Composition of asteroids

[Guest]

It's been ages since I was here but I have a question about asteroids.

I've been looking at some of the brilliant photos of asteroids which are available now. They look really solid. According to Wikipedia asteroids were formed from the original dust and gas clouds when the solar system was formed. It also says that most asteroids are made of rock. So I was wondering how the dust and gas eventually turned into rock. On earth sedimentary rock is formed from deposits of say sand and over time and with lots of pressure it eventually becomes sandstone for example. But I'm guessing that asteroids don't have a lot of gravity so where did the pressure come from to press all this dust into something like rock?

One idea I had was that the asteroids were originally part of a planet which was broken up into rocky fragments. But the information I have doesn't say that. I had this idea of an asteroid as a pile of dust held together loosely by it's weak gravity and that if you gave it  a good hard knock it would shatter. I think that's wrong so what's wrong with my reasoning?

Cheers

Steve

 

[Gfamily]

They varied - some started small and stayed small, others got much bigger by hitting and sticking together.  

If they got big enough, they had enough gravity to build up considerable pressure and heat in the interior, which allowed there to be some melting and separation of the materials - so there was differentiation of elements within the larger bodies. However, (as the Wiki says) because of the influence of Jupiter, the asteroids had too much energy to coalesce  into a single planet, rather the larger ones got shattered. 

This is why we get different types meteors, Stony, Stony-Iron and Iron - the Iron ones are the ones that came from the centre of the destroyed large meteors. 

It's still not clear (as with most things) how these things get from tiny grain-type things to kilometre size objects - but the results of the Rosetta probe around 61P Churys comet suggests that there was an aggregation of metre sized undifferentiated objects. 

What I find interesting is that analysis of meteors that are recovered from the Earth's surface can be analysed and be shown to have shattered from the Moon, Mars and even the (still) existing asteroid Vesta ( through a specific type of meteor known as (howardite–eucrite–diogenite (HED) meteorites).

[John]

Stony meteorites are divided into chondrites and achondrites. The former have not been altered due to melting or differentiation of their parent body, the latter (which includes the HED group and the Lunar and Martian meteorites) have been affected by melting and recrystalisation. 

It has been assumed that an originating body needs to be of a certain size and mass for differentiation to occur - I think a few hundreds of km is considered to be the minium diameter but this is still under discussion / investigation and I guess proximity (even temporary) to bodies such as Jupiter could cause changes in structure.

What will be interesting is when sample returns are achieved from small asteriods (eg: Bennu). If the samples include achondritic material then ideas may well change again.

 

 

 

[Guest]

Thank you both. Amongst the pictures I looked at is one on called Ryugu. It's described in Wikipedia as a pile of rubble and it does look very much like that. It's just under a kilometre across so the gravity must be quite small I think. When you look at the photo there are quite a lot of biggish boulders or rocks dotted around on the surface. I wondered if those rocks were part of the original solar system forming dust cloud. If they are, then where did they come from? Did they come from a rocky planet which broke up long before the solar system formed? This asteroid in particular does look like you could break it up with a few hard knocks.

 This came to me because I saw an article on Universe Today which said that scientists have worked out that you couldn't break up a large asteroid with a nuclear bomb. So tough luck Bruce Willis. Then I thought if they were loose aggregations of dust and rock then they should be easy to break up.

I'm surprised that an object as small as 100km diameter would be big enough to cause melting and rock formation but I guess someone's done the maths.

[Owmuchonomy]

Yes, the maths is not too difficult. Somewhere between 1000 and 2000 km radius is sufficient to also ensure that gravitational forces create a spherical object too.  When I do my outreach with the spacerocks, the questions you ask are very common and very relevant. If you like books there is a new edition of ‘Meteorites’ by Caroline Smith et al which is a good and accessible read.

[Guest]

Thanks Chris,

I've ordered the book by Caroline Smith. Looks interesting.

Steve

 

[Gfamily]

16 hours ago, woodblock said:

 This came to me because I saw an article on Universe Today which said that scientists have worked out that you couldn't break up a large asteroid with a nuclear bomb. So tough luck Bruce Willis. Then I thought if they were loose aggregations of dust and rock then they should be easy to break up.

I think the reason they said they couldn't be shattered is that the some of the energy has to go into breaking the asteroid apart, but the way that fractures propagate means there then wouldn't be enough energy to make the pieces fly apart against their own gravity. 

They actually modelled a collision between a 1km asteroid and a 25km asteroid, so not exactly the same as the film scenario. 

[Guest]

Thanks,

This kind of brings me back to my original point. You say they modelled a collision between a 1km and a 25km asteroid but the result of the collision will depend on how they modelled the composition of the asteroid.  My idea was that asteroids of that size would be no more than a aggregation of rubble held together by weak gravity rather than solid rock. It would be easier to break up a pile of rubble than a solid rock.

Steve

 

[Gfamily]

Without looking at their analysis, I can't tell what they are arguing, but I would expect the likelihood of the asteroid being shattered would depend on whether fractures propagate kms deep in to the interior, or whether they curve away from the vertical - if they are loose aggregations of rubble, the lines of least resistance may be horizontal, which would keep the core of the asteroid undamaged. 

But, as I say; without looking at the article, I can't tell. 

[JamesF]

On 23/03/2019 at 20:19, Owmuchonomy said:

Yes, the maths is not too difficult. Somewhere between 1000 and 2000 km radius is sufficient to also ensure that gravitational forces create a spherical object too.

I thought it was a fair bit less than that, I have to admit.  I have a nagging hint of memory from one of Brian Cox's programmes suggesting that it may be as low as 200 to 300km radius.  I could easily be wrong though.  I could check if I could remember which programme it was in :(

James

[Gfamily]

6 minutes ago, JamesF said:

I thought it was a fair bit less than that, I have to admit.  I have a nagging hint of memory from one of Brian Cox's programmes suggesting that it may be as low as 200 to 300km radius.  I could easily be wrong though.  I could check if I could remember which programme it was in

James

Vesta is about 500km diameter and is almost, but not quite spherical - so you're definitely in the right area. Ceres at 960km diameter is spherical, which is why it's considered a Dwarf Planet now.

[Guest]

I think that the rocky asteroids were formed from the break up of much larger objects. The biggest asteroids is Ceres which is about 1000km diameter. Most of them are much smaller. Even much smaller ones have craters on them and look pretty solid.  I think my idea of a kind of pile of rubble held together by weak gravity is wrong although I suppose there may be some like that.

 

[Alien 13]

I have often wondered about the composition of the early solar system and the thought that the material was dust and tiny grains of material but what if the supernovae that supplied this gave us huge chunks of iron and rock too that had already been super compressed.

Alan