The Earth has 2.7 km of water on its surface. Its mantle contains at least 150 ppm water, and probably significantly more depending on the amount of undepleted mantle and subducted crustal water that is present. Geologic evidence suggests that a few hundred meters of water are close to the Martian surface, but evidence from SNC meteorites indicates that the Martian mantle is very dry, containing no more than about 35 ppm water. Part of the difference in water content of the mantles of the two planets is attributed to plate tectonics. However, the Earth's mantle appears to contain at least several times the water content of the Martian mantle, even accounting for plate tectonics. We attribute the difference to two possible causes. The first possibility is that melting of the Earth's surface during accretion, as a result of the development of a steam atmosphere, allowed impact-devolatized water at the surface to dissolve into the Earth's interior. In contrast, because of Mars' smaller size and greater distance from the Sun, the Martian surface may not have melted, so that the devolatilized water could not dissolve into the surface. A second and preferred possibility is that Mars, like the Earth, acquired a late volatile rich veneer, but it did not get folded into the interior as with the Earth, but instead remained as a water-rich veneer. The perception of Mars as having a wet surface, but a dry interior, is consistent with what we know of the geologic history of Mars, which can be viewed as the progressive intrusion and overplating of a water-rich crust by dry, mantle-derived volcanic rocks.