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Cheyava Falls\u2019 fascinating two-for-one geology is what scientists call a potential\u00a0biosignature<\/i>. Biosignatures are things that are produced by living creatures. A fossil is a biosignature. The carbon dioxide we exhale is a biosignature. The screen you\u2019re reading this article on is a biosignature.\u00a0<\/p>\n
Some biosignatures are more compelling than others. Carbon dioxide may be a waste gas here on Earth, but there\u2019s also quite a lot of it on notoriously hostile\u00a0Venus. On the other hand, if Perseverance rolled over a Nokia phone one day, that would be considered fairly compelling evidence for aliens on the cusp of discovering the\u00a0Find My Phone<\/i> feature.<\/p>\n
But when you can\u2019t dropship a rover to the planet you want to study, the potential for compelling biosignatures begins to dry up. Suddenly, a detection of carbon dioxide becomes extremely appealing. But carbon dioxide doesn\u2019t mean life. And methane doesn\u2019t mean life. Even things like water and oxygen don\u2019t mean life.\u00a0<\/p>\n
Here\u2019s why: all these gasses have what we call\u00a0abiotic<\/i>, or non-living sources. Carbon dioxide is all over the place. So is water, found on every planet in our Solar System (though usually not in liquid form). Methane is a waste gas of Earth creatures, but it\u2019s also put out by volcanoes. Oxygen is produced via photosynthesis, but also by a process called\u00a0photolysis in which solar radiation breaks apart molecules, including splitting water into hydrogen and oxygen. If a planet once had an ocean, and that ocean evaporated, the world left behind might resemble Dune\u2019s Arrakis: breathable, but without a drop to drink. In fact, there are quite a few scenarios, illustrated below, in which oxygen can appear without a biological origin.<\/strong><\/p>\n This may seem like a dead end, but it\u2019s not. An astrobiologist must consider the whole picture, not just one gas at a time. From this big picture, a solution emerges: biosignature\u00a0pairs<\/i>. Some gasses simply do not coexist in nature unless something is actively producing both. On our planet, we have a bit of methane, which comes from both natural processes and human endeavors like agriculture. But oxygen and carbon dioxide don\u2019t like to hang out with methane. When they come in contact, they combine into other gasses, like water, carbon dioxide, and organic molecules. So if you see methane and oxygen together, or methane and carbon dioxide together, that is a far stronger indicator of potential life than either gas alone.<\/p>\n<\/div>\n