Spectrum of a Life Bearing Planet

Earth's atmosphere would actually be quite "unnatural" for a nonbiotic planet. It is clearly different from the nearly pure carbon dioxide atmospheres of its neighbors, Mars and Venus. The mix of nitrogen, oxygen, and water vapor is chemically unstable and would never arise on a dead planet. Without life, nitrogen and oxygen in the presence of water would combine to form nitric acid and become a dilute acidic component of the ocean. Earth's peculiar atmosphere is not in chemical equilibrium, and it succeeds in disobeying natural chemical laws only because of the presence of life. The most peculiar aspect of the atmosphere is the abundance of free oxygen. Oxygen is the most abundant element in the whole Earth (45% by weight and 85% by volume!), but in the atmosphere, it is a highly reactive gas that would exist only at trace levels in the atmosphere of a terrestrial planet devoid of life. Oxygen is a poisonous gas that oxidizes organic and inorganic materials on a planetary surface; it is quite lethal to organisms that have not evolved protection against it. The source of atmospheric oxygen is photosynthesis, the miraculous biological process that utilizes the energy of sunlight to convert carbon dioxide to pure oxygen and organic material. Ironically, it was the long-term photo-synthetic production of this poisonous gas, and life's adaptation to it, that made complex and energetic life possible on Earth. Except for the noble gas argon, all of the major atmospheric constituents are also processed and recycled on short time scales via biological processes.

Our distant alien astronomers would realize that life exists on Earth as soon as they detected, in its infrared spectra, absorption bands due to the presence of carbon dioxide, ozone, and water vapor (see Figure 11.1). Nitrogen and normal oxygen (O2) are the major atmospheric gases, but they do not produce detectable absorption effects. The telltale bands arise because of the way Earth interacts with sunlight. Earth's surface is warmed by visible sunlight, and it reradiates in the infrared. The incoming energy is in the visible region of the spectrum (near 0.5 micrometer in wavelength) where the sun, with its surface temperature of 5400°C, emits most of its energy. The atmosphere is largely transparent to visible wavelengths, and the light that is not reflected into space is largely absorbed by Earth's surface. This energy heats the surface to "room temperature"—only about 5% of the absolute temperature of the surface of the sun. Earth's surface cools itself to balance exactly (averaged over time) the absorption of sunlight by radiating infrared radiation back into space. Because of Earth's relatively cooler temperature, the bulk of this energy is in the "thermal infrared" spectral region near 10 micrometers in wavelength. Atmospheric transmission of parts of this spectral region are blocked because of absorption by certain gases. Water vapor, ozone, and carbon dioxide absorb part of the outgoing infrared radiation and block its escape from Earth. This process and these same gaseous species are the root cause of the atmospheric greenhouse effect that prevents Earth's oceans from freezing. All of these "greenhouse" molecules are minor constituents, but they cause warming effects of some 40°C above the temperature of an atmosphere that is totally transparent in the infrared. They also provide a very strong spectral signature to be seen by alien astronomers. Water makes up a few percent of the atmosphere, CO2 is currently only 375 parts per million, and ozone occurs only in the parts-per-billion range. Although rare, they absorb significant chunks of the infrared radiation streaming into space. The outgoing infrared has significant absorption dips at wavelengths of 7, 10, and 15 micrometers, respectively, for water, ozone, and carbon dioxide.

Acrylonitrile Ftir

Wavelength (micrometers)

Figure 11.1 The hypothetical infrared spectrum of an Earth-like planet with life. The abundances of water vapor, carbon dioxide, and ozone would be clues indicating that the planet was in the habitable zone of its star and that life was producing oxygen.

Wavelength (micrometers)

Figure 11.1 The hypothetical infrared spectrum of an Earth-like planet with life. The abundances of water vapor, carbon dioxide, and ozone would be clues indicating that the planet was in the habitable zone of its star and that life was producing oxygen.

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