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Originally Posted by djellison
On what basis?
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You have to start somewhere. That answer is not as flippant as it sounds
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Why would the 4 rocky planets be the same density? You must think there's a lot of idiots in this place for us to just drift past that assumption un-noticed. How, exactly, could the density (and thus, I presume you infer the bulk composition) of Mercury be the same as Mars when they formed in places two orders of magnitude different in solar radiation.
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You are assuming that the solar system condensed from a dusty ring, I'm assuming it did not. If Newton and Einstein are correct gravity is geometric and basically unstable, you almost have to start with dusty rings to get circular orbits. If on the other hand you reject the relativistic solution for Mercury's procession, it is helpful to have some explanation for Mercury's procession; and the best answer I have today is that a combination of resonant and resistive effects dampens and circularizes orbits.
You mentioned several posts ago that Bouguer anomalies are the difference between the measured gravity and the expected gravity, assuming a body is spherical. That is a simple Bouguer anomaly. The
Bouguer anomaly charts of Venus and Mars are based upon the orbitally measured gravity residual
after correcting for topographic relief, and assuming a constent density of crustal elements.
Mars volcanic mountains appear to be quite grossly over-dense (compared to the Earth), while the mountains of Venus are measurably under-dense. The deep rifts and lowlands of Mars have measured under-densities - for a while it was speculated that this is because of the presences of water; although both the atmospheric physics and visual observations are not consistent with than explanation. Meanwhile, all the chasma of Venus are over-dense. There may be geological explanations for these oddities, but they are conditions that must be observed if the Newtonian equivalence principle is faulty.
The Earths density is 5.52g/cm^3, the Moon is 3.34 g/cm^3. The combined mass and volume of the Earth and the Moon result in a density of 5.47g/cm^3 (figures below) Why do you cite a figure of 4.47, roughly, the average of the two? Why is that a valid number in this instance? It's not the density of the Earth Moon system as a whole.[/quote]
No its not. It is the (Earth+Moon)densities/2 but we both agree that number is likely meaningless - as I said, I did a best-fit curve from Mercury to Pluto, and the earth and moon straddle it; and if you assume the densities of the Earth and Moon are most likely to be the best determined masses in the solar system, the curve fit slices though at a density of 4.47 - I would be happier if the mean were 5.47g/cc^2; but nature is rarely that accomidating.
Watch Messenger and Mercury - if the topography, as modeled from orbit doesn't come off completely skewed (very under dense mountains, very overdense lowlands) you can deep six my model yesterday.