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View Full Version : Composition and structure of Hot and Cold asteroid belts?



EDG
2010-Jul-12, 10:38 PM
Our own asteroid belt is a mix of different types of asteroids. From what I can gather about 75% of the asteroids in our belt are Carbonaceous in composition (which essentially seem to be hydrated silicate bodies with some organics), 17% are Stony in composition (which seem to be "dry" silicate bodies with little water or organics, and possibly some metal content), and about 10% are Metallic in composition (mostly nickel-iron) - see http://en.wikipedia.org/wiki/Asteroid_belt#Characteristics

There seems to be a couple of things going on in their distribution - first, M- and S-type asteroids seem to be much more common on the sunward side of the belt than toward the outer part of the belt (which is dominated more by the C-types and other volatile-rich bodies). But there's also the concept that the S-types and M-types are the shattered mantles and cores of larger asteroids where differentiation occurred - so maybe there's also a process by which larger asteroids would have been more common on the inner edge of the belt (possibly because of resonances, or higher density of smaller proto-asteroids there that could grow into larger objects) too?


Anyway, my question is this - if we instead had an asteroid belt where Mercury is (about 0.4-0.5 AU from Sol), would be still have this kind of composition distribution? I think that such a belt would be almost entirely Stony or Metallic in composition since the volatiles in the C-types would all be driven away by the heat - but I'm not sure about that. I would have also said that there'd be more metallic asteroids too, but again on thinking more about it I'm not sure that's the case... unless the smaller, tighter belt would result in larger bodies forming and more differentiation, and there's more disruption and more metallic core fragments exposed?

And what if we had a belt between say Saturn and Uranus instead of between Jupiter and Mars? Would that be predominantly icy or Carbonaceous in composition? I suspect it would be (- I've not heard of Kuiper Belt objects that are stony or metallic... they might be underneath, but I suspect they're all covered by ice and volatiles that have accumulated there in the colder temperatures.

But could we ever see a belt that mostly consisted of metallic asteroids? I'm not sure that we would, unless there's a way that they could form directly as metallic bodies, without being the results of differentiation. Maybe that's possible in really hot belts?

Does anyone knowledgeable on the subject have any insights about this? Or corrections to my thought processes here?

korjik
2010-Jul-12, 11:29 PM
Well, if you hit Mercury hard enough, it would leave you a nearly metallic asteroid belt. The planet is pretty close to that anyway, all things considered.

Actually, now that I have looked, the discussion of Mercury on Wiki has some pretty good info.

EDG
2010-Jul-13, 12:40 AM
Mercury has a large metallic core that is a product of differentiation after it formed though - that doesn't necessarily mean that the majority of the original material that it formed from were metallic asteroids (and note, I'm not talking about "breaking apart Mercury", I'm talking about the stuff that it originally formed from). But that's the question really - if the original material that Mercury formed from was still around today as a belt in its current orbit, would we see asteroids that are mostly stony in appearance? Do you only get metallic asteroids as a result of the differentiation of large Stony (or Carbonaceous?) asteroids? I guess that if it's really hot/close to the sun then the heat could separate rock fom metal in small protoplanetary grains, and the rock could evaporate away (it'd be harder to do with larger objects, since a stony asteroid isn't very thermally conductive), which could result in the in-situ formation of predominantly metallic asteroids? Though I suspect that would only happen within a relatively small temperature range (hot enough to vapourise rock, but not metal).

What about if the belt was in Venus' or Earth's orbit instead (again not talking about shattering an existing Venus or Earth here, I'm talking about leaving the original material that those planets are made out of as a belt)? I suspect such a belt would be more stony/metallic, and (much) less Carbonaceous?

Ken G
2010-Jul-13, 01:01 AM
I can only give the "astro 101" kinds of explanations, because I can see from the OP you know more about asteroids than I do, but I've heard some things about formation that might help (no doubt they are gross oversimplifications). To get asteroids close to the Sun (if they don't move from somewhere else), you need to get the particles to stick together even before there is enough of them to have a significant gravity. That means you need their temperature to be low enough, which seems to have something to do with the melting point, though I don't know why they can't stick together even above their melting point (presumably the weakened interparticle forces in the liquid are insufficient to hold it together). Anyway, the melting points of metals is much higher than that of rock, so you can get metals to stick together closer to the Sun than you can get rocks to do it. It is thought this helps explain why Mercury formed with so much metal, even before it differentiated.

However, I've also heard it said that the difference seems to large in Mercury, there seems like there's too much metal. So a common theory that's invoked is that the rocky mantle of Mercury was partially stripped by a large collision, sort of like how the Moon was stripped from the Earth only moreso. It's pretty oversimplified, but it suggests that even closer to the Sun, maybe you could have gotten more metallic asteroids still.

Hungry4info
2010-Jul-13, 06:01 AM
In a planet-forming disk, volatiles survive out beyond the "ice line", which is the distance beyond which water, ammonia, etc can exist without being destroyed by the sun. Asteroids closer in are usually lacking in volatiles, whereas asteroids farther out are rich in volatiles.

As for a belt at Mercury's distance, there is a close in debris disk similar to what you describe at HD 69830. It's composition resembles P or D type asteroids, showing that this isn't a very strict rule.

EDG
2010-Jul-13, 07:54 AM
In a planet-forming disk, volatiles survive out beyond the "ice line", which is the distance beyond which water, ammonia, etc can exist without being destroyed by the sun. Asteroids closer in are usually lacking in volatiles, whereas asteroids farther out are rich in volatiles.

