In the past year or so, scientists have discovered more evidence for liquid water under the surface of Mars. They've found complex organic compounds — the building blocks of life. And they've found that methane levels in Mars' atmosphere vary with the seasons.
"Each of these things adds up to say that the probability of finding life on a world that's not our own is going up," says NASA Administrator Jim Bridenstine. "And Mars, I think, is that best opportunity in our own solar system to find life on another world."