Your News Feature 'Secrets of the martian soil' (Nature 448, 742–744; 2007) addressed the implications of the next Mars mission finding organic molecules, or not, on the surface of Mars. But the question of whether there are organics on the planet's surface, although important, is not necessarily relevant to whether there is, or was, microbial life on Mars.
With an absolute maximum reach of a metre, the Phoenix mission may do better than Viking (which in 1976 dug down about 6 centimetres). But even if the top metre is sterile, that doesn't indicate whether the planet as a whole is bereft of life.
If life had developed during the planet's first billion years, it and any accompanying organics would have had to survive billions of years of meteorite bombardments, exposure to high levels of ultraviolet and cosmic radiation, and perhaps a highly oxidizing environment. Even if life evolved to withstand such extreme conditions and managed to survive near the surface in some regions, Phoenix is surveying only one point on a planet with a dry-land area equal to that of Earth's.
Only in the past couple of decades has the extent to which life on Earth has colonized, evolved and thrived in a variety of deep ecosystems become evident. For example, recently a thermophilic anaerobic member of the genus Bacillus was obtained from a depth of about 2,700 metres below the surface in Virginia. If Earth is any guide, the question of life on Mars will remain very open until we have at least explored all the areas on Mars equivalent to those on Earth where life has been found.
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New Space (2017)
BIO Web of Conferences (2014)
Structural adaptation of the subunit interface of oligomeric thermophilic and hyperthermophilic enzymes
Computational Biology and Chemistry (2009)