[CraigS]

So, the model for the type of life they're looking for, is based on Earth's Archaea domain. These have no cell nucleus, or any other membrane-bound organs within their cells. They possess genes, and roughly similar metabolic processes to the more familiar Eukaryotes, (which posses complex structures, enclosed within cell membranes).

Archaea make use of a variety of energy sources like sugars, ammonia, metal ions, hydrogen gas or sunlight. Commonly they use CO2 to oxidise hydrogen. They don't photosynthesise. They reproduce asexually (no meiosis) and, (so far), none are known to produce spores.

So far, they've found them in lots of harsh environments, including the human gut. Their classification is based on ribosomal RNA sequencing. They contain only a single chromosome, and the smallest is estimated to have only 537 protein-encoding genes. They may contain smaller independent pieces of DNA. Probable cell fossil records date back to almost 3.5 billion years ago.

So, this is the model of life they've designed Curiosity to look for.
If archaea-like lifeforms are found, it would demonstrate that abiogenesis, (life originating from inorganic chemicals), followed Evolutionary/Natural Selection processes might happen in another habitable zone planet, other than Earth.

Would its discovery rule out Panspermia (via say meteor impacts) ?
This would seem to depend on the sub-type of archaea discovered, and how long it could possibly live in space, during transit.

But what if they don't find any bio-signs ? (This being basically, a higher percentage of biogenic C-12 bearing methane, over and above, non-biogenic C-13 bearing methane).

Does this then lead towards an evidence-based working assumption, that water-bearing planets, having life-supporting environments in the Habitable Zone, do not necessarily inevitably result in life ?

.. Now that would be a big conclusion !

Cheers