Re: mars and venus

[Stephen Murray <murray8@LLNL.GOV>]

> (1) There was no sun when the planets formed.  There was no mass in
> Keplerian orbits around the sun.  Rather, there was a huge globular
> cloud of gas and dust that became "gravitationally bound."  As this
> whole cloud began collapsing, it squished into a disk (a normal process
> for rotating, gravitationally bound stuff) and that's why planets
> essentially end up orbiting in a single plane.  At some concentration
> of matter in the central core and in the disk, "condensation" began IN
> MULTIPLE PLACES.  The sun formed and the planets formed AT THE SAME
> TIME.

Your statement of the initial conditions is exactly correct--a round, or
irregular cloud of molecular gas and dust that was slowly rotating.  We're
picking the play up in the next act, following the gravitational collapse
of the cloud.  The usual picture, supported by simulations and
observations, is that most of the mass will collapse to the center to form
the Sun, while most of the angular momentum will be contained in the
surrounding disk from which the planets formed.  The Sun and planets
certainly do form together, as you say.  In fact, I believe that the
"turning on" of the Sun is usually taken as the end of the formation of the
Jovian planets (the remaining gas is heated up and blown out of the Solar
System).

Even immediately after the collapse of the cloud, however, the gas from
which the Sun formed was concentrated at the center of the system, while
the mass of the disk was much smaller.  The gas, dust, rocks, protoplanets,
etc. in the disk therefore followed Keplerian orbits around that central
concentration of mass, which at that time might be best referred to as the
"proto-Sun."

> (2) In this picture I am not sure whether the mass that ends up in the
> planets comes from an annular ring of this disk of matter, or whether
> it comes from a more local area.  After all, the original cloud was
> more globular, and as it collapsed into a disk there was significant
> mass concentration from the globular distribution into the disk
> distribution.  The matter that ended up in the planets may have never
> had an annular distribution.

I believe that David is trying to focus in on a small piece of the problem,
to simplify matters.  The real disk of material from which the planets
formed certainly extended from inside of the orbit of Mercury to beyond
Neptune.  David is trying to look at just that part of the disk which
contributed to the formation of a given planet, which should be reasonably
approximated by an annulus.

I can't immediately say yea or nay, or why either way to David's current
picture.  In reality, resonances with/competition from/scattering by other
planets will play a role, but I don't think that those affect the basic
arguments here.  I'll be interested to look into whether or not his chosen
surface density variation with r has any strong effect (I suspect not).
Other than that, I'll have to dig out some of my old planet formation
references (which I keep at home, since that's where I do my lecture
preparation).

===============================================
  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/
===============================================