Re: Moon's Spin

[Jim Green <JMGreen@sisna.com>]

David Dockstader wonders:

> I'm lost on the gravity part.  Why should gravity have been different 4.5
> billion years ago?

I think David Bowman was saying that (David help us out here) if the Moon
was molten 5^9 years ago and in the same orbit it has now, then the then
Earth's gravity gradients would have forced the solidifying Moon into a mass
quatdrupole configuration and that configuration would have prevailed intact
to the present.  Thus if the current Earth had the same gravity gradients,
THEN the Earth would now continue to hold the solid Moon in the same
orientation it had then.

I expressed concern that assumptions that can't be verified tend to lead us
into stories which are convoluted to fit current data.  Further the same
people who talk about the above also conclude that the Moon started out MUCH
closer to the Earth than now AND if the Moon were molten so likely was the
Earth THUS the gravity gradients would have been much different at the Moon
then than they are now.  Hence there is no reason for the Earth to be able
to hold the Moon in the same orientation now as the Earth did then. 

BUT someone privately has given me to think that it might be possible for
this to happen if the mass configuration frozen into the Moon were DIPOLE --
just like the "tidal bulges" this list likes to talk about.  IE surely the
Moon is oblate with respect to the orbital axis, but suppose the Moon were
frozen oblate with respect to the Moon/Earth axis as well.  Then if the Moon
were to rotate slightly (as it does during the year), the gravitational
attraction to the front lobe would be greater than to the rear lobe and
there would be an impressed torque to bring it back.  This would be the case
if the gravitational field had spherical symetry and tht symetry would
continue to the present.

It is just the converse of this that the planetary geophysicists assume
happens between the Earth's "tidal bulges" and the Moon IE the phase lag of
the "bulges" causes a torque on the Moon to slow it down.  But of course
there are no such things as "tidal bulges" so their arguments are vacuous.
(Sorry, Folks, I couldn't resist -- write privately if you want to do a side
bar in that! -- Or maybe David or Dan want to take the heat -- I do not.)  

In any case my arguments satisfy me -- and my rampant speculation is as good
as anybody's rampant speculation. (:-)


     

Jim Green
JMGreen@sisna.com