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photonovore
Moonatic
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Reged: 12/24/04

Loc: tacoma wa
Re: Age of the Moon new [Re: THEPLOUGH]
      #5924587 - 06/16/13 10:52 PM

Surely the origin of the Moon is a relevant topic in this forum.

The hypothesis i see being touted by carl12 is the lunar capture theory (an older hypothesis from the pre-apollo era), where a (possibily pre-solar system formation) object wanders/pre-exists in/into the early/proto solar system and is eventually "captured" by Earth. Ok.

Why this is has been discounted/dismissed as a valid hypothesis:
1) Moon has been physically dated radiometrically at no more than ~4.4 by. (This is a date of *accretion*--crust solidification--which post-dates that of Earth by ~200my)
2) The chemical composition of the Earth/Moon does not match what would be expected if these were physically unrelated bodies. (ex: relative abundance of titanium-50 and titanium-47 in Moon rocks; Zhang 2012--match to Earth w/in 4ppm making a related origin highly probable)
3) Extreme improbability of capture due to angular momentum limitations, lacking a third body of sufficient (substantial) mass in the equation--for which there is no evidence.
4) Moon is an iron depleted body (core), which is a inconsistency for a body formed independently via currently understood processes.
5) Orbital inclination of Moon relative to Earth is not accounted for (among other orbital parameters inconsistent with capture theory)

There are actually *two* theories that are *currently* in competition for the title "the most probable" re; lunar formation--
1) Impact, still the preferred hypothesis
a)2x mars mass impactor (Canup, 2012)
b) smaller higher velocity impactor, coupled with fast-spinning proto earth (Cuk/Stewart 2012)
c) and other variations

2) Fission is being re-examined; if the spin rate of the proto earth is not sufficiently constrained (as heretofore, making this unlikely due to angular momentum insufficiency issues) then this theory can gain probability.(Zhang)
----------------------------------

The problem with 'popular' science education (such as documentaries on tv, many lay books etc) is that (currently leading) hypotheses are often presented very positively (as established fact) when they are anything but. What they actually represent are statements of relative higher probability based wholly upon _current data and understanding_ of that data. That's all. The field of lunar genesis is actually still relatively active...


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Mare Nectaris
Carpal Tunnel
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Reged: 03/09/08

Loc: Toijala, Finland
Re: Age of the Moon new [Re: photonovore]
      #5925031 - 06/17/13 08:40 AM

Quote:


There are actually *two* theories that are *currently* in competition for the title "the most probable" re; lunar formation--
1) Impact, still the preferred hypothesis
a)2x mars mass impactor (Canup, 2012)
b) smaller higher velocity impactor, coupled with fast-spinning proto earth (Cuk/Stewart 2012)
c) and other variations

2) Fission is being re-examined; if the spin rate of the proto earth is not sufficiently constrained (as heretofore, making this unlikely due to angular momentum insufficiency issues) then this theory can gain probability.(Zhang)




Hi all,
Sarah T. Stewart-Mukhopadhyay from the Department of Earth and Planetary Science, Harvard University, is reviewing as a co-writer the article Mardi mentioned above (Cuk & Stewart: Making the Moon from a fast-spinning Earth...).

Be well all!


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Carl_12
member


Reged: 12/05/12

Re: Age of the Moon new [Re: Mare Nectaris]
      #5925613 - 06/17/13 01:58 PM

Thank you, photonovore, much appreciated. Though you must admit you're being very selective, leaving out all the acknowledged problems with the impact theory.

To say that pre-existing planetaries would be likely in the Solar System isn't the same as touting the capture theory. Early, perhaps smaller, versions of the Earth and Moon could have both formed before the Solar System, and then experienced additional growth in the newly energized protoplanetary disk.

In fact, the impact theory was considered too improbable to be taken seriously before the Apollo rocks were dated. The dating of the rocks hasn't made the event any more probable, it merely makes the theory more coherent.

I would urge those who are seeking to plug holes in the impact theory to stop running round the epicycles and to consider a smaller, more probable impact, with a smaller Moon already in place. The impact is then a re-surfacing event rather than whole creation, all the more probable for it. The relatively small core of the Moon would support this idea, and many of the present issues with the impact theory would disappear.


