An announcement on Tuesday marked, quite literally, a watershed
moment in the history of solar system exploration. NASA
scientists said an analysis of drilled rock samples collected by
the Curiosity rover shows that ancient Mars could have supported
living microbes. It is the first time that we've discovered
actual evidence for fresh water on another planet. We've been
down this watery path before -- sort of. Back in 2004, NASA's
Opportunity rover found evidence of ancient water on Mars. For a
place to be habitable by life as we know it, it has to have
liquid water, heat sources like volcanoes or impact craters, and
carbon-bearing organic molecules. Mars had abundant volcanoes
and impacts early on, and Opportunity rover scientists finally
found smoking gun evidence for the ancient liquid water. What
that rover couldn't tell us was whether Mars had organic
molecules. But organic molecules get delivered to planets all
the time from impacts by small and large asteroids and comets
(like February's fireball impact