Yeah, that much I'm aware of... what I'm after I guess is whether there are further stages to "lacking in volatiles" as you get into the hotter environment close to a star. If you start from beyond the snow line and go toward the star, I guess one would expect the progression to go from Icy (C-type, but with lots of ices too - this would be far beyond the snow line) to C-type (nearer the snow line) to S-type (within the snow line, probably up to Venus-equivalent distance)... but if it gets really hot (around or within Mercury-equivalent distance?) then would you expect to see M-type become more dominant at the expense of S-type?



As for a belt at Mercury's distance, there is a close in debris disk similar to what you describe at HD 69830. It's composition resembles P or D type asteroids, showing that this isn't a very strict rule.

The problem there is that it's around 1 AU from a K V star, which puts it around the snow line (according to http://en.wikipedia.org/wiki/HD_69830 ) - essentially it looks like it's the equivalent of (the outer parts of?) our own asteroid belt, but it contains more material (possibly in part because it's orbiting closer to the star?). It's not just the distance from the star that's important, it's the temperature of the belt at that distance - a belt at 1 AU from a K or M V star is going to have a very different composition to a belt at 1 AU around an F or A V star because it's so much hotter around the latter at that distance. So I'm not too surprised that the belt at HD 69830 would have a generally C-type (organic and volatile rich) composition given its distance from the cooler star.

Nereid
2010-Jul-13, 01:44 PM
Yeah, that much I'm aware of... what I'm after I guess is whether there are further stages to "lacking in volatiles" as you get into the hotter environment close to a star. If you start from beyond the snow line and go toward the star, I guess one would expect the progression to go from Icy (C-type, but with lots of ices too - this would be far beyond the snow line) to C-type (nearer the snow line) to S-type (within the snow line, probably up to Venus-equivalent distance)... but if it gets really hot (around or within Mercury-equivalent distance?) then would you expect to see M-type become more dominant at the expense of S-type?
[...]

Just an observation: the melting point of olivine, as a class of mineral (common in stony asteroids and meteorites?) is 1200-1900 C; the melting point of iron (the dominant element in metallic meteorites and asteroids?) is 1530 C. Both these are much higher than the equilibrium temperature at Mercury's distance (due to solar radiation) for our Sun, so unless stonies have large amounts (relatively) of much lower melting point minerals, it would seem that there'd be no differentiation until you get much closer to the Sun (based on just this one factor).

EDG
2010-Jul-13, 05:54 PM
So it might be possible to get a mostly metallic belt if it's REALLY hot - e.g. very close to the star, or if (as the Mercury wiki article suggests) there was a hotter phase in the star's past that raised the temperatures enough at a further distance? Otherwise you'd only get metallic asteroids as a result of the breakup of larger asteroids that internally differentiated (they'd be exposed core material)?

I'm curious to know if there's a chance that belts that are closer to a star might have a greater chance of having large asteroids (the material in the belt is in a smaller volume, so the asteroids more closely packed together, so maybe they have a greater chance to accrete into 100+ km asteroids?) than ones that are further away?

Nereid
2010-Jul-13, 06:04 PM
So it might be possible to get a mostly metallic belt if it's REALLY hot - e.g. very close to the star, or if (as the Mercury wiki article suggests) there was a hotter phase in the star's past that raised the temperatures enough at a further distance? Otherwise you'd only get metallic asteroids as a result of the breakup of larger asteroids that internally differentiated (they'd be exposed core material)?

I'm curious to know if there's a chance that belts that are closer to a star might have a greater chance of having large asteroids (the material in the belt is in a smaller volume, so the asteroids more closely packed together, so maybe they have a greater chance to accrete into 100+ km asteroids?) than ones that are further away?
Another variable: the elemental composition of the pre-stellar (solar?) nebula.

While, AFAIK, the variation in composition of giant molecular clouds - wrt elements - is relatively small (the metallicities vary by much more, IIRC), small differences may become quite important in terms of the kinds of, and proportions of, stony objects which form. For example, how much do elemental compositions have to vary before olivines become merely minor minerals?

EDG
2010-Jul-13, 06:37 PM
I would suspect that olivines would remain quite common - the silicate stuff from which planets form are the constituents of basic rocks anyway, to get things like quartz and feldspars you'd need to partially differentiate them.

But that's a variable I'm not considering here. I was just interested in how changing the temperature would affect the belt that forms.

Nereid
2010-Jul-13, 07:23 PM
So it might be possible to get a mostly metallic belt if it's REALLY hot - e.g. very close to the star, or if (as the Mercury wiki article suggests) there was a hotter phase in the star's past that raised the temperatures enough at a further distance? Otherwise you'd only get metallic asteroids as a result of the breakup of larger asteroids that internally differentiated (they'd be exposed core material)?
I guess this kinda depends on the (physical) chemistry of the pre-solar nebula ... at what temperature do silicates (etc) form? How do those temperatures compare with those at which metals (mostly iron, some nickel) condense? How much of any inner asteroidal belt population would be delivered there from further out? And so on.

Why would iron only form asteroids, at high-ish temperatures?


I'm curious to know if there's a chance that belts that are closer to a star might have a greater chance of having large asteroids (the material in the belt is in a smaller volume, so the asteroids more closely packed together, so maybe they have a greater chance to accrete into 100+ km asteroids?) than ones that are further away?
Well, there's also the disruption that larger, further out, planets cause, through resonances etc. And the various radiation processes would be far more effective in the inner regions, making accumulation less likely? I'm thinking of Poynting-Robertson, and Yarkovsky, ...