Comparative Planetology and Serendipity
Science fiction gave us forests on the back side of the Moon, Martian canals constructed by advanced civilizations, and, in Venus, a throwback to happy early times: paradise. Unfortunately the Moon is lifeless, Mars is a desert, and Venus is hell. As our knowledge of the solar system has advanced, we have moved our imagination beyond its confines. The worlds of strange intelligent creatures and monstrous beasts, of great wisdom or unparalleled horror might well exist – but around some distant star, safe from the rocket probes that might render empty what fiction has filled with the riches of dreams.
A social critic may wish to know why we should then want to explore the inhospitable worlds within our rockets’ reach. Can there be, for example, any link between the exploration of Venus' poisonous atmosphere and the well being of those who breathe our own atmosphere?
There is. There are many in fact. Let me begin with one striking and important example of the serendipity of comparative planetology: the discovery of the threat to the ozone layer.
Ozone forms when oxygen molecules (O2) capture oxygen atoms (O) to combine into larger molecules, ozone (O3). Ozone acts as a nasty pollutant on the surface, particularly in the air of our large cities, but at high altitudes it absorbs ultraviolet radiation and reduces considerably the amount that penetrates the atmosphere. Thus the ozone layer protects plants and animals on the surface from excessive ultraviolet radiation that would damage their DNA and cause widespread cancer. Indeed, life was confined to the oceans for much of the history of our planet, until the level of atmospheric oxygen grew enough to form a substantial ozone layer.
Now to Venus. When NASA scientists found fluorine and chlorine compounds in the atmosphere of Venus, they investigated the chemistry of those molecules and determined the rate constants of their chemical reactions. Those rate constants were later used by Sherwood Roland and Mario Molina to discover that chlorofluorocarbons (CFCs) destroy ozone in the presence of high ultraviolet radiation. That is, they discovered that the Earth’s ozone layer might be in trouble. This discovery came as a shock to many researchers and industrialists, for CFCs had been developed precisely because they were supposed to be inert and thus, since they could not react with anything, they could not harm anything. They seemed just perfect for use in air conditioners, refrigerators, and aerosol deodorant cans.
Unfortunately, high in the atmosphere, ultraviolet radiation breaks up the CFC molecules, and the freed chlorine atoms interact with the ozone, destroying it. This discovery was confirmed by Michael McElroy, whose group had the required tools because, as Carl Sagan pointed out, they were working on the chlorine and fluorine chemistry of the atmosphere of Venus.
The presence of a large hole in the ozone layer over Antarctica was further confirmed by satellite data and later tracked and made vivid and dramatic by satellite pictures. This prompted scientists, industrialists, and governments, acting in concert, to ban CFCs, so as to significantly reduce the threat by the year 2010 (although it will take some forty or fifty years longer for the CFCs already in the atmosphere to dissipate and the ozone layer over Antarctica to recover).
This example is a beautiful illustration of the serendipity of comparative planetology. By investigating the atmosphere of Venus we transform our knowledge of planetary atmospheres; this knowledge makes us aware of a serious problem; and space technology helps us monitor the problem and provides the information needed to achieve a solution. And eventually we put the solution to the problem into effect.
Such is the link we seek between planetary science and the well being of humankind: we need to explore the solar system in order to improve our views about the Earth. And we need to improve those views so that we may deal more wisely with certain social and environmental problems that could become acute in a few decades or outright disasters in the long run.
My aim is to show that the serendipity of exploring the solar system will pay off here on Earth. In support of that conclusion I will advance the following argument. To have a good grasp of global problems and their possibly serious consequences, we need to understand our global environment. But to understand the global environment of the Earth it is important to understand the Earth as a planet. To understand the Earth as a planet, however, it is necessary to study the other members of the solar system. And, of course, to study the solar system well we need to go into space.
. Sagan, C., Pale Blue Dot, Random House, 1994, p. 222. I have adapted this section so far from Sagan’s book.