No one really knows for certain. A lot has happened to Mars in the last 4 billion years, and we still do not know much about just how active the planet is geologically, although for the last billion years, it sure looks like its been pretty quiet. Until we can 'date' the various features unambiguously by sample return missions, we do not know what the exact chronology for the various features are, or what Mars's tectonic activity has been. This is a Hubble Space Telescope view taken near opposition to show the clearest details.
Copyright 1997 Dr. Sten Odenwald
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