A lot of brain power has been devoted to establishing Biosignatures, as a means for remotely detecting alien life.
But, what if alien cows don't fart? Can we still conceive of robust generic biosignatures that are not just slavish restatements of what we think we know about the Earth?
One of my hats is as a co-PI of the Penn State Astrobiology Research Center
We used to run, in loose association, a National Science Foundation REU (Research Experience for Undergrads) program - bring in 10-12 undergrads from other unis for the summer and give them some lab experience.
As part of that, we had friday afternoon bull sessions, loose informal sessions, with an informal paper of topic for discussion assigned shotgun to a student or faculty. Those were fun.
One of the topics that came up every summer, and at most all astrobio grad seminar, is "what if there is life, but it is not photosynthetic carbon based life? Would we recognise the signature? Could we predict any signatures?"
This is a hard question: for Earth-like carbon base, photosynthesising life on a low mass planet around a main sequence star, we think we have robust biosignatures - we want to see water vapour, oxygen (or ozone as a more easily detected proxy), carbon dioxide and methane.
That is probably sufficient to indicate likely life, but almost certainly not necessary.
Why is this hard?
We currently are limited to remote sensing, which pretty much boils down to optical and near-IR spectroscopy, so we're looking at upper atmosphere molecules.
Well, first thing to look for are atmospheres out of chemical equilibrium; but that in itself is not sufficient as a biosignature, since there is significant flow of free energy from the star, and photolytic chemistry can produce chemical disequilibrium, including molecular oxygen.
Now, "we" think we understand photolytic gas chemistry, and bottom line is that if you see oxygen (or ozone proxy) AND methane, in the presence of water (and we expect some carbon dioxide as well), then we have carbon based photosynthetic life... or some funky chemistry we didn't predict.
BUT, for a large fraction of the Earth's history, there was no significant photosynthesis, and life utilized anaerobic metabolism, but it was still life.
So, what is that signature: we're talking methanogens, free-hydrogen metabolising critters, others living on sulphur compounds or metal ions. We don't even know what the Earth's atmosphere was like at different stages in the archaean, much less what the range of possible atmospheres was which was consistent with pre-aerobic life.
Next complication is very different surface conditions - we can imagine either much thicker atmospheres, or very different surface water (typically we think there may be a lot of "waterworlds" with little or no land area and deep (> 10 km) oceans) - those may have life, but with different atmospheric signatures.
All of this is still carbon based - the Big Question is can we figure out anything for non-carbon based life, assuming such is possible?
What could be possible - is there other chemisty that can spontaneously lead to self-organised complex structures which reproduce, randomise and undergo selection? What about machine life (post-organic life forms)?
Oh, one early suggestion was that freons in the upper atmosphere would be a sign of life with some intelligence and industrialization - possibly only very stupid life though (or not, depends).
In three years of BS sessions, not a single robust idea has emerged on alternative biosignatures, and I do not recall seeing any decent suggestions in the professional literature.
Anyone have any ideas?
Finally: what about anti-biosignatures? Is there a spectroscopic signature where you can just say "it is dead, Jim" and move on?
My only thought to date is that high CO2 levels combined with high SO2 levels are sure signs of death, maybe.
How far out of chemical equilibrium could you get without life? It doesn't seem to happen too much in our solar system. Not only, as Lovelock pointed out, does life have to use its environment to obtain resources and dump wastes, living things tremendously increase the surface area available for various interesting reactions. Maybe each planet should be compared to others in the same solar system.
Incidentally, oxygenic photosynthesis evolved very early on, but most of the oxygen went into oxidizing iron, producing widespread banded iron formations.
How can a planet with no land area have liquid water for a geologically meaningful amount of time? Without a carbonate/silicate weathering feedback, wouldn't it either freeze or boil? As for SO2, the dead sulfate planets in our system have their sulphate as either solid surface minerals (Mars), or H2SO4 (Venus, maybe Europa?).