Pale Blue Dot - Carl Sagan [29]
With all that oxygen you’re not surprised to discover ozone (O3) in the atmosphere, because ultraviolet light makes ozone out of molecular oxygen (O2). The ozone then absorbs dangerous ultraviolet radiation. So if the oxygen is due to life, there’s a curious sense in which the life is protecting itself. But this life might be mere photosynthetic plants. A high level of intelligence is not implied.
When you examine the continents more closely, you find there are, crudely speaking, two kinds of regions. One shows the spectrum of ordinary rocks and minerals as found on many worlds. The other reveals something unusual: a material, covering vast areas, that strongly absorbs red light. (The Sun, of course, shines in light of all colors, with a peak in the yellow.) This pigment might be just the agent needed if ordinary visible light is being used to break water apart and account for the oxygen in the air. It’s another hint, this time a little stronger, of life—not a bug here and there, but a planetary surface overflowing with life. The pigment is in fact chlorophyll: It absorbs blue light as well as red, and is responsible for the fact that plants are green. What you’re seeing is a densely vegetated planet.
So the Earth is revealed to possess three properties unique at least in this solar system—oceans, oxygen, life. It’s hard not to think they’re related, the oceans being the sites of origin, and the oxygen the product, of abundant life.
When you look carefully at the infrared spectrum of the Earth, you discover the minor constituents of the air. In addition to water vapor, there’s carbon dioxide (CO2), methane (CH4), and other gases that absorb the heat that the Earth tries to radiate away to space at night. These gases warm the planet. Without them, the Earth would everywhere be below the freezing point of water. You’ve discovered this world’s greenhouse effect.
Methane and oxygen together in the same atmosphere is peculiar. The laws of chemistry are very clear: In an excess of O2, CH4 should be entirely converted into H2O and CO2. The process is so efficient that not a single molecule in all the Earth’s atmosphere should be methane. Instead, you find that one out of every million molecules is methane, an immense discrepancy. What could it mean?
The only possible explanation is that methane is being injected into the Earth’s atmosphere so quickly that its chemical reaction with O2 can’t keep pace. Where does all this methane come from? Maybe it seeps out of the deep interior of the Earth—but quantitatively this doesn’t seem to work, and Mars and Venus don’t have anything like this much methane. The only alternatives are biological, a conclusion that makes no assumptions about the chemistry of life, or what it looks like, but follows merely from how unstable methane is in an oxygen atmosphere. In fact, the methane arises from such sources as bacteria in bogs, the cultivation of rice, the burning of vegetation, natural gas from oil wells, and bovine flatulence. In an oxygen atmosphere, methane is a sign of life.
That the intimate intestinal activities of cows should be detectable from interplanetary space is a little disconcerting, especially when so much of what we hold dear is not. But an alien scientist flying by the Earth would, at this point, be unable to deduce bogs, rice, fire, oil, or cows. Just life.
All the signs of life that we’ve discussed so far are due to comparatively simple forms (the methane in the rumens of cows is generated by bacteria that homestead there). Had your spacecraft flown by the Earth a hundred million years ago, in the age of the dinosaurs when there were no humans and no technology, you would still have seen oxygen and ozone, the chlorophyll pigment, and far too much methane. At present, though, your instruments are finding signs not just of life, but of high technology—something that couldn’t possibly have been detected even a hundred years ago:
You are detecting a particular kind of radio wave emanating