Greetings, sophonts. I arise again on the TML after surviving
the conclusion of teaching my first university class ever (thank
God that's over!), and also surviving a very nervous holiday season
with a sudden serious illness in my family (and I'm very happy to
say that my mom will live :-) I've just read four weeks of TMLs
in one sitting (gads - I must be crazy!), and will posting various
and sundry in response.
I have a post from Jim Kellerer to upload, about psionic controls,
psi shields, and other such topics; so you all can look forward to
that. I will probably get Jim's stuff online this weekend. I also
have news blips to send to Cynthia H., when I can those uploaded
and when I can find Cynthia's email address (if you read this, Cynthia,
could you do me the favor & send it to me at [email removed]?)
I know I owe Jim B. at NUSC an explanation for the formation of
gas giants vs the formation of terrestrial planets, but I will save
that for another post in the cause of trying to keep the verbosity
down to a minimum.
And now, what follows is another nerd attack. It's a little long,
I'm sorry to confess, so those of you who don't care for we overly
nerdly types, I will not be at all insulted if you decide to skip
it. ;->
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> A while back there was a thread on the TML about planetary
> atmospheres. Basically the argument went along the line that
> since the earth has a standard atmosphere, a planet with a
> more dense atmosphere in Traveller must have a
> more dense core thean Earth. I'm afraid that is not the case.
That was not at all the "argument" of that discussion, of which I
was the author. The discussion was about the formation and retention
of atmospheres during planetary formation. The central concept is
that only large and dense terrestrial planets can form and retain
atmospheres, because only those planetary bodies can exert sufficient
gravitation force on the lighter gases such that they are retained
in adequate quantities. This is not an argument per say, but the
current accepted theory in the earth and planetary science community
for how atmospheres form.
The point I was originally trying to make is that you have to have
a sufficiently large and dense terrestrial planet in order to make
an atmosphere during planetary accretion.
> any analysis of Earth's atmosphere has to take the actions of
> Earth's moon into account...The moon is so large that it is slowly
> skimming off the Earth's atmosphere, thinning it out. Tidal action
> of the moon actually gives escape volocity to some of Earth's
> upper atmosphere.
True, but it is a third or fourth order effect. Gravitation force
and weight fractionation are the primary effects in the fractionation
of the atmosphere, as governed by the Boltzman equation as discussed
by Ury (?) in 1951 <I may have that reference wrong; if anyone *really*
cares, I can dig up the reference>. While there are tides in the
atmosphere, they are not a big actor. Consider it this way: if tidal
effects were significant in skimming the atmosphere, then why, after
4 billion years of geologic time, has the earth retained most of its
atmosphere (we know it has, based on the evidence of the stratigraphic
record)? The loss of a little hydrogen, helium, and neon has more
to do with the weight fractionation of the entire atmosphere than
it has ever had to do with the gravitational pull of the moon.
> By constrast, Venus is slightly less dense than Earth (Surface
> gravity 0.9G), yet it has an atmosphere of between 10 and 25
> atmospheres (CRC handbook Chem/Physics) Why? Venus has no moon,
No, sorry. But don't take my word for it. Here for your perusal
is a pertinent passage from Fuare's emminently readable volume
on geochemistry:
"The atmosphere of Venus is composed primarily of CO2 with
smaller concentrations of N2, H2O, and trace amounts of O2, Ar,
Ne, and SO2 (Oyama et al., 1979). The atmosphere contains three
layers of clouds between 68 and 31 km above the surface. The
clouds consist of droplets of concentrated suluric acids having
diameters of about 1um. The temperature of the atmosphere increases
downward from -50 C at the top of the cloud layer to between 420
and 485 C at the surface. The atmospheric pressure is 90 bars,
equivalent to the pressure at a depth of 1000 m in the oceans of
the Earth (Glass, 1982). The high temperature and pressure of the
surface of Venus result from the abundance of CO2 (96.4%) and
H20 (0.14%) in the atmosphere, both of which absorb infrared
radiation and cuase an extreme case of the 'greenhouse effect'."
"The low abundance of water vapor in the atmosphere of Venus
is still unexplained. Venus should originally have contained as
much water as the Earth, most of which should now reside in the
atmosphere because of the high surface temperature. The low
abundance of atmospheric water vapor reported by the Pioneer Venus
spacecraft may be explained by postulating that water molecules
were dissociated by ultraviolet radiation into H2 and O2. The H2
can escape into interplanetary space, but the O2 should have
accumulated in the atmosphere of Venus. However, the observed
concentration of atmospheric O2 is much lower than expected if it in
fact originated by the decomposition of water...The O2 may have been
removed from the atmosphere by the formation of oxides, which should
form a layer several kilometers in thickness. There is no evidence,
however, for the existence of such a layer on Venus. The low
abundance of water and oxygen in the atmosphere of Venus therefore
remain a mystery."
"The abundance of CO2 and N2 in the atmosphere of Venus are
compatible with their terrestrial abundances. Most of the CO2 has
been removed from the atmosphere of the Earth by the formation of
calcium carbonate in the oceans. If all the CO2 in terrestrial
carbonate rocks were returned to the atmosphere, its mass would
increase about 70 times and the CO2/N2 ratio would be about the
same as that in the atmosphere of Venus. Therefore, the absense
of oceans on the surface of Venus has caused CO2 to remain in its
atmosphere and has resulted in the high surface temperatures."
pp. 145-146
Basically, Venus and Earth are twin planets compositionally, with
the one exception of the lack of water vapor on Venus. The higher
atmospheric pressure at the surface of Venus is due to the fact that
all the CO2 is in the atmosphere, increasing the partial pressure of
CO2, as well as making the optical depth of Venus's atmosphere much
greater than Earth's. The density of Venus's atmosphere is due to
more to surface chemistry (ie gaseous CO2) than to physics (ie, to
weight fractionation or to gravitation skimming by other
planetary bodies).
This isn't to say that the atmosphere of the Earth has been all
that stable over geologic time. The stratigraphic record contains
significant facies which could only have formed in a REDUCING
atmosphere, ie one with little free oxygen. How the atmosphere
shifted to an oxidizing environment in the precambrian is hotly
debated in paleoclimate circles.
> I also seem to remember reading an article a couple years back that
> suggested Earth may have been hit by a small planetoid. Approx
> Mars size. When the planetary core hit, it spread a large amount
> of heavier{metals over the surface of the planet. That was
> supposed to explain why there are measurable amounts of metals
> like gold, silver, platinum, lead, copper, iron, etc. in the
> crust instead of them being tied up in the core of the planet.
> Catie? You heard anything like that?
The collision theory is one of four contested theories for the formation
of the moon. It is a better theory, actually, than the other three,
but its appeal has less to do with explaining crustal abundances, but
rather, it explains the siderphillic, H2O, and voltile element depletion
of the moon itself. This is a very involved subject (ie, the formation
of the moon), so I will stop here. (S. R. Taylor's article in Geochimica
Cosmochimica Acta (1987) is a excellent spot for the compulsively
interested to continue reading on this subject.)
Gold, silver, platinum, copper, and iron are considered to be depleted
in the crust of the Earth with respect to cosmic abundances. The
reason we see these siderphillic elements at all is because the initial
differentiation of the Earth during its formation was an imperfect
process. The accretion of the Earth's core didn't manage to suck up
all of the siderophillic metals, which for us is most fortunate! :-)
That's all for now, folks. (We now return to our regularly scheduled
program of traveller weapons, psi shields, PBEM news, and the evils
of TNE ;-)
Catie Helm
[email removed] (please, use this email address only!)
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