NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/sci/space/19827

Internet Message Format | 1993-01-28 | 3.3 KB

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From: [e-mail] (fred j mccall 575-3539)
Newsgroups: sci.space
Subject: Re: Earth's rotation rate may be due to early collisions [Release 93-12] (Forwarded)
Message-ID: <[e-mail]>
Date: 26 Jan 93 20:25:35 GMT
References: <[e-mail]| <[e-mail]| <[e-mail]> <[e-mail]>
Organization: Texas Instruments Inc
Lines: 51

In <[e-mail]> [e-mail] (Larry Wall) writes:

>In article <[e-mail]> [e-mail] (Bruce Watson) writes:
>: In article <[e-mail]| [e-mail] (Ray Davis) writes:
>: ||     "A popular theory holds that the collision of a Mars-sized planetary
>: || body with the Earth threw considerable debris into orbit, which then came
>: || together to form the moon," Dones said.  "Thus, the same impact which gave
>: || Earth its spin, could also have formed the moon."
>: |
>: |How does this popular theory account for the moon having zero spin?
>: |
>: The moon rotates once a month, otherwise it wouldn't keep the same
>: face to the earth.
>:
>: The "lock" occurred due to tidal friction and took a long time to happen.

>Nevertheless, I don't know of any evidence that Moon ever did have much
>of a spin (relative to the "lock").  The fact that the back side of the
>Moon is so different from the front side would tend to argue against it.

>But to answer what I believe to be the original question, the apparent
>inconsistency of a collision giving spin to the Earth but not to the
>Moon is resolved easily.  The Earth and the Collider were already
>formed at the time.  Since they were both sizeable, it's statistically
>likely (but not certain) that after the collision the Earth would be
>left with a significant spin, just as football player getting tackled
>is likely to have his attitude adjusted, so to speak.

>Now here's the key to the conundrum.  The Collider, according to
>theory, *pulverized* both itself and a bunch of the Earth's crust and
>mantle, throwing lots of little rocks into orbit around Earth, but no
>Really Big Ones.  These eventually accreted into the Moon.  Since
>there's lots of little rocks coming from every which a way, however,
>it's statistically unlikely that the Moon would end up with a large
>residual spin, just as a football player doesn't expect to be suddenly
>upset by the air molecules that are pounding him constantly from every
>direction (in the absence of tornadoes).

Isn't there geological evidence that shows that Earth and Moon formed
(were not molten, which I would expect accretion of a body the size of
the moon to result in -- gravitational energy) at the same time?
Theat seems to me to be a death blow to any such theories of
catastrophic formation of the moon (if what I'm remembering is
correct).


--
"Insisting on perfect safety is for people who don't have the balls to live
 in the real world."   -- Mary Shafer, NASA Ames Dryden
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