Why Is Asteroid Composition Highly Varied?

[anorlunda]

A thread on this forum Plans for Asteroid Mining has more than 9000 views. From that I conclude that belief that asteroid composition must be highly varied mu thast be wide spread. I'm having a hard time understanding why that should be.

It is believed that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter ...

So unless the planitesimals formed at approximately same radius from the sun, were varied in composition, how did the asteroids become varied?

A widely accepted theory of planet formation, the so-called planetesimal hypotheses, the Chamberlin–Moulton planetesimal hypothesis and that of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets.

I would expect the original nebula to be pretty homogeneous at the same radius from the center of mass. Ditto for the planetesimals formed in that region.

So how did the planetesimals and/or asteroids become so varied? Are there other Wikipedia articles relevant to this that I haven't found yet?

 

[davenn]

hi there

You keep using the term " varied"
but varied with respect to what ? They are not too different in composition to crust/mantle/core makeup of earth


from http://www.esa.int/Our_Activities/Space_Science/Asteroids_Structure_and_composition_of_asteroids

Beyond their shape, there are a number of ways to classify asteroids. The first is by composition. The typical composition of an asteroid depends on its distance from the Sun. At the outer edges of the asteroid belt, that is between three and three and a half times further from the Sun than the Earth, over eighty percent of the asteroids are known as C-type.

The C stands for carbon and the surfaces of these asteroids are almost coal-black. These asteroids contain large quantities of carbon molecules as well as the more usual rocks and metals. They are very similar in composition to the carbonaceous chondrite meteorites that sometimes fall on Earth. It is thought that these meteorites are chippings, smashed off during collisions between asteroids.

Closer to the Sun, at just over twice the Earth's orbital distance, the proportion of C-type asteroids is only about 40 percent. Here, the majority of asteroids are grey, without the carbon material and principally made of silicate compound rock.

These too resemble a type of meteorite found on Earth, the chondrites. Although these two types of asteroid form the majority of examples, there are some that appear to be composed of little but metal and, again, there are metal meteorites found on Earth.

Aside from composition, there are other important differences in the internal structure of the asteroids. Most are solid, indicating that they must have been molten at some point in their existence. Others are 'rubble piles'. This means that they are loose collections of 'pieces', held together by the force of their gravity. These asteroids were formed in collisions.

cheers
Dave

 

[anorlunda]

These too resemble a type of meteorite found on Earth, the chondrites. Although these two types of asteroid form the majority of examples, there are some that appear to be composed of little but metal and, again, there are metal meteorites found on Earth.

Thanks Dave, sorry I wasn't clear. The excerpt from your quote above gets to the heart of my question. "some that appear to be composed of little but metal" If "some are" then it follows that "some aren't" "Mostly metal" sounds like an extreme deviation from uniform composition of near neighbor asteroids.

I can understand radial stratification, but among those closer to the sun, how did some become "mostly of metal" and others not?

I recently learned about "ore genesis" (https://en.wikipedia.org/wiki/Ore_genesis) the topic about the many processes on Earth that separate different elements and minerals. Of course, those processed don't exist in space but I'm wondering if there are analogous processes in space.

 

[davenn]

HI

do you understand why the inner planets are rocky iron and the outer ones are predominantly gaseous ?

and the reason for that is VERY different to the why the Earth's "layers" are as they are

Dave

 

[anorlunda]

Yes, as I said earlier, I understand radial stratification, meaning those close in are different than further out.

The high temperature in the inner disk causes most of the volatile material—water, organics, and some rocks to evaporate, leaving only the most refractory elements like iron. The ice can survive only in the outer part of the disk.

But it sounds like asteroids closest in are diverse in their composition. That's my question. Neighbors at the same radius from the sun being very different. There must be some process over and above radial distance.

By different I mean some rocky asteroids being mostly silicon and other rocky asteroids mostly metal. Are you saying that there was a radius that formed silicon rocky asteroids and a different radius that formed metallic rocky asteroids?

I guess my question also presumes that the amount of migration is small. By migration I mean objects that are knocked into higher or lower orbits after formation. That would cause mixing. If mixing is small, then almost all asteroids at the same radius from the sun were formed similarly and remain at their original radius. Is that untrue?

 

[Ophiolite]

A small, but important correction. The rocky asteroids, carbonaceous chondrite and chondrite, are mostly rock, not rock and metal.

If an asteroid is large enough, heat of formation coupled with radioactive heating (much more common in the early days of the solar system because of short lived radioactive isotopes) raised the temperature sufficiently for the iron content (plus siderophile elements) to separate out and form a core. Subsequent collisions could disrupt the asteroid thus providing a source of the metal meteorites.

It is also the case that the protoplanetary disc was probably not homogenous. This may account for some of the variation seen. Further (speculation), the exchange of materials within the disc mentioned, I think, by a previous poster is not necessarily completely random and might impose a systematic variation within the asteroid belt.

I do not have quantitative figures for the amount of mixing that has occurred, but I strongly suspect it is greater than what you are imagining. This is an active area of research that will certainly turn up some unexpected results in the future.

 

[anorlunda]

I just found a short note in Scientific American here, that addresses this topic.

It says, "Space rocks tend to stick with their own kind" which is what I had in mind with the OP. (I understand now that I was trying to over apply this concept.)

But it also talks about families of asteroids produced by violent collisions between larger bodies. I hadn't considered that.

Best of all, the note shows a wonderful graphic which helps illustrate both the similarities and differences in asteroid composition in the same graphic.

The URL is http://www.scientificamerican.com/a...ced-back-to-the-collisions-that-spawned-them/

 

[Dotini]

"Earth's most abundant mineral lies deep in the planet's interior, sealed off from human eyes. Now, scientists for the first time have gotten a glimpse of the material in nature, enclosed inside a 4.5-billion-year-old meteorite."

http://www.livescience.com/46337-elusive-mineral-named-bridgmanite.html

 

[lpetrich]

I recently learned about "ore genesis" (https://en.wikipedia.org/wiki/Ore_genesis) the topic about the many processes on Earth that separate different elements and minerals. Of course, those processed don't exist in space but I'm wondering if there are analogous processes in space.

Look at the processes that the article lists. Most of them can only operate on places like the Earth, places with surface liquid water. There are some that do not require liquid water, like fractionation of magma bodies. But there is a more direct way of telling: meteorite compositions. They have some variation, but as far as I know, there aren't any big enrichments of relatively rare elements like what one can find here on the Earth.

Can fractionation of magma bodies have happened in the asteroid belt? Maybe, but only for the largest asteroids, like Ceres and Pallas and Vesta. Smaller ones would not be able to trap as much heat as larger ones can -- the square-cube law. HED meteorite - Wikipedia is about some meteorites that are likely from Vesta. These are apparently igneous rocks that formed from magmas inside that asteroid.

 

[anorlunda]

Apologies to lpetrich and to Davenn, my phrasing in my comparison to Earth was unclear.

I meant to compare radial stratification of Earth's interior with respect to distance from Earth's Center, to stratification of body formation in the dust cloud with respect to distance from the Sun. To repeat, not radius from the center of the asteroid, but radius from the sun's center.

I had been trying to oversimplify things in a mental model in which radius from the center of the sun was the only major variable in asteroid and comet formation. Davenn has separately admonished me to not think so narrowly.