some sort of chemical disequilibrium, which can be exploited for metabolism. Geological processes are independent of life, and if scientists are able to constrain the geology well enough on another planet, then they know what the particular geologic equilibrium for that planet should be. A deviation from geological equilibrium can be interpreted as both an atmospheric disequilibrium and agnostic biosignature. Antibiosignatures In the same way that detecting a biosignature would be an incredibly important discovery about a planet, finding evidence that life is not present can be an important discovery about a planet as well. Life relies on redox imbalances to metabolize the resources available into energy. The evidence that nothing on a planet is taking advantage of the "free lunch" available due to an observed redox imbalance is called antibiosignatures. Martian atmosphere The Martian atmosphere contains high abundances of photochemically produced CO and H2, which are reducing molecules. Mars' atmosphere is otherwise mostly oxidizing, leading to a source of untapped energy that life could exploit if it used a metabolism compatible with one or both of these reducing molecules. Because these molecules can be observed, scientists use this as evidence for an antibiosignature. Scientists have used this concept as an argument against life on Mars. Missions inside the Solar System Astrobiological exploration is founded upon the premise that biosignatures encountered in space will be recognizable as extraterrestrial life. The usefulness of a biosignature is determined not only by the probability of life creating it but also by the improbability of non-biological (abiotic) processes producing it. Concluding that evidence of an extraterrestrial life form (past or present) has been discovered requires proving that a possible biosignature was produced by the activities or remains of life. As with most scientific discoveries, discovery of a biosignature will require evidence building up until no other explanation exists. Possible examples of a biosignature include complex organic molecules or structures whose formation is virtually unachievable in the absence of life: Cellular and extracellular morphologies Biomolecules in rocks Bio-organic molecular structures Chirality Biogenic minerals Biogenic isotope patterns in minerals and organic compounds Atmospheric gases Photosynthetic pigments The Viking missions to Mars The Viking missions to Mars in the 1970s conducted the first experiments which were explicitly designed to look for biosignatures on another planet. Each of the two Viking landers carried three life-detection experiments which looked for signs of metabolism; however, the results were declared inconclusive.