Not my analogy, theirs:

http://news.bbc.co.uk/2/hi/science/nature/5072668.stm

Seems to me this is a misguided strategy. Throw one probe at one spot on Mars and see if there currently living organisms a couple of feet under this one rock, whose chemistry so matches Earth bugs that they too use DNA and ATP. Life as we know it, Jim.

Even when the answer is no, I'm not sure we'll have been able to rule out any of the plausible scenarios for current Martian life, let alone know anything about the possibilities for past life. Given how hostile conditions are near the surface, you'd think the _last_ thing we'd find there are microbes that duplicate Earth biochemistry in detail.