NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/sci/space/16272

Internet Message Format | 1992-11-23 | 3.4 KB

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From: [e-mail] (David Ray)
Newsgroups: sci.space
Subject: Re: ROTATION OF THE MOON
Date: 23 Nov 1992 19:00:07 GMT
Organization: /etc/organization
Lines: 56
Message-ID: <[e-mail]>
References: <[e-mail]> <[e-mail]> <[e-mail]>
NNTP-Posting-Host: sunspot.ssl.berkeley.edu

In article <[e-mail]> [e-mail] (paul.h.nelson) writes:
>Since the moon is gradually receding from the earth, due to tidal effects,
>can we extrapolate backwards in time and postulate how close it has been
>to the earth and when?

Scientists have been grappling with this issue for years. It is one of the
scientific paradoxes of our lifetime.

The moon's orbit is also inclined with respect to the earth's equator, and
the orbit would have been *more* eccentric in the past -- tidal effects
are making it more circular. If you extrapolate backwards, the eccentricity
of orbit would have become impossible for any orbit to exist more than
a few hunderd million years ago. Yet, We have confirmed the Moon's age of
well over a billion years from carbon dating. So, either the moon was "captured"
into Earth's orbit well after its formation, or its orbit was significantly
perturbed after it was in Earth orbit.

The degree of tidal dissipation in the past is another large uncertainty.
We can measure the slowdown rate now, but scientists believe that the tidal
dissipation rate was much different in the past, particularly during the
ice ages. Virtually all tidal dissipation of the Moon's orbit comes from the
ocean. During ice ages, there was less ocean, more ice. The issue of "resonant"
frequency of some parts of the ocean is another issue. If only a small part
of the ocean constitutes most of the tidal dissip[ation, the effects of changes
in the tidal dissipation due to changes in ocean might not be as simple as
just the quantity of available ocean mass.

Another hitch is the lack of iron in the moon. The amount of iron tells us if
the moon came out of (or was formed near) the Earth in the formation stages
of the solar system. Normally, a body the size of the Moon should have an
iron core making up about the same % of total mass as the Earth (say within
a factor of three). But our data indicates it is almost an order of magnitude
less. So, did it not form from the same primordial mass? They can't answer
that one, either.

The only single theory that makes sense is that:
1. The earth formed from a primordial mass with a normal amount of iron
2. There was a gravatational pertubation very close to the earth at a very
   specific geologic time, after most of the iron in the earth settled to the
   center. The pertubation expelled material out of only the outer portions of
   the earth, which were depleted in iron.
3. The Moon formed from the material expelled from the pertubation, and has
   orbited the Earth ever since.
4. There has been at least one gravatational pertubation to the moon's orbit
   after it was completely formed, knocking it into its irregular orbit that
   we now observe.

dave


>
>ALso, can we pick a lower bound earth orbit, where the moon would have broken
>up and fallen into the earth, and use that as an UPPER bound for the
>age of the earth-moon system?