Volatiles

Volatiles are compounds which under normal or near-normal conditions are in a liquid or gaseous phase. Normal in this context is taken to be relative to general region where the substance is located. For example, water is volatile on Earth but water ice is a rock-hard mineral on Pluto.

When the Mars sized body, called Thea, hit Earth soon after the solar system formed, much of the mantle & crust of both bodies vaporized. A large portion of this mass went into orbit around Earth. The elements and compounds (such as titanium dioxide) with high melting temperatures condensed into liquids, then solids, first. Then those with medium high melting points condensed. However, the volatiles remained a gas long enough, that most of them spread out and were blown away by the solar wind. Thus, Luna is rich in elements and compounds with high melting points, and is low in volatile elements and compounds. Since most of the elements that make up life are volatiles, this makes them rare on Luna, and they will have to be imported.

Mars on the other hand is rich in the volatile elements, and all the common elements needed by living things are found on that planet.

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