I should start by admitting that I’m not sure this is the right term. I’ve struggled with terminology, and this is the best I’ve come up with. But if there’s a standard term, I’m pretty sure this is not it.
So what do I mean when I say ‘gravitational assortment’? Well, it’s a phenomenon that’s found in larger astronomical bodies. In outer space, and in smaller bodies, there’s not significant gravitational pull to assort matter gravitationally. Elements are mixed together more or less randomly. Although there is limited tidal assortment caused by affects at a distance of very large concentrations of mass like stars, the bulk of such assortment is done inside larger bodies: stars, planets, larger moons, etc.
So how does gravitational assortment work? Gravity pulls matter toward the center of gravity in any mass/combination of masses. All matter exerts gravitational attraction, but the more massive an object is, the greater its gravitational pull.
Note that there’s a tendency to confuse mass with sheer size. The old riddle: “Which is heavier, a pound of feathers or a pound of lead?” illustrates the matter to some degree: but it has to be restated as “Which is LARGER, a pound of feathers or a pound of lead?” to demonstrate the distinguishing factor between weight and mass: Weight=mass * density. Lead is more weighty than feathers/unit mass because it’s more dense.
Nevertheless, it’s fairly generally the case that objects that are subject to gravitational assortment are not only more massy, but also larger, than ones that are not.
Objects with large mass tend to composed of variable materials, with differing density. Over time, the denser materials tend to migrate toward the center of gravity, which, in the largest masses, is very near the physical center.
The result is that larger bodies tend to become stratified, so that more massive elements (such as ‘heavy metals’, like lead but also like iron) are well below the crust. Some of the most dense elements congregate in the core.
Conversely, less dense elements (such as aluminum) tend to be concentrated in the crust.
There are exceptions, of course. There is upwelling through volcanism, uneven heating, tidal forces, currents in the mantle, etc. And in really massive bodies, such as gas giants, ice giants, stars, etc. the elements contained in the masses may not be as variable, and in this case there wouldn’t be so great a tendency toward gravitational assortment, because circulatory forces would take precedence.
What importance does gravitational assortment have? For one thing, it means that heavy metals, though they are as common in planetary bodies as in smaller accretions, are often not as accessible as in (say) asteroids. Iron, for example, tends to congregate near the core of planets–but in many asteroids, iron is often randomly distributed–which makes it more plentiful, but less easily extracted. Plans for asteroid mining have to come up with a substitute for gravitational assortment to separate materials by density.
Another consequence is that it becomes possible to distinguish between dust which is native to the larger bodies and external dust. Thus, for example, on Earth dust which is high in iridium is pretty certain to be from extraterrestrial sources–which is useful for the diagnosis of things like meteor strikes and the speed of solar winds.
For today’s less common question, we often hear reports of research by ‘CERN’. and it occurred to me to wonder what ‘CERN’ stands for. So here it is: ‘Conseil Européen de Recherche Nucléaire’ (ie ‘European Council for Nuclear Research’).
treraia 