Re: mars and venus

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Thu, 23 Sep 1999, David Bowman wrote:

> ... 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.
> ...
> 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 ...

I am surprised at myself for never having seriously considered the
difference between the two mechanisms David describes here.  I suppose I
had always simply considered "forward" spin to be a natural--and
universal--result of the action of the second type of process.

So what *should* one expect to be the result of the first type of
process--i.e., the gradual accretion of particles that are already in
roughly circular orbits about the Sun?  Naively (perhaps), one could
simply point to the fact that the closer particles are moving faster so
that their agglomeration with slightly more distant particles should
result in a combination that spins "backwards" as David suggests.  On the
other hand, the gravitational action of the outer particle tends to
decrease the speed of the inner particle and vice versa.  So what really
happens?

To get a quick idea, I just "wrote" an Interactive Physics simulation with
two small particles orbiting a larger body in nearly adjacent orbits.  I
made them inelastic so that they could dissipate their excess energy upon
colliding and, thereafter, stick together.  Not so surprisingly (I guess),
the pair did indeed have a very small "backward" intrinsic angular
momentum after colliding and sticking together.

Few of you will probably be willing to call my research here "definitive,"
but I thought it was kind of interesting.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm