Above is a link to an amazing forum, one that I visit frequently. The thread I linked talked about microbe colonies that can survive conditions on mars for millions of years before dying out. Granted conditions on mars have likely been harsh for life for much more than a few million years. It does mean that if there is just the occasional period that the mars climate allows these microbes to multiply and spread, than there could very well be surviving colonies of martian microbes that survive from "green period" to "green period" (yes I know, there is no actual "green" on mars).
This other thread shows that there is a distinct possibility that the very process that the viking landers used to try and detect life, destroyed that life in the process. This would mean that there is the possibility that the viking landers found plenty of life, the testing system was just wrong.
I hope that they follow up with another martian mission, to try and experiment with different techniques for searching the red sands for signs of life.
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