Rabe formed in an instant when a large rock slammed into the surface of Mars long ago. It may have started as a normal, if large, impact crater, but things change. The cause of the flat floor isn't well known, but likely due to volcanic deposits filling it, plus material blown in over time.
Actually, that last bit is very important. Mars has an atmosphere, though a thin one, pushing down with less than 1% of the pressure on the surface as Earth's air does here. But it's enough.
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