IMO Levin has it nailed...we speculated about it here on FR when the rovers started moving and the tracks were seen in the pictures...there is a kind of brine mix just below the Martian surface. The rover's wheels press it into a kind of mud. In fact it was seen at the landing site. The soil looked as if it splattered like cake batter.
FWIW I think Levin's experiment on Viking did indeed find micro-organinic life. We'll have to wait and see if he gets the credit.
prisoner6