The evidence comes in the form of trace elements located in and around the Red Planet's vast northern plains, a low - lying region that might once have held a body of water large enough to blanket nearly one - third of the planet. (newscientist.com)
Up till now, the case for past oceans on Mars has been based largely on two sets of features that separate the smooth northern plains from their higher, more rugged surroundings. (newscientist.com)
In 2012, Mars Express spotted a possible ancient ocean on Mars: sediments on the planet's northern plains appeared to resemble what is seen on an ocean on Earth. (space.com)