How the layers on Mars got there
The process is relatively straight forward.
Every 26 million years or so our Solar System passed through the plane of our galaxy, the Milky Way. As a result, comets enter the inner solar system. These comets impact with planets, or they impact with minor planets, which then in turn hit the major planets.
In the case of Mars, these very large impacts release volatiles held in its surface. This inital release of volatiles triggers a further release. Then there is a period of activiy until these volatiles are again trapped in Mars surface.
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