In this comment, Astronomy SE community member David S. writes:
Just an FYI, Earth is almost 50% oxygen by weight.
How, specifically, is "Earth" defined to make this statement correct?
(Or, is the comment mistaken?)
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asked Aug 14 at 8:41
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1 Answer
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4
It's in the rocks!
The atmosphere is a tiny fraction of Earth's mass; there's no reasonable way of talking about "the Earth" that ends up with gaseous dioxygen making up even a millionth of its mass.
But the crust and mantle are almost entirely silicates - various oxides of silicon and magnesium which together are about half oxygen by mass. The mantle is two-thirds of the Earth's mass; most of the rest is the nickel-iron core (which also contains some oxygen, but less). All in all, oxygen makes up 30.1% of the Earth's mass, second only to iron at 32.1%. The "50% oxygen by weight" in the referenced comment is a little high, but I suppose that counts as "almost".
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$\begingroup$ Doesn’t the weigh less relative to it’s mass as it close to the centre of the earth $\endgroup$ Commented Aug 14 at 9:53
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1$\begingroup$ The simplest interpretation is that "by weight" is a slightly careless/sloppy attempt to say "by mass". $\endgroup$ Commented Aug 14 at 12:24
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6$\begingroup$ Yeah the commenter was almost certainly looking at a crustal elemental composition, where it looks to be about 46%. $\endgroup$– notovnyCommented Aug 14 at 12:53
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4$\begingroup$ Yea, it was my mistake, I grabbed the wrong reference, the crust is "almost" and the entire planet, well, I appreciate kind phrasing. Was too late to edit the comment, but not worth deleting it now. $\endgroup$– David SCommented Aug 14 at 17:22