NASA Goes for the Gold with Curiosity
NASA went for the Gold with the Curiosity space probe, and scored a perfect 10. Perfect start, perfect execution, perfect dismount.
But even if Curiosity does not find evidence of life, even microbial life, that does not entirely rule out life on Mars. Several billion years ago, Mars lost its atmosphere, probably due to blasts from the solar wind. (Because Mars has almost no magnetic field, the solar winds are not deflected, as they are on the earth). So the water went to 3 places: drifting into outer space, into the permafrost, and onto the polar ice caps. So if you were an alien on Mars back then, where would you go?
Into the permafrost or onto the ice caps, neither of which has been probed by instruments.
Until about a decade ago, only two habitable zone planets of any size were known to astronomers: Earth and Mars.