PDA

View Full Version : How was the Moon created?



Sp1ke
2006-Aug-25, 02:18 PM
Exploring the current thinking on how the Moon was created and why it is like it is...

I have been reading a book recently called "Who built the Moon?" by Chris Knight and Alan Butler. I'm ignoring the main thread of the book (that the Moon is artificial, made by aliens to help intelligent life develop on Earth) but it did trigger off some thoughts about the "giant impact" theory of the Moon's birth.

Knight and Butler started by criticising the current explanation of how the Moon was created, pointing out how unlikely it all was. The description of the latest theory looks fairly plausible to me but I would be interested in others' thoughts on it.

Here's what I think the theory is at the moment:

A "rogue" planet the size of Mars or up to three times larger is in the same orbit as the Earth
The rogue planet collides with Earth and part of the mantle of the Earth is ejected
The mantle coalesces into the Moon

The rocks on the surface of the Moon are similar to those on Earth, explained by the Moon's formation from the Earth's mantle.

The close match between the oxygen isotopes of the Earth and the Moon indicate that the rogue planet was created in the neighbourhood of the Earth.

The heavy elements on the Earth had already sunk to its core so that's why the Moon is short of heavy elements like iron.

Two points from Knight and Butler aren't obviously explained by this theory but are they spurious?
1) "The impact would have speeded up the Earth's rotation so a second impact in the opposite direction by another body is needed to restore it to the observed speed"
2) "The impact was not enough to have caused the 23 degree tilt of the Earth's axis"

For 1) I haven't seen anyone proposing a second impact and I can't see why the first impact would always speed up the Earth's rotation

In 2), I don't know what impact is needed to cause the tilt but presumably the other planets have achieved their various tilts, some much greater than Earth's, without any difficulty so I don't see this as valid objection.

So are we confident that we've figured it all out or might there still be some surprises in store? The Moon is unusually large compared with other moons but not very dense. How many unlikely events are needed for our theory? Are there any parts of the theory less robust than others?

grant hutchison
2006-Aug-25, 02:41 PM
1) How do they know how fast the Earth was rotating before the impact? Couldn't the impact have converted the prior spin-state of the Earth to an appropriate rate (about one rotation every five hours) which was then tidally slowed (by the moon) to its present rate?
2) I don't know the maths, but there's no requirement that the Earth's axial tilt be generated by that single impact. Its orientation presumably resulted from the accumulation of other impacts, great and small, during its formation, as well as from the impact that formed the moon.

In summary, they seem to be making some assumptions about the pre-impact state of the Earth, from which they deduce that the impact itself was insufficient / excessive to account for the Earth's present state. Unless they have a lot of theory to back up their assumptions about the pre-impact state, I'd say their conclusions were spurious.

Grant Hutchison

jlhredshift
2006-Aug-25, 02:46 PM
And, isn't it interesting that geologic science generally abhores any appeal to catastrophism, yet the number one theory is exactly that. The joke that I am making is that in the late 1800's and the mid 1950's the word "catastrophism" had a different colloquial context than it does today.

Sp1ke
2006-Aug-25, 03:12 PM
Cheers, Grant, that was my suspicion but your summary is much clearer than my thinking. The main thrust of the argument was to make the scientific explanation look unlikely at best so that they could go on to propose a much more outlandish explanation. They also make much of the coincidence that the moon is almost the same apparent diameter as the sun.

jlhredshift
2006-Aug-25, 03:28 PM
1) How do they know how fast the Earth was rotating before the impact? Couldn't the impact have converted the prior spin-state of the Earth to an appropriate rate (about one rotation every five hours) which was then tidally slowed (by the moon) to its present rate?
2) I don't know the maths, but there's no requirement that the Earth's axial tilt be generated by that single impact. Its orientation presumably resulted from the accumulation of other impacts, great and small, during its formation, as well as from the impact that formed the moon.

In summary, they seem to be making some assumptions about the pre-impact state of the Earth, from which they deduce that the impact itself was insufficient / excessive to account for the Earth's present state. Unless they have a lot of theory to back up their assumptions about the pre-impact state, I'd say their conclusions were spurious.

Grant Hutchison


I agree, and would only add that our moon and all the other planetary bodies bear witness to the random chaos that existed during the early days of this solar system and is probably diagnostic of other systems as well. I do not think that any two systems will look the same and that uniqueness will be everywhere we look. There is nothing extrodinary about the Earth Moon system other than it is extrordinary, and ours.

Kaptain K
2006-Aug-25, 03:36 PM
Knight and Butler started by criticising the current explanation of how the Moon was created, pointing out how unlikely it all was...
If I play the lottery, my odds of winning are several million to one. If I win the lottery, my odds of having won are unity.

is in the same orbit as the Earth.
An object in the same orbit as the Earth will not collide with the Earth.

The rogue planet collides with Earth and part of the mantle of the Earth is ejected.
The mantle coalesces into the Moon.
A little bit of an over simplification, but close. Most of the impactor's core and much of its mantle settle onto the Earth. The Moon is part Earth's mantle, part impactor's mantle and its core is a small part of the impactor's core.

