Signs of life on other bodies
A planet may have the right conditions for life, but it is another thing to prove that life has developed to take advantage of those conditions. One way astrobiologists look to confirm the presence of life elsewhere is by peering into the atmosphere, where living organisms are likely to leave their mark.
Take Earth’s atmosphere as an example. The air we breathe contains 21 per cent oxygen, but it shouldn’t. Molecules of oxygen above our heads react with sunlight and break apart. They then react with other molecules to make new compounds.
The fact that Earth’s oxygen levels don’t drop is because something is constantly replacing the spent oxygen: life. Photosynthetic bacteria – known as cyanobacteria – produce 50 per cent of the oxygen we breathe. Plants release most of the rest. If we found similar levels of oxygen in the atmosphere of another planet it would strongly suggest the presence of some form of oxygen-producing biology.
Methane is another possible biosignature gas, particularly when it’s present with another gas like oxygen. It is thought to be unstable in atmospheres for any length of time, so if it persists, it is being replenished somehow. In December 2014, NASA’s Curiosity rover became the latest mission to detect methane on Mars. The source of the Martian methane remains a mystery. Although its origins are more likely to be geological, a small chance remains that it has a biological source.
- FQXI podcast on ‘alien hunters’
- NASA: Simulated atmospheres
- Scientific American: Methane emissions on Mars
Questions for discussion
- Why is methane more likely to be a biosignature gas when it’s present with oxygen?
- Research how methane is produced biologically and compare and contrast with how it is produced geologically.
- PDF of Biosignatures [PDF 319KB]