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thomblake comments on Compartmentalization as a passive phenomenon - Less Wrong
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This is surely also true on the moon? The relative densities of the pen and the fluid you put it in don't change depending on the gravitational field they're in.
Gravity affects pressure affects density. To a first approximation, gases have density directly proportional to their pressure, and liquids and solids don't compress very much.
With air/water/pen the conclusion doesn't change. But an example where it does:
A nitrogen atmosphere at STP has a density of 1251 g/m^3.
A helium balloon at STP has a density of 179 g/m^3. The balloon floats.
Then reduce Earth's gravity by a factor of 10, and hold temperature constant.
The atmospheric pressure reduces by a factor of 10, so its density goes to 125 g/m^3.
But the helium can't expand likewise (assume the balloon is perfectly inelastic), so it's still 179 g/m^3. The balloon sinks.
Hmm. I actually don't know the relationship between gravity and buoyancy -- a moment with Google and I'd know, but in the meantime I'm in the position of relating to all those people who answered incorrectly.