If comets didn't give Earth its oceans..., maybe the Moon did? Options

[dvandorn]

The Messenger posted an interesting comment over on the asteroid/comet board, in a discussion about how comets are turning out to have much less water in them than we once thought:

I think it is very legitimate, at this time, to rise the question: If we need an explanation for why the earth has oceans, the collision of objects like Saturns icy moons, rather than comets as we now know them, seems a little more likely.

Now then -- something with a mass like a large outer-planet moon hitting Earth most likely *did* happen -- it's called the Big Whack theory.

I have never seen any speculations on the *composition* of the impactor that created the Moon -- the only comments I have seen are those relating to oxygen isotope abundances which indicate that Moon rocks formed from roughly the same solar nebula composition as Earth did (which I've always thought could be in doubt, since the material that made up the Moon must have a pretty large admixture of material from the proto-Earth).

If the impactor (which had a mass roughly equivalent to Mars) was something like an oversized Ganymede, with a large percentage of ice around a chondritic rocky core, is it possible that *all* of the water ended up back on Earth, while the ring from which the Moon formed lost all of it?

I don't have the proper education or tools, myself, to model a Big Whack with an impactor composed of, say, 25% water ice by mass, to see where the water ends up and how it can be disassociated from the materials that made up the Moon. But it seems to me that the question of how water in the debris cloud could have been so thoroughly removed before the Moon formed must be answered, in any event, since there had to have been terrestrial water involved -- unless we postulate that neither proto-Earth nor impactor had any water. And *that* means we have to postulate no ice-body impacts onto Earth before the Big Whack and a whole bunch afterwards, to explain where the oceans came from.

Assuming, of course, that the oceans came from impacts of ice-body objects in the first place...

-the other Doug

[RNeuhaus]

Due to the big impact of a Mars size to Earth, it seems that the Earth might loose great amount of water, even greater than a mass of land that formed the Moon because the water is less bound to the Earth than to the rocks. That is a susposition.

Rodolfo

[Rob Pinnegar]

One thing to keep in mind here:

From what I remember of some 1980s versions of the giant-impact theory, the Earth was predicted to have an atmosphere of vaporized silicates right after the impact occurred. This gives some idea of how hot the Earth would have been at the time.

[Bob Shaw]

In terms of kinetic energy, there's not going to be any difference between an ice-world hitting proto-Earth or a rocky mini-planet, so I'd expect that the established 'Big Whack' scenario would hold.

But, then we do come to the water issue - something I'd been considering, too!.

The Moon is pretty well dry - there's some water there, but in vanishingly small amounts compared to the Earth. And, as noted, both the bodies involved in the impact would have become very hot, thus baking off many volatiles. Perhaps, though, the Earth (for thus it was by this time) had become the beneficiary of simple geometry, as the Earth's surface area relative to mass is, of course, far lower than that of the Moon. We might expect both bodies to quickly cool their surfaces from the vapourised rock temperatures, and then to gradually cool down. The Moon, however, would cool far faster than the Earth, and also lose any frozen-out volatiles much more quickly, not least due to the solar wind.

The Earth, though, would be warmer for longer, and any water-bearing rocks might have a period in which to become dehydrated and to expel their water to the surface via volcanic processes, and the water wouldn't be lost from the surface as it would from the Moon. Another key factor might be the Earth's magnetic field, which would have helped stop the early solar wind from hitting the planetary surface.

Maybe the key to all this isn't so much our own planet, as the composition and history of the other rocky worlds - if there's evidence that Venus had loads of water at one point then it may make things somewhat interesting...

Oh, and then there's the (controversial) mini-comet hypothesis, where small snowballs might still be hitting our planet (but not backed up by *other* planets!).

And as for Tommy Gold...

Bob Shaw

[The Messenger]

I think the geo-dudes will tell you there is a pretty good argument that the moon broke off from the crust or outer layers of the earth - which helps explain why the moon is relatively iron-poor.

Maxwell proved that the planets could not have jelled from very fine particlate, that there had to be at least stone-sized masses. What the upper limit on the 'stone size' is, is highly debatable. With so many iron-rich silicates popping up in Kuiper belt objects, there is a lot of room for looking sideways at prevailing theories.

