How recently were different parts of Mars resurfaced by flooding, weather, etc.?
A small crater in the middle of a flood plain, like the ones in Ares Vallis near the Pathfinder landing site, must have been emplaced after the flood. Counting them gives an relative idea of when the flood occurred. Water rings in a crater, as seen in the crater within Gusev crater, suggest that the crater contained a lake sometime after it was formed. The relative geologic ages on Mars, like on Earth, have names (Noachian, Hesperian, Amazonian in the case of Mars.). On Earth, scientists eventually were able to assign absolute dates to those ages, but craters only only give relative ages.There s an excellent essay you can read for a more detailed explanation and illustrations. One striking thing about Mars is that the southern hemisphere s surface is much more ancient (heavily cratered) than the northern hemisphere s, as illustrated here.
