Re: mars and venus (long)

[Stephen Murray <murray8@LLNL.GOV>]

> I imagine that a slight difference from uniformity would have major
> effects over long time periods.

That's sort of what I was originally thinking--material from further out
comes in moving more quickly in its orbit than the protoplanet, and hits
the outer limb preferentially, the reverse happens for material pulled in
from smaller orbits, and hence the protoplanet gets a preferential prograde
spin.

The references that I've found (which generally give only oblique
references to numerical simulations) seem to imply that, instead, the
collisions are more random.  Objects get scattered into the "feeding zone"
(or channel as you referred to it) with enough random velocity that when
they eventually hit the planet, they hit any ol' place on the exposed
hemisphere with equal probability.  The only net spin is thus due to random
deviations, which are larger for larger impactors (and hence the obliquity
argument for small impactors).

> ...  I wonder... might this effect control the size of planets?
> A small planet would experience a more uniform bombardment and might
> slow down its spin, while a large planet would quickly accumulate spin and
> start losing material.

The problem here, I think, is that none of the planets, even the Jovians,
are spinning anywhere near their breakup speeds.  If they were spinning
that rapidly in the past, I know of no mechanism that would slow them down
to their current rates.

> (I wonder if such a phenomenon occurs in Saturn's rings?  If it does, then
> perhaps some of the moonlets in Saturn's rings should be spinning
> dangerously fast.  If they don't spin much at all, then that would
> indicate that the deposited material strikes them uniformly.

Saturn's rings are a somewhat different beast.  Self-gravity among the ring
particles cannot play a significant role in their evolution (they're inside
Saturn's Roche Limit).  Gravity therefore shouldn't have a significant
focussing effect upon the collisions, and there can be no feeding zones.
Collisions result from random encounters, with random velocities possibly
pumped up by gravitational kicks from Saturn's moons.  I think that this
would lead to very different behavior for ring particles vs. protoplanets.

===============================================
  Stephen D. Murray
  Physicist, A Division
  Lawrence Livermore National Laboratory
  phone: (925) 423-9382   FAX:    (925) 423-0925
  email: sdmurray@llnl.gov
  web page: http://members.home.com/murraysj/
===============================================