The third datum we generate for a system (main world) is the hydrographics percentage. This doesn’t mean water; on some worlds this could be another surface liquid (Titan, for example, has lakes of liquid methane).
Do note that it explicitly means “liquid”. A world covered partially or entirely by ice should have a hydrographic digit “0” if no liquid oceans are present.
The Hydrographic stat is determined by 2d6-7, adding the world size code. Modifiers are:
- Size 0 or 1: Hydrographics 0
- Atmosphere code 0, 1, A, B, or C: -4
- If atmosphere is not D, or F (the rules state “of a type thick enough to retain water”): -2 for “hot” temperature; -6 for “roasting”.
I am not quite sure about the reasoning behind some of that, but it’s just as well since the “hard science” rules I am using replaces (adds? the rules aren’t even clear on that) this with the following:
- If Size is 3–4 and Atmosphere is A the DM is –6.
- If Atmosphere is 0–1 the DM is –6.
- If Atmosphere is 2–3, B or C the DM is –4.
This still seems broken: Assume a vacuum world with a size of 6 (Size 5, 2d6-7+5, roll a 2). Roll 12-7+5 for Hydrographics, results in 10, negative DM 6 meaning 40% liquid water. This could be avoided if the presence of water included ice, but as I am trying not to change what the UWP means, I can’t really make that call.
A different approach
Atmospheric pressure is vital to determining whether or not liquid surface water exists on a world. The reason hydrographics is not based off of the atmosphere, I think, is because the atmo digit doesn’t just encode atmospheric pressure, it also encodes composition – and to differing amounts for different values, too! An exotic atmosphere is not more likely to produce surface water, for example. Unfortunately, conversely, similar size doesn’t necessarily mean similar atmospheric pressure. We need not look further than Venus for proof of that.
Roll 2d6-2. Do not add size directly, instead use the following modifiers:
- Atmosphere 0 or 1: Hydrographics is set to 0.
- Atmosphere 2-3 (very thin): -4
- Atmosphere 4-5 (thin): -2
- Atmosphere 8-9 (dense): +2
We’ll disregard planetary temperature and weird atmo codes for now. This gives us the following basic distribution:
Is this realistic? I have no idea, but it seems reasonable plausible to my un-educated eyes and it’s certainly a smoother system than theTraveller default.
A quick sanity check seems to work out:
- Mercury: Size 3, Atmo 0: Automatic Hydro 0.
- Venus: Size 8, Atmo A: No surface water possible due to temperature.
- Earth: Size 8, Atmo 8: Hydro 7. 11.11% chance for that, which works just as well.
- Mars: Size 4, Atmo 1: Automatic Hydro 0.
- Europa: Size 2, Atmo 0: Automatic Hydro 0, but surface is covered in ice.
- Titan: Size 3, Atmo A: At most a “few percent” are covered by lakes, which is a hydro of 0.
Surface liquids for atmospheres A, B and C are difficult to judge for a layman like myself.
Atmosphere D (high pressure oxy-nitro) should probably be rolled as “dense”. Despite the high pressure the world is still very earth-like or it would be classified with atmo digits A-C.
Atmosphere E (thin atmosphere that collects in fissures and other depressions) should likely have an automatic 0-1, maybe 2 at most. Anything higher defeats the idea behind this world type. That would be a DM of -8.
Atmosphere F is a completely special case, it’s basically a catch-all for “everything that’s very exotic”. Set Hydro to 0 as a safe default, then roll according to actual conditions.
Planets that are very hot or very cold will likely have little or no liquid water. I’ll tackle that point in the next post. Meanwhile – if you have any feedback, I’d appreciate it!