Plaut said the amount of water in the Martian past may have been the equivalent of a global layer hundreds of meters deep, while the polar deposits represent a layer of perhaps tens of meters.The Scientific American says that geological artifacts like channels suggest that Mars was once "very wet". According to Plaut, the frozen water at the poles only accounts for maybe 10% of the water that was once on Mars. He says that perhaps more water exists below the surface, or that most of the water simply evaporated into space over time.
"We have this continuing question facing us in studies of Mars, which is: where did all the water go?" Plaut said.
"Even if you took the water in these two (polar) ice caps and added it all up, it's still not nearly enough to do all of the work that we've seen that the water has done across the surface of Mars in its history."