I'll warn you now, this is a bit long, and on the nerdy side. You
won't insult me any if you want to skip it.
While I have an infrequent idle moment, I wanted to address one of
the topics that Corran Webster brought up. (hi Corran, welcome to
the TML!)
In order for a planet to have an atmosphere, it must be both large enough
and dense enough to be able to capture and retain the gaseous phase
during its formation. The earth and Venus are the two planets in our
system with significant atmospheres. Let's look briefly at some planet
stats:
planetdiameter, kmmass, gdensity, gm/cc
mercury48783.3e265.44
venus121004.87e275.25
earth127565.98e275.52
moon34767.35e253.34
mars67866.44e263.94
(Well, yeah, the moon is technically a satellite, but compositionally,
it's part of the earth/moon "system" and is of the same compositional
geochemical material as the earth's lithosphere...I just couldn't neglect
it.)
The large satellites of Jupiter vary in size from ~3000 to ~5000 km
diameter, and from ~2 to 3.5 g/cc density, by comparison.
So, it looks like the upper limit for terrestrial planet-sized bodies
is around 5.5 g/cc density, though it certainly is within the realm
of planetary geochemistry to accrete a larger Fe/Ni core to make a
denser planet. The upper limit for a planetary body would be the
density of pure iron (in this system) or pure nickel (in some other
system), I would suppose, assuming that you could really accrete a
planet without retaining the other elements (which I believe would
be close to impossible, though most of the science in Traveller is
impossible, so why worry about it?)
Anyway, for planetary bodies smaller than the Earth or Venus, a decent
atmosphere is not viable, because the gravity would be insufficient
to retain low vapor pressure gases.
Incidentally, one of the great puzzles of planetary science is why
Venus does not have a significant proportion of water vapor in its
atmosphere. It is large enough to retain water vapor, but yet its
atmosphere is mostly CO2. The science types are still arguing about
it...
Now about Corran's proposed UPP scale, based mostly on planetary
composition, which translated to rocks, solid cores, molten cores,
etc. I would like to make a small correction to his view of
planetary composition.
When you have planetessimals colliding to make terrestrial style
planets, there are a number of compounds available which can be
classified as siderophilic (mostly iron/nickel), lithophilic (oxides
and silicates), chalcophilic (oxides and sulfides), and atmosphilic
(low vapor pressure compounds, in the forms of ices). These
four broad categories are the left-overs from accreting a
sun. They hang out as planetessimals and get busy colliding
and accreting. As they accrete, they segregate by density.
The siderophilic (ie iron-loving) elements sink into a core (always
molten at first). The lithophics form a mantle. The chalcophilics and
some of the oxides float to the top. The atmosphilics try their damndest
to outgas. The heat of collision drives the atmosphilics into a vapor
phase. If the body is large enough and dense enough, the vapor phases
can't escape, and form an atmosphere.
So essentially, every terrestrial planet has a siderphilic (ie iron-rich)
core topped by a lithophilic (ie silicate-rich) cap. The lighter oxides
and sulfides may not be present, depending on whether the growing planet
was large enough to capture the lighter, less-abundant planetissimals
as it accreted. (This is a simplistic model, especially when you consider
that Io, at 3640 km diameter, is sulfide rich! I suppose the moral of
the story is that coincidence probably plays a large role in the capture
of planetissiamals...)
Now every core is molten to begin with. What keeps a core molten is
governed by how many radioactive elements sank with the other siderophiles
in forming the core, and how big the planet is. The radioactive elements
decay and give off heat, keeping things warm in the core. The planet size
governs how fast the heat of the core is lost. The bigger the planet,
the slower the heat loss.
The Moon is not just a rock. It has a solid iron/nickel core (once
molten, now cooled), and an outer layer of lithophilic silicates (which
are identical in isotopic signiture to the earth's silcate mantle!).
Venus still has a partially molten core, like the Earth's. We know
this for certain now, since the Magellan data has confirmed that
mantle convection occus on Venus (and hot cores drive mantle convection).
I know this post is getting really long, but you hard core science
fiction types may want to take a look at the Magellan data. It's
been published in two special "green-cover" Journal of Geophysical
Research (JGR) issues dated this year. Really cool pictures, folks!
The first good pictures ever made of the surface of Venus (done with
radar imaging). Check it out! The pictures alone are worth your
while.
Anyway, back to planet cores. Any really old terrestrial planet,
regardless of size, will lose the heat of its core eventually, and
the core will solidify. The point: solid vs molten core is a function
of age and size and radioactive content, so I would amend Corran's
scheme and lump the solid and molten core planets together. I think
I would devide the UPP's to relect whether a planet was large enough
to retain an atmosphere able to sustain life instead.
Corran's observation that gas giants were often "conveniently" close
to terrestrial planets is a very good one. The way elements segregate
in a forming solar system means that the gas giants will be farther out
and the terrestrial planets will be further in. Gas giant shouldn't be
close to the terrestrial planet zone, at least as we understand planetary
science today. This doesn't mean, of course, that a gas giant can't
have satellites that are terrestrial planetary bodies, because they
can and do, as the satellites of Jupiter amply demonstrate.
Overall, I think Corran's idea of UPPs for planets is a good one. I'd
like to see something like it for Traveller. And with that parting
comment, I'll quit boring all you good folk with this latest nerd-attack
of mine... ;-> (Gee, Scott, I guess geology is good for something
afterall!)
Catie Helm, goechemistry-nerd
[email removed]