On Mar 20, 8:31 am, Andrew Usher wrote:
Some time ago I came across this page
(http://arcbuilder.home.bresnan.net/PCLMaster.html) (I don't remember
how) and was quite interested. Though unfortunately we have only our
own solar system to study right now, contemplating the possibilities
is certainly worthwhile.

I had once imagined making such a list myself but was never confident
of my knowledge.

As I was reading it, I found many apparent inaccuracies - some I'm
sure
of, and others I'm not quite so sure of. I have written up my
criticisms and comments in a rather long list below. I hope you might
read it - I have done this kind of thing before, and you shouldn't
take
offence - after all I wouldn't even have bothered if I considered it
totally worthless.

You might want to take a look at my recent post 'Liquid ammonia
in space'
(http://groups.google.com/group/sci.a...thread/thread/
27a0eb982fef9030/76a1f9a71e472f5a#76a1f9a71e472f5a)
as some of my comments below pertain to it.

Well, here's the list. To read it you need a copy of the PCL open,
as the list is indexed by order in that.

If any of my points are clearly wrong, please tell me, and provide
literature references if
possible - I'd like to know more about it, in any case.

This post is adapted from an e-mail I sent to the author of the PCL,
John Dollan, two weeks ago, to which he never replied. The following
list is what I showed him.

The referenced scheme (and your objections) appear to concentrate on
many variables but leave others out; frinst a body's distance from its
primary, how long it's been there, and the characteristics of the
primary. I mean, take Venus and snuggle it up close to Sol and you'll
get something more Mercury-like, but toss it out near Neptune and its
atmosphere will become just another layer of weird ices. Then there're
comets (those that do close approaches to their primaries)- they're
(broadly speaking) ice/rock aggregates until they get warm, then they
grow atmospheres and maybe even (small) bodies of liquid volatiles.
Well, are they fundamentally different from comets that stay way out
there? How about comets that come near and get trapped, then "evolve"
into "asteroid swarms" as they lose their volatiles? Do they deserve a
different classification for each stage in their evolution? Should
classifications be limited to stable states of planets etc.? What does
"stable" mean in this context considering that the system primary can
change, thus altering the characteristics of orbiting bodies?


Mark L. Fergerson