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Mercury looks like the Moon, and so it makes me think about a question: is it possible that Venus and Mercury were the a same planet originally, and a giant impact with that planet made it split into Mercury and Venus (like with the Moon and Earth)?

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    $\begingroup$ I would speculate that the heavier elements in Mercury fits better with it having formed close to the Sun, than as the smaller fragment having caught the heaviest parts of Venus. The Moon is pretty low density. And I can imagine problems with impact debris reforming to a planet, not around Venus, but far away in heliocentric orbit. But little is known about both objects. M is hard to reach and V is hard to survive on. $\endgroup$ – LocalFluff Apr 1 '16 at 11:08
  • $\begingroup$ From an orbital mechanics point of view I cant see how you can go from orbiting a planet to orbiting the sun without some sort of divine intervention. $\endgroup$ – Dean Apr 1 '16 at 12:48
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    $\begingroup$ Densities: Mercury 5.4 g/cm^3 Venus 5.2 Earth 5.5 Luna 3.3 Dense stuff sinks. It'd be odd for the moon, Mercury, to be denser than the planet, Venus. $\endgroup$ – Wayfaring Stranger Apr 1 '16 at 13:58
  • $\begingroup$ Any large rocky body with no atmosphere is likely to look similar to the Moon and Mercury. Their appearance is dominated by impact craters. That tell you much about whether it's a (former) moon. $\endgroup$ – Keith Thompson Apr 3 '16 at 19:58
  • $\begingroup$ @WayfaringStranger nailed it. Mercury's enormous iron core relative to it's size means it couldn't possibly have been formed by a Thia like impact. It was probably impacted itself or it may have lost much of it's surface by being too close to the sun, but it's likely always been a planet. Compare Mercury's core to the Moon's core. They're nothing alike. $\endgroup$ – userLTK May 20 '16 at 2:05
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This was originally going to be a comment, but it ran too long, so I'm making it an answer.

Some models argue that the scenario of a satellite of Venus escaping like this is unlikely. Alemi & Stevenson (2006) have explored the possibility of a prior Venusian moon, starting from the assumption that Venus would not have been able to avoid a giant impact. Here's their sequence of events:

  1. A large body collides with Venus in a similar manner to the proposed Earth-Theia collision.
  2. Debris from the impact moves outwards into a disk surrounding Venus,
  3. A moon coalesces from the disk, and begins to slowly recede because of tidal acceleration.
  4. Another large body hits Venus. It reduces Venus's angular momentum, reversing its rotation.
  5. The moon spirals into Venus as it undergoes tidal deceleration, finally colliding with it again.

One of the tricky things about testing this model is that the authors say that there would not necessarily have been drastic composition changes, meaning that it would be hard to analyze the planet's surface and see if there is evidence supporting the double impact hypothesis. So far, there have not been tests.

It is certainly true that Venus could have suffered other impacts - the model does not preclude that. There are a couple problems with Mercury arising from such a collision:

  • Other impacts could have ended up with the same result as the original moon.
  • The chances of many more impacts aren't too high.
  • Solar tides would likely have destabilized the orbit of any moon larger than a few kilometers in diameter (see Sheppard & Trujillo (2009)).
  • MESSENGER determined that Mercury has a high potassium/thorium ratio on its surface, which would seem to disprove any events involving extremely high temperatures, including any giant impact variant.

Of course, if we accept that Venus could have captured a moon, only the third objection remains - still a strong point against the survival of a satellite, even by itself.

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  • $\begingroup$ The second impacting body might eject the moon on its flyby. Nevertheless, the element ratios are way off. $\endgroup$ – Joshua Jul 2 '17 at 23:40

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