Hard to argue that this is anything but clear evidence that Mars was once a wet planet.

NASA’s Curiosity Finds Surprise Clues to Mars’ Watery Past

It’s the sort of terrain where, on Earth, I’d not be surprised to see a fossil shell or footprint. That’s doubtless too much to ask *here,* but the evidence of water just sitting there like that is pretty spectacular.

Feel free to try to imagine “other ways of knowing” providing this.