Re: Moon's Spin

[George Spagna <gspagna@rmc.edu>]

At 10:44 PM 2/18/97 EST, you wrote:
> I think the composition of the moon suggests that it did not come from a
> comet or an asteroid.

The current "best model" suggests that the Moon originated as a consequence
of a grazing impact between a young Earth and a Mars-sized planetessimal.
This accounts for lunar composition being similar but not identical to
terrestrial surface rock.


>  It seems
> quite clear, for example, that the heat with in the earth today was generated
> by radioactive decay and that residual heat of formation makes only a minor
> contribution. I find no compelling reason to accept either hot or cold
> formation. 

It's even more complicated  -  some researchers suggest that the formation
of the outer planets ("gas giants") was via collapse, very similar to the
formation of the Sun itself.  The inner planets appear to have grown by
accretion.  Accretion still releases substantial energy for heating, though
for large bodies probably not enough to bring about melting.  However, the
pressure in the center of a large accreting body should be sufficient to
raise the temperature.

> I think the jury is still out & we need to keep an open mind.

A good practice for any scientist!

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

I believe the argument is that the Moon was closer to Earth in the past,
hence the tidal interaction was substantially greater (its an inverse cube
relation, as you recall.)  For a prograde orbit the tidal interaction
transfers angular momentum from rotation into orbital motion.  Both Earth
and Moon slow their rotation, but Moon falls into tidal lock first. 

>

George Spagna           **********************************************
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