The Significance of Oxygen
Venus has often been called Earth’s sister planet. The two planets are similar in size and coalesced from similar material within the solar nebula. However, if you were to compare the atmospheres of all the terrestrial planets, you would find that the atmosphere of Venus is more like Mars than Earth. Earth's atmosphere is composed almost entirely of nitrogen (~78%) and oxygen (~21%). In comparison, Venus and Mars have atmospheres that are predominantly composed of carbon dioxide (~95%). Currently, Earth is the only planet known to have substantial amounts of oxygen in its atmosphere. This is a markedly unexpected result. Oxygen is famous for its highly reactive character. If any excess oxygen exists in a planetary atmosphere, it would be expected to react with other materials. For example, oxygen interacts readily with the iron minerals found in rocks. It makes the iron “rust”, resulting in red minerals like those typical of Mars. In addition, it is probable that these types of reactions would happen over a few millions of years, a geologically short time period. So, if you were an alien searching for life in the universe and you identified a planet with excess oxygen in its atmosphere, you might suspect that something unusual might be happening to continuously replenish that oxygen.
And what would that “something unusual” be? On our planet, it is life. Many living creatures on Earth take up molecules like carbon dioxide (CO2) and release oxygen. Photosynthesizing organisms like cyanobacteria and plants are primarily responsible for the abundance of oxygen in the atmosphere. One hypothesis about the history of life on Earth suggests that early Earth had very little oxygen in the atmosphere. It wasn’t until photosynthetic microorganisms evolved that an excess of oxygen could exist. The oxygen that was produced by these organisms was molecular oxygen (O2), which reacted with other O2 molecules in the upper atmosphere to create ozone (O3), the molecule that absorbs ultra violet (UV) light and protects life on Earth. Once there was an ozone layer and sufficient oxygen in the atmosphere, more complex life could evolve.
We’ve already alluded to one reason why oxygen is important to life. When it exists as the molecule we call ozone, oxygen is very useful in protecting life on the surface of Earth. It is thought that life was restricted to the oceans prior to the formation of ozone because large of amounts of water can also block UV light. In addition to serving as a planetary shield, oxygen is important to the chemical processes of many organisms on Earth. When organisms break down organic molecules, such as sugar, to get energy to live the chemical reaction is called respiration. In most complex organisms like ourselves, the process is called aerobic respiration – aerobic meaning in the presence of oxygen. The general reaction for this can be written as:
C6H12O6 + 6O2 ® 6CO2 + 6H2O
Notice that molecular oxygen is on the left hand side of the reaction. Molecules other than O2 can be used instead (such as methane, nitrate, and sulfate). However, out of all the possible molecules, molecular oxygen provides the most free energy. Organisms evolved to take advantage of this extra energy. Consequently, most complex organisms perform aerobic respiration as opposed to anaerobic respiration – anaerobic meaning without oxygen.
Knowing that excess atmospheric oxygen can be a signature of life, and knowing that oxygen can provide an energetic advantage for living organisms, it makes sense that one of the things scientists will look for on other planets is oxygen. We are finally close to the technology that would allow us to detect planetary atmospheres that have been modified by life processes. There are several future space missions, such as Terrestrial Planet Finder, which will focus on examining extrasolar planets for such clues for life.