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...

[ngunn]

With the greatest of respect to our two sceptics, and fully accepting the objection they raise, there is a world of a difference between 'improbable' and 'impossible'. The early history of the solar system must have been an endless parade of individually improbable events. It is virtually certain that some improbable things have happened to each and every one of the surviving planets at some time - the problem is not whether but which improbable events occurred. Also, we don't see today all those protoplanets that didn't make it, only the 'lucky' ones. So, just for fun, I think it's worth kicking this around a bit more.

After the putative collision with Earth it's not hard to imagine proto-Mercury entering an inclined, highly elliptical orbit with its aphelion close to Earth's orbit and perihelion close to where it is now. It's most immediate problem would be avoiding fruther close encounters with Earth. Now, since the collision would have had to decelerate the Mercury-precursor it might also have accelerated the proto-Earth, leaving it too in a somewhat elliptical orbit with perihelion close to the point where the collision occurred. This would have greatly reduced the probability of any subsequent close encounter. Still there is a need for a mechanism to lower the aphelion of proto-Mercury, both to take it out of danger from Earth and to move it gradually to where it is now. The inclined proto-Mercury would have continued to encounter the remaining protoplanetary disc in two places - near aphelion and perihelion. The former would have been comparatively clear, having been swept out by the proto-Earth. The latter (near Mercury's present orbit) would not, there having been no planet there hitherto to sweep it clear. Mercury would have had its work to do clearing this zone, reducing both its inclination and its eccentricity (in the form of a lowered aphelion) whilst getting severely battered in the process.

Now I know orbital dynamics cannot be done from an armchair - especially mine - but I'd appreciate some more detail from the sceptics (or hard data from Mercury) before abandoning this intriguing idea completely.

[karolp]

How about seeing this on SPACE.COM in a year's time, with a similar headline and an explanation aking to ngunn's? As proved by research on exoplantes, a "clockwork" planetary system as ours is a rare exception and we are not sure whether it looked like that in the past. Remember theories about Jupiter migrating inwards or Uranus and Neptune having elliptical orbits? By the way, is it at all POSSIBLE to have a rocky planet form that close to the Sun in a stable orbit?