Astrobiology was launched as a scientific discipline at the beginning of the twenty-first century. Its stated goal is to find extraterrestrial life. But there’s a problem. How does one go about identifying the signature of life on some distant planet? While a number of such signatures (or indicators) have been proposed, the only one universally agreed to be a reliable indicator is the presence of large amounts of free oxygen in the planet’s atmosphere.
Earth’s atmosphere contains 21 percent oxygen by volume. This enormous amount of oxygen makes possible the existence of large-bodied advanced animals. It’s also responsible for establishing an ozone shield that protects all surface life from deadly ultraviolet radiation emanating from the Sun.
All this oxygen arises from plants and bacteria that engage in photosynthesis. While scientists have identified non-biological pathways for generating free oxygen in Earth’s atmosphere, all of these mechanisms deliver only the tiniest quantities to the atmosphere. Moreover, these limited quantities are highly ephemeral (transient). Without enormous quantities of photosynthetic life-forms existing for billions of years, there can be no large amounts of free oxygen in a planet’s atmosphere. Consequently, astrobiologists are persuaded that if they find the spectral signature for oxygen in a planet’s atmosphere, they will then have proven that abundant life exists on that planet. But recent research by MIT planetary scientist Feng Tian challenges the free oxygen signature for life.1
In an article published in the Astrophysical Journal, Tian illustrates that the existence of abundant free oxygen in a planet’s atmosphere doesn’t necessarily indicate the presence of life. He points out that planets the size of Earth and somewhat larger will begin, as Earth did, with thick atmospheres dominated by carbon dioxide. But continuous bombardment of a planet’s atmosphere by soft X-ray and hard ultraviolet radiation causes carbon dioxide to be dissociated into carbon and oxygen, which allows carbon to escape thermally. Tian notes that stars less than half the mass of the Sun radiate copious amounts of soft X-ray and hard ultraviolet radiation throughout their nuclear burning phase, which lasts more than ten billion years. (The Sun and stars with similar masses do so for only 0.75 billion years of their nuclear burning phase.)
Oxygen does not escape thermally from the atmosphere as easily as carbon does. It has a heavier atomic weight, plus, the advantage of easily forming a double oxygen molecule. Therefore, a lifeless planet bombarded by its star’s X-ray and ultraviolet radiation could, without the benefit of any biological activity, end up with significant amounts of free oxygen in its atmosphere.
Tian and his colleagues calculated that more than four billion years ago when the Sun was radiating lots of soft X-rays and hard ultraviolet light, the Martian atmosphere contained about 2 percent oxygen by volume due to solar radiation impinging its atmosphere.2 Likewise, planets orbiting low mass stars at distances where surface liquid water would be possible will contain a few percent oxygen by volume in their atmospheres for many billions of years, even if no life at all exists on them.
These research results mean that no longer can the detection of free oxygen in the atmosphere of a planet be considered an unambiguous signature for the existence of life. Hopefully, Tian’s achievement will become disseminated widely enough to temper premature announcements of the discovery of extraterrestrial life and, thus, the implication of evolutionism’s triumph.
- Feng Tian, “Thermal Escape from Super Earth Atmospheres in the Habitable Zones of M Stars,” Astrophysical Journal 703 (September 20, 2009): 905-9.
- Feng Tian, James F. Kasting, and Stanley C. Solomon, “Thermal Escape of Carbon from the Early Martian Atmosphere,” Geophysical Research Letters 26 (January 2009): citeID L02205.