Mercury - a left-over of the crash that created the Moon?, Highly speculative but maybe worth it Options

[karolp]

Having agreed upon the Moon being created by a grazing collision with a Mars-sized object, could we give any thoughts to what happened to the impactor afterwards? Since it was a grazing collision, it might have been melted but not destroyed, only with some material ripped from it (and from the Earth) which ended up in Earth orbit to coalsce into what we know as the Moon today. But what happened to the impactor after it passed the Earth? Could it still hang around somewhere in the Solar System? I suppose it would bear some significant markings after the event, for instance have its outer layers stripped. But wait a second... Mercury DOES have its outer layers stripped off, with an unusually high mean density resulting from a core which could be considered oversized for such a small planetary body. In the wake of MESSENGER beginning to reveal Mercury's secrets in January, could anyone bother to give any thoughts to this idea? I am not sure whether it had been put forth previously or not, I am just curious if it could make any sense to have the impactor impact the Earth in a grazing manner and then end up parked in an elliptical orbit close to the Sun, with its outer layers stripped and an "oversized" original core left inside...

[Phil Stooke]

Quite right, EGD. The hardest part is getting the object to where Mercury is now. I'd say it was effectively impossible.

Phil

[JRehling]

Quite right, EGD. The hardest part is getting the object to where Mercury is now. I'd say it was effectively impossible.

Phil

Yep. As a solid rule, when an object takes part in an interaction (violent or otherwise) at a certain distance from the Sun, then the object's eventual orbit will leave it passing through that same distance in the future. So the most mercurylike orbit that could result from an collision at/with the Earth would be an orbit with perihelion at 0.4 AU and aphelion at about 1.0 AU. Without there being another large body at about 0.4 AU, there would be no way to draw that aphelion all the way in from 1.0 AU to 0.4 AU. (Which is a huge distance in gravity-well distance.) A second large (but inelastic) collision at 0.4 AU would have some potential for doing this, but then the other body would have to be a very significant fraction of the earth-collider in mass. Which upends the premise of Mercury (today) consisting largely of matter that took part in the Earth-Moon collision.

You could degenerate the premise and allow for just *some* of current-Mercury to have been matter from the Earth-Moon collision, but if you degenerate it sufficiently, you probably make it trivially true (ie, if you only stipulate that some grams/kilograms of the matter from that collision eventually "accreted" into/onto a Mercury that was already at 0.4 AU. But there's no way to get the bulk of the mass to end up at 0.4 AU. As they say in New England, You can't get there from here.

Venus is not the answer to making it work, either. While Venus could bend the orbit of something orbiting between 0.4 and 1.0 AU, it -- also -- could not drop the aphelion to 0.4 AU. You'd end up with a body with an aphelion at 0.7 AU, and getting that to drop down to 0.5 AU is, again, a massive change in orbital velocity.