<p>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 <a href="https://en.wikipedia.org/wiki/Asteroid_belt#Formation" rel="noreferrer">wikipedia</a>:</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>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.</p>
<p>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 <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JA083iA06p02637" rel="noreferrer">Kessler and Cour-Palais 1978</a>:</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>This process is now known as the <a href="https://en.wikipedia.org/wiki/Kessler_syndrome" rel="noreferrer">Kessler syndrome</a>, or collisional cascading.</p>

CC BY-SA 4.0
2022-02-15T17:44:30.753Z
2022-02-15T17:44:30.753Z
Connor Garcia
<p>You imagine a cloud of molecules that is very gently pulled together by mutual gravity.  This is not how things happened.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2022-02-16T11:51:15.230Z
2022-02-16T11:51:15.230Z
Stig Hemmer
<p>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 (<sup>26</sup>Al), with a half-life of 717 thousand years, is the key suspect with regard to this heating.</p>
<p>There are lots of signs that <sup>26</sup>Al 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 <sup>26</sup>Al 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 <sup>26</sup>Al by itself.</p>
<p>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.</p>
<p>References:</p>
<p><a href="https://adsabs.harvard.edu/pdf/1977ApJ...211L.107L" rel="nofollow noreferrer">Typhoon Lee, D. A. Papanastassiou, and G. J. Wasserburg. &quot;Aluminum-26 in the early solar system-Fossil or fuel&quot; <em>The Astrophysical Journal</em> 211 (1977): L107-L110</a></p>
<p><a href="http://www.psrd.hawaii.edu/Sept02/PSRD-Al26clock.pdf" rel="nofollow noreferrer">Using Aluminum-26 as a Clock for Early Solar System Events</a></p>
<p><a href="https://iopscience.iop.org/article/10.3847/1538-4357/ab9a38" rel="nofollow noreferrer">Brandt AL Gaches, Stefanie Walch, Stella SR Offner, and Carsten Münker. &quot;Aluminum-26 enrichment in the surface of protostellar disks due to protostellar cosmic rays&quot; <em>The Astrophysical Journal</em> 898, no. 1 (2020): 79</a></p>

CC BY-SA 4.0
2022-02-16T16:05:24.257Z
You would need a great deal of heating for a kilometre-sized bodies wouldn't generate enough gravity to become differentiated via convection or buoyancy effects, though. I'd expect the results to be very porous and rather mixed. Collisions would compress the results (into various exotic minerals due to shocking?) but still don't explain where things like metal-heavy bolides come from. What am I missing?
David Given
CC BY-SA 4.0
2022-02-16T16:08:29.050Z
...although I have just seen the wikipedia reference to larger bodies like Vesta and Ceres that certainly would produce differentiated bodies. These could then break up to expose fragments of the metal-rich core. Smashing up something the size of Vesta would be a ridiculously violent event, though (and let's not talk about Ceres...).
David Given
CC BY-SA 4.0
2022-02-17T06:43:25.017Z
What you are missing is the square-cube law. The surface from which an object radiates heat is proportional to the square of the object's radius while the amount of heat generated by radioactive material within an object is proportional to the cube of the object's radius. Estimates of heating from $^{26}\text{Al}$ vary, but many authors are of the opinion that the heating could have been immense once objects reached a kilometer or so in diameter.
David Hammen
CC BY-SA 4.0
2022-02-18T02:20:19.147Z
Very cool reference on local 26-Aluminum production, thanks !!
giardia
CC BY-SA 4.0
2022-02-15T18:33:11.860Z
2022-02-17T13:36:01.317Z
2022-02-17T13:36:01.317Z
David Hammen
<p>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.</p>
<p>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?</p>
<p><a href="https://i.sstatic.net/fz9dP.jpg" rel="noreferrer"><img src="https://i.sstatic.net/fz9dP.jpg" alt="enter image description here" /></a></p>
<p>(Osiris-Rex's photo of Bennu's surface)</p>

<asteroids><planetary-formation>
How is asteroidal rock formed?
David Given
# How is asteroidal rock formed?

(Asked by: username_0 on 2022-02-15)

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?

[![enter image description here](https://i.sstatic.net/fz9dP.jpg)](https://i.sstatic.net/fz9dP.jpg)

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

**Tags:** asteroids, planetary-formation

---

## 3 answers

### Answer by username_3 on 2022-02-15. Score: 11 | Accepted answer

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](https://adsabs.harvard.edu/pdf/1977ApJ...211L.107L)

[Using Aluminum-26 as a Clock for Early Solar System Events](http://www.psrd.hawaii.edu/Sept02/PSRD-Al26clock.pdf)

[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](https://iopscience.iop.org/article/10.3847/1538-4357/ab9a38)

> **Comments:**
> **username_0**: You would need a great deal of heating for a kilometre-sized bodies wouldn't generate enough gravity to become differentiated via convection or buoyancy effects, though. I'd expect the results to be very porous and rather mixed. Collisions would compress the results (into various exotic minerals due to shocking?) but still don't explain where things like metal-heavy bolides come from. What am I missing?  
> **username_0**: ...although I have just seen the wikipedia reference to larger bodies like Vesta and Ceres that certainly would produce differentiated bodies. These could then break up to expose fragments of the metal-rich core. Smashing up something the size of Vesta would be a ridiculously violent event, though (and let's not talk about Ceres...).  
> **username_3**: What you are missing is the square-cube law. The surface from which an object radiates heat is proportional to the square of the object's radius while the amount of heat generated by radioactive material within an object is proportional to the cube of the object's radius. Estimates of heating from $^{26}\text{Al}$ vary, but many authors are of the opinion that the heating could have been immense once objects reached a kilometer or so in diameter.  
> **username_4**: Very cool reference on local 26-Aluminum production, thanks !!  

---

### Answer by username_1 on 2022-02-15. Score: 10

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](https://en.wikipedia.org/wiki/Asteroid_belt#Formation):

> 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](https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JA083iA06p02637):

> 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](https://en.wikipedia.org/wiki/Kessler_syndrome), or collisional cascading.


---

### Answer by username_2 on 2022-02-16. Score: 2

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.


---

