Re: mars and venus

[Stephen Murray <murray8@LLNL.GOV>]

> regarding:
>
> > Lois Breur Krause asks:  Why does Venus rotate backwards?
> > > > > > > > > > > > > > > > > > > > > >
> >
> > I doubt there's a definitive answer to the question, but it seems plausible
> > that Venus suffered a major collision while it was forming which left it in
> > its current rotation state.  BTW - Uranus is also in an odd (compared to the
> > rest of the planets) rotation state, with its equator inclined about 90
> > degrees to the plane of its orbit, for probably the same reason.
>
> Although I have not studied the planetary science literature on the
> subject (and consequently what I'm about to say may be taken as entirely
> misguided ignorance) it seems to me that Venus' slow backward rotation
> state should be *normal* for an inner planet formed gently by accretion
> of many small rocky planetesimals that orbit the Sun in an inhomogeneous
> band of material whose largest initial chunks gradually accumulate all
> the rest of the material in the band.

(remainder omitted for brevity)

> David Bowman
> dbowman@georgetowncollege.edu

My understanding is quite the opposite.  As you point out, both Terrestrial
and Jovian planets formed originally by the accretion first of rocks, then
of planetesimals.  Once the Jovian planets had cores of a critical mass
(10-20 Earth masses), they began to rapidly accrete gas from the
surrounding Solar nebula.  Gas was pulled in from larger orbital radii, and
hence higher orbital angular momentum, and from smaller orbital with lower
orbital angular momentum (relative to the planetary core).  That gas would
then naturally form an accretion disk around the planetary core that
rotated in a prograde sense (counterclockwise when the Solar System was
viewed from above the North Pole of Earth).

To zeroth (-1st?) order, I believe that accreting planetesimal and
smaller-sized objects will deliver angular momentum to a protoplanet in the
same manner.  Objects coming in from larger orbital radii will have higher
angular momentum, ie. they'll be moving more rapidly than the protoplanet.
They'll also tend to collide with the "outer" hemisphere of the
protoplanet, and so will tend to spin it up in the prograde sense.  The
reverse will apply for objects pulled from orbits closer to the Sun.  It's
therefore no surprise that Terrestrial and Jovian planets almost all spin a
prograde sense, and Venus is the oddball.

I belive that the difference between this picture and John's simulation is
that, unless I misunderstood his message, he simulated the collision
between two similarly-sized objects, the difference of whose orbital radii
was less than the sum of their radii, ie. even without mutual gravity they
would have collided.  In the accretion picture, a larger object accretes
smaller ones from much further away.  I suspect that what John may have
done is to simulate the formation of Venus!

For awhile, I'm not sure when, some folks thought that Venus' retrograde
spin might be the result of a sort of tidal lock with Earth.  Venus is
close to being in a resonance, in the sense that at every inferior
conjunction it presents almost exactly the same face to Earth.  It's not a
perfect resonance, however, and I don't think that anybody could figure out
how the tidal forces between Earth and Venus could possibly be strong
enough to alter the spin of Venus (or why Earth would show no effect).

===============================================
  Stephen D. Murray
  Physicist, A Division
  Lawrence Livermore National Laboratory
  L-022, P.O. Box 808
  Livermore, CA  94550
  phone: (925) 423-9382   FAX:    (925) 423-0925
  email: sdmurray@llnl.gov
===============================================