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Mercury - a left-over of the crash that created the Moon?, Highly speculative but maybe worth it
nprev
post Dec 8 2007, 11:33 PM
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Hmm. Thanks, JR.

Well, how's this, then: Mercury formed where it is, has always been there. During the Sun's T Tauri phase & subsequent period of greater luminosity (as much as 30% higher than that of today), Mercury lost all of its volatiles, including a significant fraction of slightly heavier elements that the outer planets retained. After differentiation & the LHB, what's left is basically a ball of molten iron with a relatively thin shell of silicates.

Problem: Earth's average density is actually higher than that of Mercury, in fact the highest of all the Sun's planets. If the preceding scenario was correct, you'd think that Mercury would be denser...


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JRehling
post Dec 9 2007, 04:58 AM
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QUOTE (nprev @ Dec 8 2007, 03:33 PM) *
Hmm. Thanks, JR.

Well, how's this, then: Mercury formed where it is, has always been there. During the Sun's T Tauri phase & subsequent period of greater luminosity (as much as 30% higher than that of today), Mercury lost all of its volatiles, including a significant fraction of slightly heavier elements that the outer planets retained. After differentiation & the LHB, what's left is basically a ball of molten iron with a relatively thin shell of silicates.

Problem: Earth's average density is actually higher than that of Mercury, in fact the highest of all the Sun's planets. If the preceding scenario was correct, you'd think that Mercury would be denser...


Mercury is actually slightly denser in terms of its constituents (higher average atomic mass). Earth is "literally" denser only because with its greater size, it is more compressed. Really, the densities of Mercury, Venus, and Earth are in a virtual tie (at the top of all major bodies in the solar system), with the differences not being clearly significant.

But I have always thought of it about as you state, that generally the trend of hotter = less volatiles should have a chance to act also in the realm of the solids. As a variant, maybe at the protoplanet stage there were more protoplanets with more iron inside 1.3 AU than between 1.3 AU and 4 AU, so as accretion proceeded, there'd be a random chance of two worlds in similar parts of the solar system getting different amounts of metal, with the inner three planets happening to get about the same amount, but Mars quite a bit less.
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vk3ukf
post Dec 28 2007, 08:15 PM
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Hi, I never realised that Mercury has such a strange orbit around the Sun, I just downloaded a gravity sim and speed it up, and I thought there was something wrong with it. Mercury apears to have a highly eliptical orbit that rotates as well.
Or is my grav sim a bit bonkers.
If Mercury had formed where it is now, would the orbit not be a lot more circular?
I think back to the theories that were developed when they were looking for planets and a chap came up with the planetary orbit resonance theory, that predicted a planet between Mars and Jupiter, but they only found asteroids. His numbers fit fairly well.
It was called the Titius-Bode law.
http://metaresearch.org/solar%20system/eph/eph2000.asp
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JRehling
post Dec 28 2007, 08:22 PM
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Yes, Mercury has a pretty notably eccentric orbit. It's so deep in the Sun's gravity well that the tugs that other planets might exert are comparatively quite minimal. But an eccentric orbit is no less stable than a circular one. Presumably, random accretion events tend on balance to circularize orbits, but once accretion stops, the circularization stops, too, and Mercury's orbit is actually a lot less eccentric than most extra-solar planets, not counting those that are so close that tides have circularized them.

Since Mercury's rotation has been synchronized with its revolution (3:2 ratio of periods), there was probably some tidal circularization of its orbit as well, but it's much farther out than your typical hot Jupiter.

The "rotation" ("precession" is the preferred term) of Mercury's aphelion is actually due to relativistic effects, not the pull of other planets, and was one of the early points of validation for General Relativity.
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qraal
post Oct 26 2008, 05:40 AM
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Mercury is like a Mars minus a big chunk of mantle. A high speed collision between a proto-Mercury and another object might just put Mercury into an eccentric orbit and blow the bulk of the mantle away into space.

I find it curious that the Moon-forming impactor, Theia, struck Earth with almost zero hyperbolic excess - plus its chemical composition was too akin for it to have formed far away from Earth's radial position. Could Theia have been a Lagrange point planet that formed in Earth's orbit? Could proto-Mercury have formed in Venus's? And what whacked into Mars to wipe off a hemisphere?
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