The Geological Society of America published an article in June that implies Mars had an early climate that involved "widespread precipitation and runoff". This was catalyzed by high-resolution photographs released by NASA's Mars Reconnaissance Orbiter, which revealed fossilized rivers in the Arabia Terra region. Below is a topographic image of the Martian surface, with Arabia Terra near the center. That large green region of runoff is cited as being roughly the size of Brazil. Its western border appears to be a curvy cut of blue, indicating the now dry canyon of Valles Marineris, a canyon more than eight times deeper than the Grand Canyon.
Image credit: NASA/JPL/MOLA Science Team.
Valles Marineris starts at the foot of Olympus Mons, the highest mountain in the solar system (no coincidence then that Valles Marineris is also its deepest canyon). Four billion years ago, when Mars' climate was vastly different, the eastern side of Olympus Mons must have been a huge precipitation zone, collecting so much snow that the warming of the planet caused Valles Marineris to erode at tremendous rates. Prior to the melting, there must have been an oceanic wind current that brought storms to this bay of the Southern Highlands, where it was continuously blocked off by the mighty facade of the 88,000-foot mountain. It's not a stretch to presume that the entire northern hemisphere of Mars used to have an ocean, while the southern hemisphere was a single continent.
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