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How can water be chemically trapped in rocks?

My astronomy text says: "Water is also locked into the chemical structures of many Earth rocks. In that sense, even the Sahara Desert is a veritable swamp compared to the arid, bone-dry surfaces of Venus and Mars." What does "locked into the chemical structures" mean? Isn't water a solid, liquid, or gas? I could understand if there was a miniscule amount of solid, liquid, or gaseous water PHYSICALLY trapped in the rock, but what do they mean about water being CHEMICALLY locked into the rock?

There are two ways that water can be included in rock. It can exist as a solid, liquid or gas trapped in pore spaces (holes) in the rock. Water molecules can also be trapped and included as part of the crystal structure of the rock.

In the first case, the water exists in holes in the rock, as you describe in your question. We think that most of Mars (the equatorial region) can't support this kind of pore-space water trapping in the upper part of the surface because the planet is dry and the atmospheric pressure is low. New Mars Odyssey results show that there may be this kind of water trapping near the poles of Mars, however.

In the second case, the water is actually bound up in the rock, changing the mineralogy. For example opal is actually quartz (SiO2) with water molecules in the crystal structure. So the chemical formula is then SiO2*H2O. Other good examples include the clay minerals. These minerals have places in their crystal structure that can accomodate water molecules. Clays consist of layered sheets of atoms (for example Si, Al, Mg, O, etc.) that are bound together. The sheets themselves are only loosely bound together, and in many cases water molecules can become trapped between these sheets and can bind loosely to the sheets above and below. So when these clays get wet, the water is actually trapped by the crystal and becomes bound; it's not just trapped in holes in the rock. Some clays are extremely expansive (they can accomodate several layers of water moleules in between layers of silicates) and can cause serious damage when they absorb water. On Earth, clays are very common. Mars has such a dessicating environment that we think clays (if there are any) are probably isolated in specific regions. The Mars Odyssey craft also shows some hydrogen detections near the equatorial regions, and some people think that these may be places where water is trapped chemically in minerals.

December 2002, Lynn Carter (more by Lynn Carter) (Like this Answer)

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