As we search for life on other planets, particularly exoplanets, we look for signals or indicators that are uniquely characteristic of life. But what makes for a good biosignature? Since oxygen plays such an important role in life as we know it, especially advanced life, some scientists suggest we should simply look for the spectral signature of large amounts of oxygen in the atmospheres of other planets. At first glance this seems to make sense, as Earth is the only planet in our solar system with plenty of oxygen in its atmosphere, and the only one with a thriving biosphere on its surface.
But it may not be that easy. Norio Narita and co-authors recently identified a process that does not involve biology, which can still produce the amounts of oxygen we have in Earth’s atmosphere. The process involves titanium oxide, an oxidized metal, which catalyzes the splitting up of water into oxygen and hydrogen when a planetary surface is exposed to ultraviolet radiation. According to the scientists’ estimates, titanium oxide could make up as little as 0.05 percent of surface materials on an exoplanet to produce oxygen levels comparable to what we see in Earth’s atmosphere.
Louis Irwin and I have proposed a different method of producing huge amounts of oxygen on Europa-type worlds. That ice-covered moon is exposed to a tremendous amount of radiation from Jupiter. If the moon were hit by a large asteroid, the icy crust would melt and liquid oceans would appear on its surface, while its extremely thin atmosphere would fill with water vapor. The radiation would split the water into oxygen and hydrogen; the hydrogen would escape to space, while the oxygen would be retained. Of course, it may turn out that Europa actually contains life, but the process of producing lots of oxygen in the atmosphere could be completely unrelated, and would not in itself tell us whether there is life in Europa’s subsurface ocean.
Also, if we consider the early history of life on our planet—the first few hundred million or perhaps a billion years—life thrived in the absence of oxygen. Even today, oxygen is toxic for many microbial organisms. Does complex multicellular life like us absolutely require an oxygen atmosphere? This is a difficult question to answer, and is the subject of intense scientific debate at this time.
So what to do? Oxygen by itself is not a suitable biosignature. But oxygen together with methane or some other reducing compound that, in the absence of life, would be expected to react with oxygen and remove it over time, would be a powerful indicator. Oxygen on Earth is constantly produced by photosynthesis, and methane is produced by certain type of microbes that live under oxygen-poor conditions. If the reducing compound (methane in Earth’s case) and the oxidizing compound are consistently produced anew, then both can be produced in large enough amounts to be measured. And that would indeed be a strong biosignature on another planet.