Re: mars and venus

[David Bowman <dbowman@TIGER.GEORGETOWNCOLLEGE.EDU>]

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.  It seems to me it is the fast
forward (but tilted) spins of the Earth and Mars that suggest a major
collision for each of them in a late stage of their formation.  In the
Earth's case the Moon is supposedly a remnant of that collision.  I don't
know what to think about Mercury's slow forward 3-2 tidally locked spin
and eccentric orbit.  I don't know if any major collisions would be
needed for it or not.

The outer planets each (except Pluto if you want to call it a planet) are
supposed to have formed by gravitational collapse of a local eddy of gas
and protoplanetary material from the solar protoplanetary disk onto a
nucleating rocky core that presumably also formed by accretion. Thus
their rapid forward spin, prograde orbiting rings and prograde orbits of
most of their moons are remnants of the initial relative angular momentum
of each eddy and are miniature versions of the formation of the Sun and
its inner planets except for the effects of the huge mismatch in ambient
temperature and total gravitational field strength between these two
cases.  I suspect that the Uranian system very well may have been subject
to a major disruptive event (e.g. collision episode) in an early stage of
its formation to cause its sideways orientation.  Presumably most of the
wierdly orbiting moons of the gas giants are mostly captured planetoids
that happened to be tresspassing and suffered a near miss with some other
previously bound moon (undergoing a three body interaction with the moon
and the central planet). They may have come from the ejecta of previous
collisions and near misses occurring elsewhere in the early solar system.

Would anyone who actually knows the current consensus of the planetary
science community actually care to comment and straighten out my
misconceptions?

David Bowman
dbowman@georgetowncollege.edu