Why does it appear that the heavier elements (carbon, nitrogen, oxygen, silicon, nickel, iron) are concentrated in the planets rather than the sun? I realize that this may not actually be the case, as Jupiter and Saturn may be mostly hydrogen, as may be Uranus and Neptune. Is it simply that gases like hydrogen and helium could not condense into little packets the size of Earth, and so were gobbled up by the sun and the gas giants, while heavier elements that could form into solids such as silicon, oxygen, iron, and nickel condensed into planets where convenient but also fell into the sun and larger gas giants in proportion?
So far as the smaller "terrestrial" planets go, they're not massive enough to hang on to large quantities of hydrogen and helium. So they would have had more, but lost it (and probably didn't "catch" so much as they formed).
Think about the refractory nature of the varios elements as well as the condensation temperature profile in the initial solar nebula. Also the chemical affinity of some elements for others is a big effect in some cases.
Uranus and Neptune are nowhere near as hydrogen/helium rich as Jupiter and Saturn.
Check out a paper by Lodders. It think it's about the year 2000 and it's on astro/ph.
By mass, the sun is 70% hydrogen, 28% helium, and 2% other elements. The sun's total mass is 1.989e30 kg. 2% of that is 3.978e28. The Earth's mass is 5.972e24. So, there's over 6,600 Earths-worth of elements heavier than helium inside the sun.
As mentioned previously, free hydrogen & helium on Earth can escape into space more readily than it can on more massive planets like the gas giants.
I think the mass of the planets is not the only factor for the presence or absence of lighter elements on the planets. It also depends on the surface temperature of the planet. Higher the temperature higher the velocity of the molecules and hence most likely to escape.
In other words closer the planet is to the sun thinner the atmosphere due higher surface temperatures. Hence if the Jupiter happens to be one of the inner planets it would not have probably held that much Hydrogen.
There is a remarkable agreement between the condensation temperature (the temperature at which a solid or liquid of a given species forms) and the abundance of those elements vs. planetary location. For instance, if we group the compounds we observe based on the thermodynamic equilibrium condensation temperature, you would have the following temperature scale:
K Species 1600 Al, Ca oxides REFRACTORY 1200 Mg Silicates, Iron metal 1000 Feldspars 750 Iron sulfide 400 Magnetite 200 Water 150 Ammonia 100 Methane 70 Liquid nitrogen, other similar gases VOLATILE
With refractory being high temperature molecules, and volatile being low temp molecules.
Now if you use an adiabat (or a slight variation thereof) to decrease temperature with distance, you'll find that the chemical abundances in the planets correspond very well with the location of where a given temperature would be. This is especially true of the trace elements- if you look at the Martian meteorite trace element abundances, you'll see that the more volatile elements are in greater abundance. This is due to Mars' distance from the sun (1.52 AU as opposed to 1 AU). Likewise, the gaseous giants contain even more volatile compounds- Jupiter is rich in water, and may have formed due to the presence of the snow line, Saturn and Uranus have water and methane, Neptune is rich in Methane, Pluto and other Kuiper bodies have solid methane and N2 ice. You can add this to the list:
K Species 1600 Al, Ca oxides *Mercury 1200 Mg Silicates, Iron metal *Venus 1000 Feldspars *Earth 750 Iron sulfide *Mars 400 Magnetite *Asteroid Belt 200 Water *Jupiter, Saturn 150 Ammonia *Saturn, Uranus 100 Methane *Uranus, Neptune 70 Liquid nitrogen, other similar gases *Pluto, comets
Reference and general equilibrium calculations are described here, as is a temperature-radius profile: M.A. Pasek et al. 2005, Icarus, in press.
I understood this was a consequence of solar system formation. During the formation of the sun, it goes through contractions and explosions, which blast material out into the broader gravity well (heavy elements are of course themselves the products of solar fusion). Light elements can be blasted quite a distance while the heavier elements slow down sooner (as they are climbing the solar gravity well).
Heavy elements also have more mass, and thus more gravitational attraction, than the lighter elements. The heavy elements become the kernels of the developing planets, and then attract light elements once some gravitational bulk had been built up.
Hi Contracycle- I'm not a solar system physicist, but I'm pretty sure that matter is not blasted out of the sun very much during stellar formation, at least not enough to influence planetary evolution. The initial cloud from which the solar system was formed was nothing too special, mostly H2 and He, with a smattering of light elements. The cloud collapsed, most matter forming the sun, but with enough angular momentum to keep a significant portion of matter in a disk surrounding the protosun. This cooled and some of the regions where matter concentrated (due to condensation) later formed planets.
Additionally, there's no real chemical evidence for heavier elements going farther than lighter elements. Uranus and Neptune both contain more heavy elements than Jupiter and Saturn, yet they are a significantly greater distance from the sun. Earth is also the most dense planet, so it contains the heaviest elements, yet Mercury and Venus are both closer.
The phenomenon you describe is important for carbon stars, though. Carbon stars continuously expell sizable quantities of carbon-rich matter, which we observe in pre-solar grains in meteorites.