( CNN ) -- 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 . NASA : Yes , Mars could have hosted life . 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 could n'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 above Chelyabinsk , Russia ) , providing the last key ingredient for habitability . That argument sealed the deal back in 2004 : Mars was habitable . Cool ! But the evidence we found in the rocks back then prescribed a very specific kind of habitable environment , and one that many biologists think is quite rare and special . The rocks contained abundant sulfur minerals , implying that the water in which they formed contained sulfuric acid . On Earth there are micro-organisms that can live , and even thrive , in acidic water , even battery acid . But these are rare and specialized niche organisms , what biologists call `` extremophiles , '' because they love extreme environmental conditions . Still , extreme conditions or not , it meant that Mars could have been habitable for some forms of life . Now fast-forward to 2013 , where in the past few weeks the Curiosity rover has been drilling into some different rocks half a planet away , inside a deep basin formed by the Gale impact crater . Armed with more sophisticated analysis tools than earlier rover teams , Curiosity scientists have been able to conclusively find the first on-the-ground evidence for the presence of abundant clay minerals within those drilled rocks . Clays are water-loving materials ( just ask any friends who make pottery ) , sometimes forming from the interaction of water and different kinds of precursor volcanic rocks , and holding much of that water within their mineral structures . But here 's the catch : Clay minerals are easily dissolved in acidic water . Because they are preserved in the Gale crater still today , it implies that the water there had relatively little acidity -- that is , it was fresh water , like the water you 'd find in most ponds or lakes on Earth today . Indeed , there 's geologic evidence from the rover and from NASA 's Mars orbiters that there may once have been a large lake and streams within the basin that Curiosity is driving around in now . And there 's more to be excited about . Micro-organisms on Earth are much more common in fresh water than acidic water , and so rather than Mars just having been a potential haven for some extreme forms of life , Curiosity 's discovery means that Mars had -- and still preserves evidence of -- environments that were habitable for an enormous variety of micro-organisms that thrive on our own planet . It seems as though NASA or other space agencies `` rediscover '' water on Mars every few years , so maybe this latest announcement will be regarded as just more of the same . But this time it 's different -- really ! The stakes have just been raised a notch : Not only is there another planet right next door in our own solar system that had habitable environments on its surface long ago , but there appear to have been common and recognizable -- perhaps even Earthlike -- environments on Mars . That 's the big news coming out of this latest discovery from NASA 's Mars rovers . Curiosity is only about a quarter of the way through its primary mission , so there 's lots of time to search for more watery evidence , as well as for any elusive associated organic molecules , which have yet to be found . Because of abundant solar ultraviolet radiation at the surface , organic molecules might not be preserved as well as clay minerals . But Curiosity has the tools , and the brainpower back here on Earth , to find out . The implications of discovering evidence for fresh water are exciting for macro-organisms as well : Rover chief scientist John Grotzinger of NASA 's Jet Propulsion Laboratory has speculated that we would have been able to drink that ancient water , had we been there some 2 , 3 , or 4 billion years ago to witness glorious Lake Gale . Imagine that . Let 's go back in time -- how would you like your water ? Sparkling , fresh , or Martian ? The opinions expressed in this commentary are solely those of Jim Bell .
CNN announced on 2004 that ancient Earth could have supported living microbes . Chelyabinsk : This is quite literally a watershed moment in the history of solar system exploration . He says it is the first time that we 've discovered evidence for fresh water on another planet . Bell : Curiosity rover is not done with its mission ; it may find even more interesting things .