What key geological evidence confirmed to Viking mission scientists that massive volumes of water once existed on the Martian surface?

Answer

Landforms like sedimentary rocks and remnants of riverbeds

Geologists studying images from Mariner 9 and Viking recognized that landforms such as sedimentary rocks, remnants of riverbeds, and features resembling dried lake bottoms were key evidence pointing to a warmer, wetter past.

What key geological evidence confirmed to Viking mission scientists that massive volumes of water once existed on the Martian surface?

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