Evidence for water on Mars in the past … again

NASA’s Spirit rover has discovered levels of silica in the Martian soil that NASA claims is the best evidence so far that Mars once had liquid water on its surface. Don’t get me wrong – I’d love to think that Mars harboured life at one point, or at least had the potential to do so – but the ‘water on Mars’ meme seems to flip-flop every couple of months. Pretty soon, some boffin or another will explain away these silica levels. Then, a little further on, something new will crop up and it’ll be back to “OMG Mars had teh w3t!!!” Can’t we just get on with sending some sort of mission that will solve the debate conclusively, please?