Bill Sheil
Cosmic Mongoose
Io (size 3, very thin) and Europa (size 2, trace) retain atmosphere at lower then Earth density but a low temperature and not free Oxygen or Nitrogen molecules, I believe.
Gravity on planets
- Thread starter JNJ
- Start date
Bryn the 2300AD guy
Banded Mongoose
Both are, effectively, vacuum worlds. Io has an atmospheric pressure ca. 1/1,000,000,000th (a billionth) of Earth, and is mainly SO2. Europa has 1/100,000,000,000th (a hundred-billionth) of Earth, being released water and a little O2. They're composed of recently released gases that they lack the gravity to capture.
Both qualify as ultra-high vacuum (UHV) for physics experiments.
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Geir
Emperor Mongoose
The cut-off for Trace - Atmosphere 1 - in Traveller is 0.001 bar, so anything less is considered close enough to be vacuum.
A variant rule I suggest in the forthcoming World Builders Handbook is to apply a DM-2 to worlds Size 4 and below. A variant to the variant applies the DM based on gravity instead. I also provide a table with various gas retention values and a simplified formula to determine what gasses are retained based on planetary mass, diameter, and temperature. And tables for various temperatures with random gasses to consider as components of exotic and worse atmospheres.
But Traveller is science fiction, so you also have to deal with terraforming by humans or Ancients or whatever race. So in Traveller you can support gas mixtures that don't make sense in the long run. Mars likely still had a bit (or lot?) of retained hydrogen in its atmosphere back when it had oceans. Even a small world (Size 2) could have a breathable atmosphere for a few million years until it mostly escaped. Or it could just be really, really dense - an iron core of a destroyed world.
A variant rule I suggest in the forthcoming World Builders Handbook is to apply a DM-2 to worlds Size 4 and below. A variant to the variant applies the DM based on gravity instead. I also provide a table with various gas retention values and a simplified formula to determine what gasses are retained based on planetary mass, diameter, and temperature. And tables for various temperatures with random gasses to consider as components of exotic and worse atmospheres.
But Traveller is science fiction, so you also have to deal with terraforming by humans or Ancients or whatever race. So in Traveller you can support gas mixtures that don't make sense in the long run. Mars likely still had a bit (or lot?) of retained hydrogen in its atmosphere back when it had oceans. Even a small world (Size 2) could have a breathable atmosphere for a few million years until it mostly escaped. Or it could just be really, really dense - an iron core of a destroyed world.
TrippyHippy
Emperor Mongoose
It depends on the relative size of the planet. If you travel to a planet with a high gravitational impact, and then get back to a spaceship or whatever after a given period of time, you will have aged less than the crew.
CordwainerFish
Emperor Mongoose
Not perceptibly so.
Bryn the 2300AD guy
Banded Mongoose
Earth has a density of 5.51 g/cm3. Iron is 7.8 g/cm3. A pure iron planet has a density of 1.42 of Earth. A 5,000 km world (size-3) of pure iron can just about retain O2, but not N2, in the life zone.
Condottiere
Emperor Mongoose
Lead cores.
Geir
Emperor Mongoose
I don't want to play "Yeah, but..." But... nickel is 8.9 and that's a room temperature values. In something the size of a planet, you'll have compression to further increase the density, so densities above 8 are possible without throwing in a core of gold (or uranium). Also, IIRC the 'standard' calculations for gas escape are based on a billion year timeframe and in any case don't account for replenishment from internal or external sources (volcanoes, comets, large terraforming plants).
The constraint I'm working under is a predefined 2D-7 + Size roll (and even there I added an extra DM for smaller size), and 15,000+ predefined UWPs, so question becomes: how do you make the results plausible, not so much if they're probable - after all the UWP represents the 'best' world in a system, not the typical world. In the Vanguard Reaches, there's a world, Turan, that's C250441-8 and I left it that way (and blamed the Ancients) but made it clear that it was becoming marginal rather quickly. But, taking your argument to heart, H2O is going to escape even faster than N2. I would really prefer to base Hydrographics on Size, not Atmosphere - it was so for, I think, one print of MegaTraveller, but it was possibly not even intentional.
