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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
The computer simulation (or was it just an illustrative graphic?) that I remember seeing ages ago showed most of the impactor, including all of its core, being assimilated by the proto-Earth. But nobody really knows what happened, and it's a particularly timely season for Mercurial speculation. Go for it, I say! Let the imagination loose until more hard facts come in.
ElkGroveDan
The lunar formation theory is predicated on very specific orbits and masses for the bodies both prior-to and after the encounter. So there wouldn't be any missing material to create another planet. Now you could argue that there was a larger initial mass in the system, but then you have to take into account different orbits, velocities and angular momentum of the bodies which would affect a whole host of other things going all the way back to the stability of the system during the formation of the Earth and it's impactor twin in the first place.

Also keep in mind that this event was an unlikely planetary cue ball in the corner pocket. To account for Mercury from this event you'd need a secondary lucky shot to put it in a place where it wouldn't be swallowed by the Sun our fly off to the far corners of the solar system. If you've ever played with a speeded-up planetary system simulation program, its really quite difficult to place a body in a longterm stable orbit. It's an interesting thought, but I don't think it works here.
karolp
Thank you for your swift answers. However, I did not mean forming Mercury from leftovers but simply SURVIVING, with its outer layers stripped off in the grazing impact with the Earth. I also thought that after passing the Earth it might have undergone some kind of gradual orbital decay caused by some finer debris near the Earth-Moon system, which started spiralling towards the Sun and dragged on Mercury. It is now in a stable but ELLIPTICAL orbit. Also, it bears a strong resemblance to the Moon in many respects. I realise this is just speculation based on certain facts with a tiny little bit of intuition, but maybe it is a good warmup for focusing on investigating Mercury next year.
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
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.
ElkGroveDan
QUOTE (ngunn @ Dec 7 2007, 01:24 PM) *
but I'd appreciate some more detail from the sceptics.

The only thing I would try to emphasize again is the level of precise detail that went into the modeling of the lunar birth event and the need for it all to balance out before and after. It's not like someone just said "Hey I bet a Mars sized body smacked into the Earth creating the Moon". Such things as the masses of the original bodies, their orbits, chemical compositions, and angular momentum, etc are very unique and precise. If you try to account for something the size of Mercury as an additional product of this event, everything changes in the initial calculations and you are forced to throw out the simulation entirely and start over; i.e, with this new theory you aren't building on the prior theory you are completely rejecting it.

From a 2001 Space article:

For 25 years, scientists have pondered a theory that the Moon was created when an object the size of Mars crashed into Earth less than 100 million years after the Sun was born, some 4.6 billion years ago. The general idea has been run through the paces and massaged into shape and is now the favored explanation.

But attempts to model cousins of that theory on computers generate inexplicable side effects.


The point being that much smaller alterations to this theory blow it all to hell.

A more recent 2004 article
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?
JRehling
QUOTE (Phil Stooke @ Dec 7 2007, 08:52 AM) *
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.
MarsIsImportant
Could Mercury have been created from a planetary collision with Earth? No. It is not possible. Mercury is too far down the gravity well for that to be possible.

I once thought that maybe a collsion with Venus created Mercury, explaining both Mercury as a core and Venus with a very slow retrograde rotation. I don't know how this is possible either. There would need to be a massive body closer to the Sun for this to be possible. There isn't any.

But Mercury crashed into something. Perhaps there was once a planet closer to the Sun but has since disappeared for whatever reason. Maybe there once was a Hot Jupiter too close to the Sun and it eventually evaporated? Highly unlikely.

More likely, there were once many planetary objects closer to the Sun and most got either thrown into the Sun or out into the far reaches of the solar system very early in the Solar System's history. The interactions would be far too complex to figure out. More objects would also mean that there would more likely be collisions. So you get the Earth-Moon system, the Venus Retrograde rotation, and the stripped down core of Mercury all from different collisions early in the solar system's history. Where did these object go? Some of them became part of the remaining planets. Others thrown out would be difficult to identify as such even if we were able to find them.

But then we have the recent evidence that suggests that these huge collisions are fairly rare. It's been calculated that they may only occur in 1 out of 20 star systems. So something is not correct. Perhaps Mercury is not a stripped down core of a planet that it appears to be. But that is difficult to believe too. Confused? I am.
edstrick
A mega impact cause for Mercury's high density has been seriously proposed, backed up by numerical simulations similar to those for a lunar origin by mega impact. In this case, a head-on impact, maybe at higher speeds, onto an object with lower mass and lower escape velocity blows a large fracton of the crust and mantle to escape velocity. Some in recaptured over time, but more is gravitationally scattered to impact Venus or be lobbed into higher eccentricity orbits by Venus, etc. It eventually ends up in the sun or scattered throughout the solar system and beyond. It's plausible, Messenger's crust composition mapping and geophysical studies will show if it's more or less plausible. Be very hard to prove.
ngunn
QUOTE (ElkGroveDan @ Dec 7 2007, 10:17 PM) *
with this new theory you aren't building on the prior theory you are completely rejecting it.

