroids that are all orbiting the Sun at high speed. Mutual attraction is nearly irrelevant, instead chance leads to collisions when orbits intersect.

The relative speed of two intersecting orbits can anything from nearly zero to quite high. In the first case, rocks accumulate, in the second case, they scatter.

Either way, forces involved are large. More than enough to compact rocks, and even melt them. After a number of impacts at varying speeds, you end up with something like your picture.