FOUR/2 Hydrosphere

STEP ONE: Check if the world has a hydrosphere on 4.2.1. If it has none then skip to FOUR/3.

STEP TWO: If 4.2.1 gave a result of "Liquid" or "Ice Sheet" then determine the extensiveness of the hydrosphere by rolling on 4.2.2.

STEP THREE: If a "liquid" or "ice sheet" hydrosphere is present then determine the water vapour factor of the atmosphere.

Table 4.2.1 Hydrosphere Determination

Base Surface Temp   Type
Outer Zone     Crustal
≤245°K     Ice Sheet
246°K - 370°K     Liquid
371°K - 500°K     Vapour
≥501°K     None

Table 4.2.2 Hydrosphere Extensiveness (%)

1d10  Planetary Radius (km)
≤2000  2001-4000  4001-7000  ≥7001
1  None  None  None  None
2  None  None  2d10  2d10
3  None  1d10  1d10 + 20  2d10 + 20
4  None  1d10  1d10 + 30  2d10 + 40
5  None  1d10 + 10  1d10 + 40  1d10 + 60
6  1d10  1d10 + 20  1d10 + 50  1d10 + 70
7  1d10  1d10 + 30  1d10 + 60  1d10 + 80
8  1d10 + 10  1d10 + 40  1d10 + 70  1d10 + 90
9  5d10  1d10 + 50  2d10 + 80  100
10  10d10 + 10  10d10 + 10  100  100

Table 4.2.3 Water Vapour Factor

Wv = [(T - 240)/ 100] x H x 1d10

... where: T is the base temperature (in °K)
H is the hydrosphere percentage

(Treat negative Wv as zero.)


This only considers water. Other liquids may form pools or oceans on cold or hot worlds. Methane on cold worlds, for instance, or sulfur on hot worlds. High pressure can be a prerequisite for non-water oceans.


4.2.1 lists five different kinds of hydrospheres:

None: The world has no water at all. There may be small deposits of ice in polar locations on an airless worlds.

Vapour: The world has water in the form of water vapor only. As water vapor seldom is very stable in an atmosphere, this is likely a small part.

Liquid: The world has a potential for liquid water.

Ice Sheet: The world can have water on the surface, but generally only in solid state. Some water may be permafrost, but ice sheets and ice caps are a definite possibility on worlds with more than 5% hydrosphere. Thick enough ice sheets may cover unfrozen oceans heated by tectonics.

Crustal: The world is an outer zone world which has not endured lunar density separation, and ice is a major part of the moon, mixed with silicates and other ices/frozen gasses. These worlds do not have a normal "hydrosphere".


If a world has low atmospheric pressure (approximately 0.006 atm) liquid water cannot exist. This means that liquid water is not possible on airless or near-airless worlds. The atmospheric pressure is determined in 4.2.3. Remove hydrospheres on these worlds - water may be as permafrost or ice caps.


Further developments will affect the base temperature of a world, and that means that a ice sheet world can become a ocean one. Or vice versa.


According to theories how atmospheres and hydrospheres form, they are generated by 1) cometary infall in the early system and 2) volcanic outgassing. A larger world can generate more atmosphere and water, and thus larger worlds have more extensive hydrospheres. In general.


Large impacts on icy worlds may melt the icy crust. In the same way, a large impact may bring dust into the atmosphere and cool off the world.


In extreme cases, the eccentricity of a world may be enough to boil water at closest separation and freeze it at farthest separation. The vapour itself will add to both greenhouse effect and modify the albedo, so this can get complex.


These icy worlds seldom have real hydrospheres unless they are density-separated moons.


Ammonia-rich worlds may sustain liquid water below 273°K, down to about 180°K. The ammonia mixes with the water and acts as a defrosting agent. This is most likely around cool stars.


Small amounts of water can be found in polar locations on hot airless worlds. Water can also be chemically bound to rock, or exist in subsurface strata.