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View Full Version : Is there a difference in the composition of the Sun compared to rest of solar system?



WaxRubiks
2006-Jul-14, 02:44 AM
The Sun is supposed to be mainly hydrogen and helium but the rest of the planets are made from heavier elements, is this correct and if so why the disparity?

Jeff Root
2006-Jul-14, 04:32 AM
Yes, it is correct.

Hydrogen and helium were blown away from the Sun by solar
wind during the early stages of planet formation. To the extent
that hydrogen and helium were already part of the inner planet's
atmospheres, they got blown off of the planets. This doesn't
necessarily happen in all star systems. It depends on how active
the star gets and how massive the planet is. The Sun was likely
a very active T-Tauri star for a while after it became luminous.

-- Jeff, in Minneapolis

Bynaus
2006-Jul-14, 09:02 AM
It is interesting to mention that the sun has more heavy elements than all the planets combined... 1% of it is "metals", elements heavier than helium, this means, ~10 Jupiter masses worth of heavy elements! The heavy elements of all the planets combined don't even make out 0.1 Jupiter masses.

Tobin Dax
2006-Jul-14, 09:32 AM
It is interesting to mention that the sun has more heavy elements than all the planets combined... 1% of it is "metals", elements heavier than helium, this means, ~10 Jupiter masses worth of heavy elements! The heavy elements of all the planets combined don't even make out 0.1 Jupiter masses.

That's only for the same reason that a full 10-gallon bucket has more water than a full 1-gallon bucket.

Also, the ratios of heavy elements found in asteroids, planets, etc., are pretty much the same as the those ratios are in the sun.

TriangleMan
2006-Jul-14, 04:29 PM
In addition hydrogen and helium are light enough to escape Earth, so the vast majority that was with the Earth when it formed eventually escaped into space (as it would on any of the inner planets).

Warning: trying to Google "helium escape velocity" will get you a lot of young-earth creationist websites. They like to argue that helium can't escape from Earth, therefore the lack of it in the atmosphere shows that the Earth is young. :rolleyes:

ngc3314
2006-Jul-14, 05:10 PM
The Sun is supposed to be mainly hydrogen and helium but the rest of the planets are made from heavier elements, is this correct and if so why the disparity?

The more massive and colder a planet, the more sunlike its composition. Galileo's descent probe found that the Jovian atmosphere is as identical to the spectroscopically derived composition of the solar photosphere as the experimenters could be confident in concluding. Hydrogen and helium ar ethe major players by mass, so whether a planet hangs on to them (see the escape velocity arguments) makes all the difference.

neilzero
2006-Jul-14, 05:12 PM
My guess is we are not sure. The outer surface of the 4 inner planets and Earth's moon are mostly elements heavier than helium. The outer surface of the Sun and the 4 gas giant planets is mostly hydrogen with about 8% helium. Even the interior of Earth competition is educated guess. Some doubt has been recently cast on our guesstimates of the interior composition of comets, asteroids, moons etc.
The disparity may be less than we think. The lack of hydrogen and helium on bodies lighter than Neptune is likely due to the solar wind blowing away the light gases from the upper atmosphere. Neil

PhantomWolf
2006-Jul-17, 01:47 AM
Warning: trying to Google "helium escape velocity" will get you a lot of young-earth creationist websites. They like to argue that helium can't escape from Earth, therefore the lack of it in the atmosphere shows that the Earth is young.

Huh? Surely if it couldn't escape then there would be a lot, not very little.... Surely a far better argument is that it escapes very fast and therefore the lack of Helium in the Atmosphere is not able to be used as a measure of the Earth's age one way or the other.

Ken G
2006-Jul-17, 02:38 AM
I think a point that hasn't been brought out is that hydrogen and helium don't as readily form 'sticky' molecules that are good at building planets. Instead they tend to form "volatiles" like H2, H2O, NH4, and CH3, which easily become gaseous and are not good ways to build planets. Since the more massive elements form the strong superstructures of metals and dust, they don't get blown away as easily by the solar wind as the volatile gases.

Tobin Dax
2006-Jul-17, 03:15 AM
I think a point that hasn't been brought out is that hydrogen and helium don't as readily form 'sticky' molecules that are good at building planets. Instead they tend to form "volatiles" like H2, H2O, NH4, and CH3, which easily become gaseous and are not good ways to build planets. Since the more massive elements form the strong superstructures of metals and dust, they don't get blown away as easily by the solar wind as the volatile gases.

Atomic helium tends to not form anything at all, as your examples show so well. :)

(edit for clarity)

94z07
2006-Jul-17, 03:26 AM
He in trapped subterranean reserves is the by-product of radioactive decay. I bet cold fusion alchemists/hoaxers wish they could find a little He.

PhantomWolf
2006-Jul-17, 03:30 AM
Atomic helium tends to not form anything at all, as your examples show so well.

If you give it a kick it will. ;)


Helium can form unstable compounds with tungsten, iodine, fluorine, sulfur and phosphorus when it is subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma. HeNe, HgHe10, WHe2 and the molecular ions He2+, He2++, HeH+, and HeD+ have been created this way. This technique has also allowed the production of the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces . Theoretically, other compounds, like helium fluorohydride (HHeF), may also be possible.

Tim Thompson
2006-Jul-17, 03:32 AM
It is my understanding that the main reason for the lack of volatiles for the smaller, terrestrial planets, is that the fierce stellar wind of the infant sun blew it away. The small cores of stuff that are now the inner planets were not big enough to hang onto lots of H & He. However, the atmospheres of the gas giant planets have relative elemental abundances that are similar to the sun, and meteorites are known to have elemental abundances that are the same as the sun, which is taken to mean that they sample the original makeup of the pre-solar nebula.

Ken G
2006-Jul-17, 03:32 AM
Atomic helium tends to not form anything at all, as your examples show so well.
Yeah, those "nobles" keep to themselves. Not a very good strategy for sticking around long in the circumsolar environment!