Early in Martian history, liquid water energetically carved the surface, forming channel systems that look remarkably similar to river valleys and drainage networks on Earth. Exactly how these channels formed—by rainfall, snowmelt, or seepage from underground springs — is often debated.
The answer has important ramifications about the early Martian climate. Clues about the source of the water may indicate the shape, layout, and scale of the various tributaries in a channel system. [More at link]