[dvandorn]

See, this is where we need some better modeling of the Big Whack to see what behaviors the models predict, over a set of possible variables.

For example, the energy of such a collision would obviously have been enough to vaporize a large amount of the impactor (which, at the size of Mars, is no mean feat). Would the proto-Earth have been completely re-melted and remixed, and thus proceeded through an either complete or partial re-differentiation? The core of the impactor is supposed to have "sunk" to the Earth's core and added its mass of (presumably) nickel and iron to our own -- wouldn't that require a significant, if not total, re-melting of Earth?

How much of the debris ring/disk that formed around the new Earth was composed of the impactor, and how much of materials from the proto-Earth? What materials would have been thrown farthest, and why? What materials would have re-impacted the new Earth, and what would have been pulled into the newly forming Moon? Is there some "magic line" in the size, shape and mass of the debris disk below which its water would be shared between the two new worlds, and above which the lucky Earth retained it all? (Remember, there is no hint that *any* water was ever retained by the Moon in its formation; its polar water ice seems to have been accumulated over the eons from subsequent ice-body impacts.)

Just how discrete would the impactor and terrestrial materials have remained after the impact? How thoroughly would the materials have been mixed? If the impactor had originally accreted in a different part of the solar nebula and had a different oxygen isotope ratio than the proto-Earth's, would the mixing be complete enough to create an amalgam isotope ratio that would, by necessity, be the same between the Moon and the new Earth?

Recall, too, that the Moon formed relatively close to the new Earth (something like 12,000 miles away when it accreted, IIRC), and that a tremendous amount of angular momentum was imparted into the system by the collision. The new Earth was rotating every 10 hours or so at that point. What effect would such angular momentum have on the gravity field, the magnetic field, etc.? And how would partial or total re-melting have affected the Earth's magnetic field, anyway? And how would magnetic field fluctuations and gravity field perturbations (especially as the new Earth was in the process of "swallowing" the impactor's core) have affected the materials in the debris disk?

Since the Moon formed so close to the new Earth, wouldn't it have orbited within the outer van Allen belt? How long did it take for the Moon to traverse the outer Belt? Or did Earth's disrupted core even generate a magnetic field for thousands, or even millions, of years after the impact? What affect would the total disruption of Earth's magnetic field (if that's what happened) have had on the re-accretion of water onto Earth from the debris disk?

Did all of this happen before or after the Sun hit its T-Tauri stage and sent powerful winds out to strip all of the rocky planets of their primordial atmospheres? If the Big Whack occurred before the T-Tauri winds, then could water on Earth have survived the stripping of the primordial atmosphere? If not, wouldn't that mean that Earth had to have accumulated all of its water well *after* the Big Whack?

Questions, questions, questions... I see all of these factors floating in front of me, provocative -- and I don't have the tools available to me to investigate the probabilities. It's frustrating!

-the other Doug

[hendric]

Did all of this happen before or after the Sun hit its T-Tauri stage and sent powerful winds out to strip all of the rocky planets of their primordial atmospheres? If the Big Whack occurred before the T-Tauri winds, then could water on Earth have survived the stripping of the primordial atmosphere? If not, wouldn't that mean that Earth had to have accumulated all of its water well *after* the Big Whack?

I always assumed the water came after the Big Whack, simply because the Moon was well in place during the LHB, and that many of those impactors came from KBOs tossed around by the outer planets, thereby giving us good sources of water etc.

[Bob Shaw]

I always assumed the water came after the Big Whack, simply because the Moon was well in place during the LHB, and that many of those impactors came from KBOs tossed around by the outer planets, thereby giving us good sources of water etc.

The problem with that is the lack of water-altered minerals on the Moon!

Oh, and there *does* appear to be some water in the depths of the Moon - there was a paper on it in the JBIS at one point, pointing out that it's likely to be there, but you can't reach it (though Stephen Baxter wrote fictionally of doing so).

Bob Shaw

[Guest_BruceMoomaw_*]

There is a veritable swarm of abstracts on this subject from the coming LPSC and EGU meetings -- every single one of them presenting a radically different rival theory. More details later; but it looks as though it will take a LONG time to settle the origin of inner-planet water -- if we ever settle it.