A good piece of evidence is that the dust of Mars is significantly composed of 'goethite', essentially hydrated iron rust. There's an awful lot of dust, and if you do the work backwards you can get a decent idea of how much water it took to make it happen.
Furthermore, most liquids found at reasonable temperatures aren't all that voluminous, and a number of the minerals scanned and inspected on Mars show evidence of water contact.
Basically, there's not much else it could be. :) That said there's some really cool theorizing on underground carbon dioxide flows that might explain some washouts seen later in Mars's life.
Almost! The blueberries are hematite, Fe2O3, an iron oxide. Goethite is FeO(OH), an iron oxyhydroxide. Both strongly indicate water went in to making them but the blueberries imply they were submerged in water whereas goethite doesn't necessary require anything but water vapor.
Furthermore, most liquids found at reasonable temperatures aren't all that voluminous, and a number of the minerals scanned and inspected on Mars show evidence of water contact.
Basically, there's not much else it could be. :) That said there's some really cool theorizing on underground carbon dioxide flows that might explain some washouts seen later in Mars's life.
http://en.wikipedia.org/wiki/Goethite