A few days ago, some folks (I don't archive the TML, so I don't
remember who, though I'm pretty sure wildstar was one of 'em) were
talking about revising the UPP system to have an integrated
"composition" digit that would include atmosphere type and planetary
makeup.
I just looked at an article on Titan (the second-biggest satellite in
the Sol system, bigger than Luna), and it seems that the postulated
relationship between surface gravity and atmospheric pressure is
contradicted by astronomical observation.
Titan has a radius of 2575 km (that's a diameter of 5150 km or 3219 miles, for
a size digit of 3), a mass 0f 0.022 Earths, a surface gravity of 0.14 g, and a
surface pressure of approximately 1.5 bars. That's 50% *more* pressure than
Earth, on a *much* lighter planet. So much for the idea that atmospheric
pressure is directly related to planetary mass or surface gravity....
Titan's atmospheric composition is not precisely known, but the
primary gas is nitrogen (pp around 1.3 bars), the secondary is methane
(around 0.15 bars), there might be some argon (pp disputed), and there
are traces of all sorts of nasty nitrogen compounds, from hydrogen
cyanide to cyanoacetylene. There are almost no oxygen compounds (as
befits a strongly reducing atmosphere like that); carbon monoxide
might have a partial pressure as high as 0.2 millibars.
However, current astronomical speculation on Titan fits the
previously-expressed theory of "everything has a core." With a
density of 1.88 grams per cubic centimeter (0.34 Earth densities), the
_World Builders' Handbook_ gives Titan a designation of "icy body." The best
current estimates give Titan a 1000 km radius core of liquid iron and ferrous
sulfide, surrounded by layers of silicate rock, clathrates, and various ices,
with the exterior being a miniscule crust of water-ammonia ice floating on
(possibly liquid) water-ammonia.
Obviously, the WBH nomenclature of "icy body"/"rocky body"/"molten
core"/"heavy core" needs revision; however, any revision will not be able to
encompass atmospheric pressure as well.
(BTW, I've assigned Titan a tentative atmosphere digit of A: exotic-irritant.
Any comments?)