Why Do Elements Form Clumps Instead of Randomly Distributing?

[bobsmith76]

The elements above iron are formed in a supernova. If that is the case, I don't see why we should find lumps of gold, lumps of uranium, lumps of cobalt, lumps of nickel. Why isn't it the case that all elements are just swarming around randomly with normal distribution.

I put this under general physics rather than atomic physics because I felt that this was more a general question then one that dealt specifically with the atom.

 

[phinds]

The elements above iron are formed in a supernova. If that is the case, I don't see why we should find lumps of gold, lumps of uranium, lumps of cobalt, lumps of nickel. Why isn't it the case that all elements are just swarming around randomly with normal distribution.

I put this under general physics rather than atomic physics because I felt that this was more a general question then one that dealt specifically with the atom.

I'd guess it's because different elements have different densities and so settle together when the planet is still liquid.

 

[Simon Bridge]

Ores form in seams - gives you a hint... when a planet is formed, more massive elements tend to end up closer to the center, but not always - it would be all very mixed as you'd expect. The billions of years pass. Planets are not static over that time frame - the crusts rise and fall, the molten stuff circulates, strata gets laid down, gasses end up in an atmosphere and so on. It's a big long sifting process ... and the different elements and compounds get redistributed by weight and according to how they interact with life and water.

Also remember that the gas cloud our solar system formed from must have been enriched from several supernovae - which would not have been quick or lead to an homogeneous mix of elements.

 

[bobsmith76]

thanks for the answer. i really appreciate it.

 

[Simon Bridge]

No worries - it was a valid and intelligent question. It is quite a challenge to wrap ones mind around the kinds of time-scales we deal with in cosmology and what that means to processes that are statistical in nature. You can start off with something very random and end up with quite intricate structures ... not intuitive.

Of course, while still a condensing gas, more heavy elements will gravitate together faster than lighter elements - putting the solids in the middle of the emerging macroscopic body. Elements that get close enough together to form chemical bonds will also tend to accrete on other elements according to their electromagnetic fields - leading to particular combinations appearing more than others... this adds to the different clumping effects.

 

[Danger]

No worries - it was a valid and intelligent question.

Roger that. I've been kicking around for a long time, and that particular matter never crossed my mind. I had to really stop and think about it when I read it.
Excellent response, Simon.

 

[cmb]

Might it be expected, then, that a planet/moon/asteroid that has no vulcanic or biological processes, or is at the point of a common liquid-gas transition (water or methane, for 'rain' and weathering effects), should have a more even distribution of elements?

I think there are still a few interesting, unanswered questions here -
1) why do meteorites tend to have elevated levels of iridium, compared with earth
2) why are comets made up of water
3) if all the rocky planets are made up of the same original space-dust that formed the solar system, do they have the same ratios for all non-gaseous elements (which I believe false, because here on Earth we were blessed with an abundance of uranium/primordial plutonium elements which has kept the core molten, which I believe is unlike, e.g., Mars).

 

[Danger]

1) no idea
2) they aren't
3) no idea

To clarify #2: Most comets incorporate some amount of frozen water, but most of the ice is methane, ammonia, C0₂ and other assorted unpleasant things, with a lot of rocks wrapped around it.

 

[cmb]

Thanks for the clarification on #2.

I think I should have said;

2) why do comets have a particular composition of ices

 

[Simon Bridge]

Roger that. I've been kicking around for a long time, and that particular matter never crossed my mind. I had to really stop and think about it when I read it.

Part of the reward for participating in forums like this when someone asks a naive-sounding question that you'd never think of yourself - but turns out to need careful thought. You deepen your own knowledge that way.

Might it be expected, then, that a planet/moon/asteroid that has no vulcanic or biological processes, or is at the point of a common liquid-gas transition (water or methane, for 'rain' and weathering effects), should have a more even distribution of elements?

... depending on the formation processes ... the processes would differ and produce different clumping.

Our moon, for example, is very similar in composition to the Earth (apart from the lack of free gasses and surface water or smooth dust or grains - already a difference). This is likely because it formed in close conjunction with the Earth - say as the result of a big collision ...

Asteroids formed by similar processes to planetary formation ... the bigger ones have a kind-of potato look to them, suggesting a once molten state after a collision. This process mixes stuff up allowing materials to reclump by momentum and chemical reactions.

So let's look at the questions:

1. they are not.
http://www.lpi.usra.edu/science/kring/epo_web/meteorites/composition.html
You are thinking of the K-T boundary Iridium layer.
Remember, meteorites are the bits that don't burn up in the atmosphere.

2. comets have quite a wide composition
http://www.Newton.dep.anl.gov/askasci/ast99/ast99090.htm
... however, if they did not have a goodly proportion of volatiles, you would not be able to see them as a fiery trail in the sky. i.e. they would not be comets.

Imagine you have something like Ganymede collide with something like Europa - what you get will be a lot of chunks of different kinds of stuff ... including big chunks with lots of water in it. Collisions, breaking up, reforming, is all part of the general mixing that happens right up to the present day.

3. they formed through different processes - Mars cooled so fast because it is much smaller than the Earth and further away from the Sun. The ratios of elements is pretty consistent across bodies in the solar system ... but not every body has every thing.
http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements
http://ijolite.geology.uiuc.edu/08SprgClass/geo593K11/ClassProject/Halliday01_SSR_Mars.pdf

The early solar system was likely a condensing disk of gas, irregularly seeded from supernova remnants, and spinning. The odd rogue planet wanters through from time to time. This is not going to produce much uniformity over the 5-billion years it took to get to this state. For all the reasons already mentioned, we do not expect a randomly shuffled even mix of elements to remain uniform. The fundamental forces have a sieving/sorting effect. Similarly you don't expect uniform distribution of material in the pan, when you are panning for gold: sloosh the muck around a bit and they quickly sort out. Uniformity would actually be the surprise.

Bear in mind that we have mostly indirect knowledge of the various bodies in the solar system (that includes of the Earth - nobody has drilled past the crust). We have enough to be confident of the broad strokes, pretty happy with many of the details, but the exact formation of specific structures is still poorly understood.

By analogy, it is a bit like looking at a stalactite - we know a lot about how stalactites form but if you want to know how a particular lump got in a particular place you are going to draw a blank.

In this forum I can only tell you general stuff and point in something like a helpful direction. You realize these are big topics?