In September 2012, NASA announced its Curiosity rover had identified gravel made by an ancient river in Mars' Gale Crater.
Then in March 2013, scientists found chemical ingredients for life — sulfur, nitrogen, hydrogen, oxygen, phosphorus, and carbon — in powder that Curiosity had drilled from rock near the ancient streambed.
"A fundamental question for this mission is whether Mars could have supported a habitable environment," Michael Meyer, who worked as the lead scientist for NASA's Mars Exploration Program at the time, said in a press release about the finding. "From what we know now, the answer is yes."
In the following years, evidence has mounted that the planet was once home to a vast ocean.