@martzel86: …with a change in its chemical conditions, it would be [compressible]... I’d say physical, not chemical, conditions.
The effect of air pressure (which arises in the first place from gravitation) would only change the density of gas-filled objects like balloons or certain foams. Gravity itself should have no effect on density except for objects immense enough to experience tidal forces, which would tend to decrease density by outward tension.
In comparing earth and moon, note that (1) the moon’s gravity is 1/6 that of earth; and (2) the moon has no atmosphere, so its “air pressure” is always zero.
Permit me to quote from Wikipedia’s article on density:
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In general, density can be changed by changing either the pressure or the temperature. Increasing the pressure always increases the density of a material. Increasing the temperature generally decreases the density, but there are notable exceptions…
The effect of pressure and temperature on the densities of liquids and solids is small. The compressibility for a typical liquid or solid…roughly translates into needing around ten thousand times atmospheric pressure to reduce the volume of a substance by one percent.
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Note that the atmosphere on Venus, whose gravity is about 90% of earth’s, is almost 100 times that on earth—equivalent to diving in the ocean to a depth of 1 kilometer. Even at this crushing pressure, however, density would (according to quote above) only increase by 1/100 of 1% = a factor of 0.0001.