Accepted. (edit - something funny going on with the quote boxes here)

Quote fom J Rehling:
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.)


It doesn't have to have been a single event. Scattering a lot of smaller objects and colliding with some of them might have done it. Similar processes have been invoked to account for giant planet migration. What would the region around 0.4 AU have looked like before there was a planet there? We have no idea.

QUOTE (MarsIsImportant @ Dec 8 2007, 06:46 AM) *
More likely, there were once many planetary objects closer to the Sun and most got either thrown into the Sun or out into the far reaches of the solar system very early in the Solar System's history. The interactions would be far too complex to figure out. More objects would also mean that there would more likely be collisions. So you get the Earth-Moon system, the Venus Retrograde rotation, and the stripped down core of Mercury all from different collisions early in the solar system's history. Where did these object go? Some of them became part of the remaining planets. Others thrown out would be difficult to identify as such even if we were able to find them.


Actually I think the scenario quoted above is the most likely. The Mercury-from-Earth-collision idea probably suffers from trying to account for too much with a single event. There is no need to be parsimonious with our proposed collisions. Nevertheless it's interesting to think about. If it actually turned out to be true - well, the computer simulation people would just have to start again. I'm sure they'd come up with a model eventually.smile.gif Myself, I'm going to be very sparing with the word 'impossible' until we have a lot more data. Mercury ahoy!!
Rob Pinnegar
QUOTE (ngunn @ Dec 8 2007, 10:52 AM) *
Actually I think the scenario quoted above is the most likely. The Mercury-from-Earth-collision idea probably suffers from trying to account for too much with a single event.


Yeah. It's a neat idea and is very imaginative. But it won't work in practice.

Even if a grazing collision (between proto-Mercury and Earth) could somehow produce the Moon while sending Mercury plummeting inside Venus's orbit, there's still the problem of keeping it there (i.e. close to the Sun).

To do that, you need to postulate a second major impact on Mercury -- and an impact of that type happens to be one of the events this theory is attempting to replace.
nprev
Beginning to wonder here if the detailed history of our Solar System (or any other, for that matter) may be too complex in terms of variables to effectively model. Wouldn't be as difficult if we knew even the total quantity of significant protoplanet impactors to any degree of certainty, but apparently it only takes one to radically alter the end state after two or three or two hundred have already done their jobs.

IMHO, solar system formation is a very chaotic process that may be quite difficult to define even stochastically. We're beginning to see hints of this with the eccentric--in our view--orbital states of some exoplanets. My own guess about Mercury is that it was originally an Earth-sized body in a circular solar orbit that got whacked at least once (and probably many more times) after its formation & differentiation until its rocky outer layers thinned out to where they are today; the large core is an artifact.
JRehling
QUOTE (nprev @ Dec 8 2007, 01:20 PM) *
My own guess about Mercury is that it was originally an Earth-sized body in a circular solar orbit that got whacked at least once (and probably many more times) after its formation & differentiation until its rocky outer layers thinned out to where they are today; the large core is an artifact.


Keep in mind that Mercury only has 6% of the mass of the Earth. If the other 94% were whacked away, there ought to be an unexplained planet-sized mass somewhere. And the Sun would be an unlikely destination for much if any (Mercury to the Sun is a bigger step in gravitational well-space than Mercury to Pluto).

Collisions are much more likely to stick things together than to break things apart. Imagine the impactee as a dartboard and the impactor as a dart. Just about anywhere the dart strikes the dartboard, almost all of the material will end up absorbed by the impactee. It's only for the most glancing blows that a large fraction of the material will escape. And since the impact process isn't TRYING to aim for the edge, the center will receive many more hits than the edge. (Of course, the gravity of the impactee will slightly increase the center-aiming tendency.) So on balance, worlds getting smacked by impactors are going to gain mass over time, not lose it.

It would take a single, rare, large impact to take a lot of a world's mass away. In the best case we know of (Earth-Moon), it was only 1.2% of the whole that ended up getting peeled away. We don't have any precedent for a planet-sized body losing much of its mass, and there's good reason to suspect that that would be very rare, and would almost never happen via multiple losses of moderate size, because for every "loss" event, there'd be far more "gain" events.
nprev
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...
JRehling
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.
vk3ukf
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
JRehling
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.
qraal
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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