[RPG] What are the Pathfinder environment rules for combat on the surface of the moon

Low Gravity

The Distant Worlds Campaign Setting actually has a few rules about low/no gravity:

Low Gravity (×1/2): Characters can jump twice as high and as far and can lift twice as much. Movement speed is unaffected. Each range increment for ranged weapons is doubled. Creatures that fall within an area of low gravity take 1d6 points of damage for every 20 feet fallen.

No Gravity (×0): Without magical flight, moving in an area with no gravity is difficult. A character with a surface to push off from can move up to half speed in any direction. A double move or charge can be performed this way, but not a run. A character can move at his full climb speed or his full land speed by succeeding at a DC 20 Climb check as long as he remains adjacent to a surface with sufficient handholds. He adds his Dexterity modifier (minimum 0) on this Climb check in addition to his Strength modifier. Once a character starts moving, he continues moving at the same speed in the same direction each round without using an action until he latches onto an object to stop himself, pushes off in another direction, or creates thrust somehow (each of which requires a move action). Creatures with nonmagical means of flight lose the ability to fly for 2d6 rounds after entering a no gravity area. A character in a no gravity environment can lift and carry 10 times his normal limit. Ranged weapons have no maximum range, and their range increment distances are multiplied by 10.

Since the moon has about 1/6 the gravity of the Earth, the rules for 1/2 Gravity suits us better for the desired effect.

As a house rule, you could extend the jump bonus and ranged weapon increment by 3 times what is listed, so a character could jump 6 times as high and lift 6 times as much weight, while ranged weapons would reach 6 times as far. This rule could be extended to any gravity that is different from earth, on a planet that has 1/10 the gravity, you could jump 10 times as high, while on a planet that has twice the gravity, you could only jump 1/2 what you could jump on earth.

Do notice that, despite the name, the rules for no gravity can actually be used on both to Zero Gravity and enviroments such as outside of a space ship or on the surface of an asteroid (Microgravity), since they do allow your character to move if you have a surface to push off from.

No air

The Enviromental Rules cover the lack of breathable air, or water, for aquatic creatures, or whatever you breath to stay alive:

Suffocation

A character who has no air to breathe can hold her breath for 2 rounds per point of Constitution. If a character takes a standard or full-round action, the remaining duration that the character can hold her breath is reduced by 1 round. After this period of time, the character must make a DC 10 Constitution check in order to continue holding her breath. The check must be repeated each round, with the DC increasing by +1 for each previous success.

When the character fails one of these Constitution checks, she begins to suffocate. In the first round, she falls unconscious (0 hit points). In the following round, she drops to –1 hit points and is dying. In the third round, she suffocates.

The space is not cold

Since the atmosphere of the moon is practically nonexistant, creatures on the surface of the moon are subject to the enviroment of space, which, unlike popular belief, is not exactly cold, that will depend on the surrounding stars (in our case, the Sun).

The temperature on the surface of the moon will depend on whether those creatures are on the bright side of the moon (about 123°C, or on her shadow (which is about -150°C).

But creatures are subject to the lack of warm from a planetary normal atmosphere, and will eventually lose body heat.

Again, the Enviromental Rules have a set of rules regarding cold enviroments, though these rules were designed for cold weather rather than space travel.

If the creatures are on the moon's shadow, you could say they are dead within a few turns unless they have cold immunity somehow.

Extreme cold (below –20° F) deals 1d6 points of lethal damage per minute (no save). In addition, a character must make a Fortitude save (DC 15, +1 per previous check) or take 1d4 points of nonlethal damage.

If they are on the moon's bright side, they should actually be vulnerably to Extreme Heat instead of cold, and are probably vulnerable to solar radiation. But rules as written, they only really need Energy Resistance (cold) 6, as these rules don't make a distinction between -20°C and -150°C.

In severe heat (above 110° F), a character must make a Fortitude save once every 10 minutes (DC 15, +1 for each previous check) or take 1d4 points of nonlethal damage. Characters wearing heavy clothing or armor of any sort take a –4 penalty on their saves. A character with the Survival skill may receive a bonus on this saving throw and might be able to apply this bonus to other characters as well (see the Survival skill in Using Skills). Characters reduced to unconsciousness begin taking lethal damage (1d4 points per each 10-minute period).

For solar radiation, there are published rules on the Technology Guide aswell that you could use those on top of the rules for Extreme Heat. The rules for radiation, however, do not account for a creature that has fire resistance, so you might want to reconsider using them if the character is resistant/immune to fire damage.

Vacuum

The Starsoul Sorcerer Bloodline has an ability called Breaching the Gulf:

(...) once per day you can teleport a single creature within 30 feet into the void of space if it fails a Will save. The save DC is equal to 10 + 1/2 your sorcerer level + your Charisma modifier. The target can attempt a new saving throw as a full-round action each round to return. While trapped in the airless void, the target suffers 6d6 points of cold damage per round and must hold its breath or begin to suffocate.

However, this is one ability and does not cover the enviroment in general.

This topic on paizo's forum has interesting ideas on how to handle vacuum situations. One of which is from d20 Future and i will quote here:

On the third round of exposure to vacuum, a creature must succeed on a Constitution check (DC 20) each round or suffer from aeroembolism (“the bends”). A creature that fails the save experiences excruciating pain as small air bubbles form in its bloodstream; such a creature is considered stunned and remains so until returned to normal atmospheric pressure. A creature that fails the Constitution check by 5 or more falls unconscious.

The real danger of vacuum comes from suffocation, though holding one’s breath in vacuum damages the lungs. A character who attempts to hold his breath must make a Constitution check (DC 15) every round; the DC increases by 1 each round, and on a successful check the character takes 1 point of Constitution damage (from the pressure on the linings of his lungs). If the check fails, or when the character simply stops holding his breath, he begins to suffocate. In the next round, he falls unconscious with 0 hit points. The following round, he drops to –1 hit points. On the third round, he drops to –10 hit points and dies.

In my opinion, though this rule is unofficial and for a different d20-based system, it does accurately simulate the desired effect.