The rocks on the surface of the Moon are similar to those on Earth, explained by the Moon's formation from the Earth's mantle.
Small quibble.The Moon and the Earth's (current) mantle are a mixture of Earth's (original) mantle and impactor's mantle.

The close match between the oxygen isotopes of the Earth and the Moon indicate that the rogue planet was created in the neighbourhood of the Earth.
See above.

The heavy elements on the Earth had already sunk to its core so that's why the Moon is short of heavy elements like iron.
See above. The Moon's core is the little bit of the impactor's core that escaped being absorbed by the Earth.

Jason Thompson
2006-Aug-25, 04:01 PM
The main thrust of the argument was to make the scientific explanation look unlikely at best so that they could go on to propose a much more outlandish explanation.

That's a favourite tactic of people with ideas against the mainstream or conspiracy theorists. Their stance is that if the official version of events is not absolutely watertight then the whole thing becomes a free-for-all and any explanation is equally valid.

They fail to recognise, or want their readers to fail to recognise, that this is never an either/or case. They want to portray a system whereby either we have a theory that explains everything in a completely watertight fashion or else the playing field is entirely level and one explanation is as good as the other. Reality works differently. A theory is accepted if it explains the observations better than any of the others. The 'aliens created the Moon' theory presumes something for which there is no evidence: the (prior) existence of an alien civilisation with the capability of building and/or repositioning planet-sized objects. That alone shuld be enough to discard it. Their response to this would likely be that the Giant Impact Theory presumes the existence of a Mars-sized body that collided with Earth, and there is no evidence that any such object existed. While this is true, there is evidence that large rocky objects can be formed in a solar system, since we happen to have other examples of them, and the solar system is riddled with evidence that impacts happen. We even had the privilege of watching one in 1994 as Jupiter got peppered with bits of a comet.

So, is the GIT watertight? No, it's still a work in process. But it does explain the observations of the Moon and what it is made of beter than any other theory without having to invent alien civilisations.

Having said that, their two main objections are total crap, since both require knowledge of conditions from prior to impact, as well as a knowledge of exactly where and from what direction the impact took place, neither of which they can actually have.

korjik
2006-Aug-25, 04:05 PM
1) How do they know how fast the Earth was rotating before the impact? Couldn't the impact have converted the prior spin-state of the Earth to an appropriate rate (about one rotation every five hours) which was then tidally slowed (by the moon) to its present rate?
2) I don't know the maths, but there's no requirement that the Earth's axial tilt be generated by that single impact. Its orientation presumably resulted from the accumulation of other impacts, great and small, during its formation, as well as from the impact that formed the moon.

In summary, they seem to be making some assumptions about the pre-impact state of the Earth, from which they deduce that the impact itself was insufficient / excessive to account for the Earth's present state. Unless they have a lot of theory to back up their assumptions about the pre-impact state, I'd say their conclusions were spurious.

Grant Hutchison

Point one and point two are directly related. Adding two angular momentum vectors gives you a third vector. Any change in rotation rate will be accompanied by a change in axis.

grant hutchison
2006-Aug-25, 04:14 PM
Point one and point two are directly related. Adding two angular momentum vectors gives you a third vector. Any change in rotation rate will be accompanied by a change in axis.Indeed.
Interesting, then, that the authors of the book feel the impact was both too large to allow the Earth its present slow rotation rate, while also being too small to account for the axial tilt.

Grant Hutchison

jlhredshift
2006-Aug-25, 04:21 PM
And yet Venus is pretty much tipped over and has no moon or debris.

Sp1ke
2006-Aug-25, 04:24 PM
Kaptain K, you said

An object in the same orbit as the Earth will not collide with the Earth.

I think the GIT (love that acronym!) says that the oxygen isotopes point to the impactor having originated in our neighbourhood. So how could the impact happen? I can see that if the other object is in the same orbit, it must be going at the same speed so can't collide (obvious now I think about it). But could it have been created sufficiently far away to be able to hit us, but close enough to be classed as having been created in the same neighbourhood as the Earth?

phunk
2006-Aug-25, 04:40 PM
IIRC 2 similarly sized objects in the same orbit isn't a stable system, they will eventually collide.

Nereid
2006-Aug-28, 01:03 AM
One of pieces of evidence for GIT (or, if you prefer, 'the big splat') is Luna is 'dry' - not only does it lack the obvious volatiles (water, methane, ammonia, ...), but also bound forms of water (e.g. hydrates), etc. It also lacks minerals with low boiling points (most of those we've been able to analyse are highly refractory).

Another is the cratering history of bodies which have preserved craters (or, if you prefer, the detailed stats on crater densities). Combined with age data on the relevant rocks (Earth, Moon, Mars - via meteorites), a size-frequency graph of impactors can be produced (actually, many such, by epoch).

Then there's numerical modelling, using all kinds of inputs.

Put it all into the oven, bake for an hour or month, and what do you get? That GIT is plausible - it's not extremely freakish, it fits (most of) the input data well (or very well), etc.

Best of all, there is no alternative that comes even close to matching the wide range of relevant inputs.