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David Knisely
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Reged: 04/19/04

Loc: southeastern Nebraska
Re: Age of the Moon new [Re: Carl_12]
      #5925628 - 06/17/13 02:08 PM

Quote:

Thank you, photonovore, much appreciated. Though you must admit you're being very selective, leaving out all the acknowledged problems with the impact theory.

To say that pre-existing planetaries would be likely in the Solar System isn't the same as touting the capture theory. Early, perhaps smaller, versions of the Earth and Moon could have both formed before the Solar System, and then experienced additional growth in the newly energized protoplanetary disk.

In fact, the impact theory was considered too improbable to be taken seriously before the Apollo rocks were dated. The dating of the rocks hasn't made the event any more probable, it merely makes the theory more coherent.

I would urge those who are seeking to plug holes in the impact theory to stop running round the epicycles and to consider a smaller, more probable impact, with a smaller Moon already in place. The impact is then a re-surfacing event rather than whole creation, all the more probable for it. The relatively small core of the Moon would support this idea, and many of the present issues with the impact theory would disappear.




Mardi isn't being selective. All of the theories of the moon's origin have their problems, but given the data currently available, the impactor idea does provide somewhat of an answer to some of the compositional similarities between Earth and moon rocks. Dynamically, it has also been shown to be possible using extensive computer simulations, so it is better supported than it was years ago. The capture hypothesis is less likely dynamically, but is probably still in the running, although capturing the existing moon into Earth orbit via an impact is somewhat less likely than something just impacting the Earth and creating material which would eventually collect together to form it in the first place. It would be better if you didn't accuse people of "running around epicycles" and just stick with the actual facts of each theory. That way, you would be less likely to ruffle any feathers here. Clear skies to you.


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UND_astrophysics
sage


Reged: 01/19/13

Re: Age of the Moon new [Re: Carl_12]
      #5926130 - 06/17/13 06:47 PM

Quote:


I would urge those who are seeking to plug holes in the impact theory to stop running round the epicycles and to consider a smaller, more probable impact, with a smaller Moon already in place.




But this is not your original assertion. You implied earlier that the moon was formed prior and independently of the original circumstellar disk, based on your thinking that because there were a lot of craters on the moon, it must be older than Sol system (an invalid conclusion)
I am totally lost as to where your reasoning is going, because your original premise is morphing from post to post into something else.


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Carl_12
member


Reged: 12/05/12

Re: Age of the Moon new [Re: UND_astrophysics]
      #5931141 - 06/20/13 11:39 AM

Quote:

Quote:


I would urge those who are seeking to plug holes in the impact theory to stop running round the epicycles and to consider a smaller, more probable impact, with a smaller Moon already in place.




But this is not your original assertion. You implied earlier that the moon was formed prior and independently of the original circumstellar disk, based on your thinking that because there were a lot of craters on the moon, it must be older than Sol system (an invalid conclusion)
I am totally lost as to where your reasoning is going, because your original premise is morphing from post to post into something else.




I've been quite consistent throughout, that planetary objects must pre-date the Sun by the very nature of the processes by which the Sun came to be.

That said, the state that those planetaries were in at the moment of stellar ignition is unknown to us, further evolution would be more than likely in the no doubt energetic solar nebula. Growth through impacts and resurfacing would be plausible.

I would also add that the age of the Moon's surface is far from proven as we haven't dated the very oldest structures, likely to be found in the ancient highlands on the other side. The margin of 200my difference from Earth, as reported in this thread, doesn't seem so great and could easily be overcome.

In short, I'm being flexible on the detail due to paucity of evidence, but consistent on the main point.


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photonovore
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Reged: 12/24/04

Loc: tacoma wa
Re: Age of the Moon new [Re: Carl_12]
      #5933978 - 06/22/13 12:28 AM

There isn't any reason why noritic anorthosites would vary significantly in age between farside/nearside highlands since the oldest of these plutonic rocks from either hemisphere would be reasonably expected to date to the same stage of the Moon's geologic history (crystallization stage of the lunar crust, which current data indicate to have been a global event).

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