Hot molten rock, called magma, is generated deep down. It rises until it meets cooler rocks that contain hydrogen-bearing minerals. These minerals decompose when heated by the magma and the hydrogen is released, dissolving in the magma and forming water. The magma continues toward the surface, where it erupts from a volcano and its water is released in the form of vapor. Think of the bubbles escaping a soda when you open the bottle.
Scientists say water from Earth's interior reaches the surface in a similar fashion.
Massachusetts Institute of Technology researcher Timothy Grove, working with McSween, makes fake Mars rocks in his laboratory. By subjecting these synthetic rocks to high temperatures and pressures, he can figure out how much water would be contained in magmas at the time their crystals formed. The only way you can reproduce the unique chemical composition of these minerals is to have water present. Click here for more news and information about Mars.