The Tail Wagging the

Before moving on to consider how the atmosphere interacts with the Earth's surface, we should make a few additional comments about several of the remarkable numbers displayed in Table 5.2. On the low-abundance side, the last two entries in the table for hydrogen and helium are not a particular surprise, since we have already seen that these light elements are easily lost from the atmosphere (see Figure 5.11).

The ozone (O3) abundance is even smaller than those of hydrogen and helium, and yet its presence is absolutely vital to the Earth's surface life. Figure 5.12 (lower panel) shows that it is the tenuous stratospheric ozone layer that straddles Earth at altitudes between 15 and 50 km, which absorbs the potentially lethal-to-life solar uV radiation. The ozone layer is dynamic in the sense that ozone is continuously created and destroyed. Ozone is produced through a three-component recombination reaction in which O + O2 + M ) O3 + M, where M is an atmospheric molecule that takes away the excess energy liberated during the reaction. in contrast, ozone can be destroyed by a whole host of processes. Photodissociation will destroy ozone through the reaction O3 + E(photon) ) O2 + O, where E(photon) corresponds to the energy carried by a short-wavelength photon (i.e., those corresponding to UV radiation). It is also destroyed by reactions with, for example, free hydrogen, nitrogen oxide (NO2), chlorine (Cl), and bromine (Br).

One particularly tenacious group of ozone-destroying agents are the chlorofluorocarbon (CFC) compounds. Invented by American engineer and chemist Thomas Midgley in 1928, CFCs were initially hailed as a significant and versatile industrial product— which, it must be said, they are. Their legacy, however, has been a near-environmental disaster. Not produced in nature, all of the ozone-destroying CFC compounds that permeate the Earth's atmosphere have been placed there by human industrial activity since the 1930s. Incredibly, in less than 60 years after their invention, the global effects of CFC emissions were measurable.

The first indications that something had gone badly awry was the discovery of a hole in the ozone layer over Antarctica in 1985; a corresponding ozone hole over the Arctic was soon thereafter

Figure 5.12. The top panel shows the energy flux of the Sun and Earth as a function of wavelength. Since the Sun has an effective temperature of 5,780 K, most of its energy is radiated in visual wavelengths. The Earth, in contrast, has an effective temperature of about 300 K, and it radiates most of its energy into space at infrared wavelengths. The middle panel shows the effect of the Earth's atmosphere. Although visible light can pass unhindered through the atmosphere, short-wavelength ultraviolet and long-wavelength infrared radiation are either completely or selectively absorbed. The lower panel shows the major atmospheric-absorbing components. Water vapor is the strongest absorber at infrared wavelengths, followed by carbon dioxide and methane. Oxygen and ozone (O3) are the main absorbing components at ultraviolet wavelengths. Figure prepared by Robert A. Rohde for the Global Warming Art Project (www.globalwarmingart.com).

Figure 5.12. The top panel shows the energy flux of the Sun and Earth as a function of wavelength. Since the Sun has an effective temperature of 5,780 K, most of its energy is radiated in visual wavelengths. The Earth, in contrast, has an effective temperature of about 300 K, and it radiates most of its energy into space at infrared wavelengths. The middle panel shows the effect of the Earth's atmosphere. Although visible light can pass unhindered through the atmosphere, short-wavelength ultraviolet and long-wavelength infrared radiation are either completely or selectively absorbed. The lower panel shows the major atmospheric-absorbing components. Water vapor is the strongest absorber at infrared wavelengths, followed by carbon dioxide and methane. Oxygen and ozone (O3) are the main absorbing components at ultraviolet wavelengths. Figure prepared by Robert A. Rohde for the Global Warming Art Project (www.globalwarmingart.com).

discovered as well. All of a sudden, the free UV protection provided by the ozone layer was in doubt. In a rare coming together of nations, the Montreal Protocol was established in 1987 to curb CFC production and emissions, and it has been (well, mostly)

successfully held to. Detailed numerical model calculations, however, suggest that the Antarctic ozone hole won't fully recover until 2050 or even later.

In the case of the Earth's ozone layer, it is clear that a little goes a long way, and the lesson for the would-be terraformer is that it is not always necessary to build elaborate protective structures when the careful regulation of relatively minor atmospheric gas components can do the same job in a much more efficient way. The other lesson, of course, is that atmospheric chemistry is far from simple and that important atmospheric components can be altered, for good or ill, on timescales of just a few decades. This lesson is also double edged in the situation of CFCs, since they are also highly efficient greenhouse gases and may well have an important atmospheric warming role to play when terraforming Mars.

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