World Size, Surface Gravity, and Atmosphere.

How many times have you looked at a UWP and asked yourself; "How does a world the size of Mercury have a standard
atmosphere?"  Ok, maybe that's a bit of an exageration, but not much.  According to some sources even Mars doesn't have
the gravity to retain a standard atmosphere for long, and it has a surface gravity of .38G.

There are several possible answers.  Mass of the atmosphere, percentage of oxygen in the atmospere, and density of the
planet.

First an atmosphere on a planet with a lower gravity will not be as dense.  This lower air pressure at "sea level" means
that less oxygen will get through the membrane of the alveoli in the lungs.  If you compensate by raising the percentage
of oxygen to above the 21% standard more oxygen will reach the blood stream thus acting as a "standard atmosphere".

There are several limitations on how far you can go raising the oxygen content of an atmosphere. First, as you might
imagine if the oxygen level is above 25% smoking will be prohibited planet-wide regardless of law level.  Not to mention
unshielded electrical devices, blow-torches, and back yard barbecues.  Secondly, oxygen is an irritant and an oxydizer.
It can sting the eyes, contribute to skin problems, accelerate rusting of metals, and do a host of other things including
causing the paint to peel right off your nice habitation module.

And density.  Mercury is size 3, Mars is size 4, but they both have about the same surface gravity about .38G.  That's
because while Mercury's density is about 1, Mars' density is 0.71, or only 71% of Earth.  Now what if Mars had a larger
core, or an abundance of Heavy Metals, giving it a density of 1.4?  Well, then it'd have a surface gravity of 0.7G.  Now
if we gave it an atmosphere that massed 25% more than that of Earth, and had 5% more oxygen, then the amount of oxygen
that entered the bloodstream of the average human would be roughly 92% of what you'd get at Earth sea level, or about
what you'd breathe if lived up on a mountain.

So if you want to do a little math and make your world make sense, you can work with these two factors and maybe even
convince your players that the UWP isn't all that crazy after all.  First work on the density.

For example:

	Planet
Roll	Density	|
2	.5	|	After you either roll up or pick the density it's fairly simple to multiply it by the normal
3	.6	|	surface gravity then by the mass of the atmosphere then by the % of oxygen in the atmosphere
4	.7	|	minus the standard 21%.
5	.8	|
6	.9	|	For example size 4 world (.5G), Density (1.4), atmosphere 25% more massive than standard
7	1.0	|	(1.25), and 26% oxygen (1.05).  Like so:
8	1.1	|
9	1.2	|					.5 x 1.4 x 1.25 x 1.05 = 91.875% partial pressure of oxygen
10	1.3	|
11	1.4	|
12	1.5	|

Disclaimer:	I am not a doctor or a med-student.  If you are please do not email me and rant about how my
		numbers are off by 1.2% or something.  This is simply and attempt to add a little bit of sense
		and detail to the Traveller worlds, which often make no sense at all.

		Oh and kids, don't try this at home!