All organisms must consume something, react it with something else and excrete the products. They live off the flow of energy.

If a planet has a source of energetic chemistry that would naturally react with something else and be quenched, life can exist by standing between these extremes and mediating the energy transfer.

These energy flows may be either secular, or circular. By this, I mean that either the planet as a whole is slowly changing in its state (e.g. oxidation) or, both reactants are being regenerated by other processes.

Consider a redox reaction. If, say, hydrogen is being continuously vented from and around volcanoes, then life can survive by mediating the reaction with CO2 to produce reduced carbon (partially reduced carbon = sugar/cellulose), plus water. Over time, the CO2 gets used up. Eventually it's all gone and that particular form of life dies out - or evolves to use different reactants.

Alternatively, the reaction products may be cycled away and regenerated - the reduced carbon re-oxidised somewhere, and the water split back to hydrogen and an oxidant, which can theoretically be channeled back to oxidise the reduced carbon.

(You can pose the above with a nominal oxidant instead, although you need to react with something different to live off the energy flux).

Now, suppose we observe such a planet. This is Lovelock's greatest contribution to astrobiology (or second greatest, if you happen to be a Gaia fan). You would observe partial reduction products of the CO2+H2 reaction, such as CH4 and CO. These would be relatively free to escape from the subsurface in normal circumstances.

Alternatively, you might observe sites of regeneration of the reagents - something splitting water to H2 + O2 (on Earth, this is now done by photosynthesis, with substantial release of free oxygen). Or something reconverting reduced Carbon to oxidised Carbon (and if this was connected to the atmosphere, it would perhaps show as CO).

We don't see any of this on Mars, so we know for an incontrovertible fact that right now there is negligible biological activity in any location on Mars that is in contact with the atmosphere.

OK, so that leads to part 2. Can we sequester all this reactivity in the subsurface, so that there is no sign at the surface?

Perhaps - it requires some special conditions, such as a thick impermeable layer under the ground (e.g. permafrost) that has essentially always existed. It says that life on Mars does not and will not come to the surface unless we dig deep, and it will die rapidly if exposed and deprived of its energy flux.

And, it requires active energy cycles under the surface. This requires hot areas and cold areas and hence active volcanism. (or a parasitic end-game of a hydrothermal system shrinking deeper and deeper into the planet, feeding a trace of life as it goes deeper and deeper, leaving behind an e.g. oxidised crust above the zone of life.)

We don't see signs of *ongoing, active* volcanism - Mars DOES have a long history of volcanism continuing to recent times and probably will have volcanic episodes in the future, but these seem to occupy around 1% of geologic time, with long rests in between - planetwide. Bursts of activity for 1 Ma are followed by stasis for 99 Ma!

So, if life is living off this flux, and wholly sealed underground, it is likely dormant right now (and has a powerful ability to wait between meals).

Or, Mars is totally dead.

This implies that:

1) It's biologically safe to colonise the surface of Mars
2) Only by deep drilling will we have any hope of finding extant life on Mars
3) A long search is likely before we might find signs of earlier life on Mars