Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Atomic-S on 14/02/2012 01:04:04
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Recently at a mineral show I observed a collection of meteorites. They were all heavily pocked with indentations of assorted sizes. I asked the exhibitor if these were formed when the meteorites became heated on entry into the atmosphere. I will not bother you with her answer inasmuch as, after I had asked this, it occurred to me that she could be just a seller and I knew nothing of her scientific credentials. So I will ask the question here instead: Are the pockmarks in meteorites caused by atmospheric entry, or did they already exist in space?
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I am not quite sure what you mean it would be nice to have a visual example. However if you are talking about metal meteorites that have been lying around for a long time and have cavities in them like a rotten tooth. The answer is probably rust because some of the metal bits rust away and others do not giving some indication of their original formation processes. I will try to find a picture of what I am talking about and add it to this reply.
I have now added a picture of a corroded meteorite
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The features you seem to referring to are called regmaglypts. They are, as you suspected, a consequence of ablation during the meteorite entry through the atmosphere. They are characterisitc of iron-nickel meteorites. Stony meteorites do not possess this feature.
Soul surfer has provided an excellent exmaple of the phenomenon I think you are talking about. However, this is not a 'rusting' phenomenon, soul surfer. The variations in chemistry at a small scale are not sufficient to enable such a mechanism.
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The meteorites I saw were like the one pictured to some extent, yet were generally different in that there were basically no unpitted places. I am unsure what that might signify. In the pictured meteorite, the depth of the holes as well as the existence of significant smooth areas between that are essentially holeless, makes it hard for me to understand how this configurated arose from hot gases, unless those gasses came in significant measure from within the meteorite. Is that a possibility? Like when certain volcanic rocks are, upon collection, then placed in a furnace, causing them to swell up into a foam due to intenal gasification.
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I would also like to point out, in the picture, some of the smaller holes: note how some of them cluster in groups, in which they tend to line up in rows. Why these tiny holes that are much smaller than the others, if outside hot air were the only force acting? If, however, external heat caused some kind of internal gas pressure at some inhomogeneity, an arrangement like this is easier to understand. Interestingly enough, the arrangement in which these smaller holes are laid out closely resembles the arrangement in which much larger craters on moons and planets are laid out.
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Which suggests that they both may have been formed by similar processes.
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If you look at a small rocky or metallic meteorite the dents can't be due to hits from smaller objects because the gravitational pull of such a small object is very small and it can't generate the force required to have large dents.
In case its a fragment of a larger object, still its very hard to have such deep holes in a body.
Most of the holes are made when it falls through the atmosphere and burns up, some of the areas has less dense material and that is evaporated as soon as meteorite starts burning, though some of the dents might be due to collusion between two objects but majority is due to burning on entry into the atmosphere.
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Given the similarity of appearance between the meteorite holes and the craters seen on asteroids and othe objects, could we then assume that the craters on those objects were (in many but probably not all cases) formed similarly? Obviously I do not mean that asteroids were shaped by traveling through the Earth's atmosphere, but rather than there was some kind of an analogous thermal process from whatever cause?