Expert: Courtney Seligman - 10/1/2010

Question
Courtney,

I often hear of asteroids as being pebbles and rocks and boulders that never became a planet.

If they are solid rock, though, wouldn't they had to have once been part of something large enough to have enough gravity to form solid rock?  And would asteroids that are mostly iron had to have once been part of something not only large, but molten as well, to allow the iron to settle out?

If this is true, then would dating asteroids gives the age of these process on a doomed planetoid, rather than the actual age of the solar system?

Thanks,
Paul

Answer
In the early solar system, any rocky body more than a couple of hundred miles across would have melted for one reason or another (there is a detailed discussion of this on my website, at http://cseligman.com/text/ssevolve/melt.htm). When they cooled and resolidified, they would become chunks of rock (on the outside) and metal (on the inside). Rocky bits blasted off them by subsequent collisions are the source of the rocks, irons and such which run into us (meteorites). So they did form like planets; just not like the major planets, which were large enough to sweep up all the remaining debris and be more or less alone in their individual orbits. These could be called "differentiated" asteroids.

There are also asteroids that are loose accumulations of relatively fragile rubble, which were never part of something big enough to melt. As far as nothing of geological interest took place on or in them since the start of the Solar System, these would be
called "primitive" asteroids (or meteoroids).

Studies of rocks (meteorites) which fall to Earth from space which were part of differentiated asteroids tell us about the nature of such objects. Depending upon the kind of minerals in the meteorites, we can tell the approximate date at which they resolidified, and the length of time since they were blasted off their source asteroids (the former by studying radioactive materials and their decay products, the latter by studying cosmic ray tracks in the surface layers of the meteorites). In general, the asteroids melted around 4.4+ billion years ago, but most of the pieces were knocked off them within the last 200 million years.

Studies of meteorites which were never part of any asteroid, or were part of a primitive asteroid, can be used to determine the physical conditions in that part of the Solar System at the time it formed, and the time at which that occurred (around 4.5 billion years ago). In fact, when you read a discussion of the origin of the Solar System (such as that at http://cseligman.com/text/ssevolve/ssorigin.htm), most of the hard facts about dates and physical conditions come from the study of primitive meteorites; whereas most of the story of the melting and differentiation of the planets is derived from studies of meteorites knocked out of differentiated asteroids.

I think this takes care of your question, but to make sure I clearly answered your last question -- the radioactive decay studies show that all the differentiation took place near the beginning of the Solar System; but as you supposed, it is possible to determine the approximate time since the collision which knocked a particular piece out of its parent asteroid.

With respect to this idea, there is a group of asteroids, whose orbits are just outside the orbit of Mars, which share physical and chemical characteristics which make it clear that they were originally part of a single larger asteroid. These are named after the currently largest member of the group (unfortunately, the name escapes my memory at the moment). Based on orbital simulations, we estimate that around half of all large asteroid impacts in the inner Solar System during the last 200 million years were due to pieces from that particular collision (the other half being due to random bits, or the results of other collisional groups).

If this doesn't take care of your question, or raises more questions, please feel free to contact me again. Also, I apologize for any confusing grammar or spelling, as I have to get up very early, and don't have time to carefully edit this answer.