NASA Rover Finds Old Streambed on Martian Surface 
A quick Mars timeline:Mars was formed around the time Earth was, but it was blessed with only 11% of Earth's mass and less than 40% of Earth's gravity field. Shortly after cooling solid, its "Noachian Era" was similar to proto-earth: warm, a thick atmosphere, plenty of liquid water on the surface, and probably a significant magnetic field.But this era was still during the era of the Late Heavy Bombardment, a time in which the last dregs of the solar system were still settling out. Large asteroids still pounded the planets with regularity.Unlike Earth, Mars had trouble maintaining its liquid iron magnetic field. Since it's much smaller, it cooled and thus congealed faster. And there's growing evidence that asteroid impacts were able to drive enough heat beneath the surface that interior convection was quelled, leading to a fragmented magnetic field.Without an adequate magnetic field to deflect solar wind, the atmosphere was prone to shedding off pieces of itself into space. This was amplified by the lower gravity which meant holding on to lightweight gasses was even harder.Over time, Mars cooled to the point where the major forms of tectonics ceased. The water locked up beneath the ground, rusted out pulverized basalt dust from the asteroid impacts, and frizzled in the radiation-baked atmosphere, floating off.The seas and lakes dried, the rain stopped, and that... was that.Three billion years later, we arrive on the scene and find out we have a little sibling. Then we send robots. We hope to find life, or evidence that it once lived. Characterizing how water worked in the Martian past is a part of answering that question.