One of the proxies used for paleoclimate is the ratio of oxygen-18 to oxygen-16 in ice cores and in sedimentary rocks. The idea is that water molecules with oxygen-18 generally evaporate less readily than oxygen-16, and condense more readily; and this proclivity varies with temperature. One can then use the deviations of oxygen-18 concentration from a standard ratio to say something about Earth's temperature in the distant past.
Why do light water molecules evaporate more readily? It makes some intuitive sense that lighter molecules would evaporate more easily. But how does this square with the equipartition theorem and the idea that all molecules have the same amount of kinetic energy at a given temperature?
It would seem to me that because the intermolecular forces between two water molecules are independent of their mass, it should take a certain amount of energy to remove a water molecule from its neighbors in the liquid phase and move it far away. This energy requirement doesn't depend on the mass of the molecules, only on how the force on it varies with distance ($W = \int \vec{F} \cdot d\vec{r}$.) So by this argument, molecules with the same amount of kinetic energy would have the same likelihood of evaporation. And by the equipartition theorem, the translational kinetic energy of a water molecule is only dependent on the temperature, not on the mass of the molecule.
EDIT: Just to pre-empt the "it's velocity that matters, not energy" argument: I agree that lighter molecules would, on average, have a higher velocity. But from classical physics, we know that applying a fixed force to a smaller mass leads to a larger change in velocity, while a larger mass gets a smaller change in velocity. That would imply that the "escape velocity" required for a heavier molecule would be smaller; since it's heavier, its motion would not be affected as much by the force it feels from the other molecules. This would imply that while heavier molecules are moving more slowly on average, they also don't need to be moving as quickly to escape. In other words, the velocity threshold for evaporation should be lower for heavier molecules. These two effects (lower speeds and lower threshold for evaporation) should cancel each other out.
Where does my logic fail?