Pale Blue Dot - Carl Sagan [79]
There’s also a Mars connection. With Viking we found the surface of Mars to be apparently lifeless and remarkably deficient even in simple organic molecules. But simple organic molecules ought to be there, because of the impact of organic-rich meteorites from the nearby asteroid belt. This deficiency is widely attributed to the lack of ozone on Mars. The Viking microbiology experiments found that organic matter carried from Earth to Mars and sprinkled on Martian surface dust is quickly oxidized and destroyed. The materials in the dust that do the destruction are molecules something like hydrogen peroxide—which we use as an antiseptic because it kills microbes by oxidizing them. Ultraviolet light from the Sun strikes the surface of Mars unimpeded by an ozone layer; if any organic matter were there, it would be quickly destroyed by the ultraviolet light itself and its oxidation products. Thus part of the reason the topmost layers of Martian soil are antiseptic is that Mars has an ozone hole of planetary dimensions—by itself a useful cautionary tale for us, who are busily thinning and puncturing our ozone layer.
(2) Global warming is predicted to follow from the increasing greenhouse effect caused largely by carbon dioxide generated in the burning of fossil fuels—but also from the buildup of other infrared-absorbing gases (oxides of nitrogen, methane, those same CFCs, and other molecules).
Suppose that we have a three-dimensional general circulation computer model of the Earth’s climate. Its programmers claim it’s able to predict what the Earth will be like if there’s more of one atmospheric constituent or less of another. The model does very well at “predicting” the present climate. But there is a nagging worry: The model has been “tuned” so it will come out right—that is, certain adjustable parameters are chosen, not from first principles of physics, but to get the right answer. This is not exactly cheating, but if we apply the same computer model to rather different climatic regimes—deep global warming, for instance—the tuning might then be inappropriate. The model might be valid for today’s climate, but not extrapolatable to others.
One way to test this program is to apply it to the very different climates of other planets. Can it predict the structure of the atmosphere on Mars and the climate there? The weather? What about Venus? If it were to fail these test cases, we would be right in mistrusting it when it makes predictions for our own planet. In fact, climate models now in use do very well in predicting from first principles of physics the climates on Venus and Mars.
On Earth, huge upwellings of molten lava are known and attributed to superplumes convecting up from the deep mantle and generating vast plateaus of frozen basalt. A spectacular example occurred about a hundred million years ago, and added perhaps ten times the present carbon dioxide content to the atmosphere, inducing substantial global warming. These plumes, it is thought, occur episodically throughout Earth’s history. Similar mantle upwelling seem to have occurred on Mars and Venus. There are sound practical reasons for us to want to understand how a major change to the Earth’s surface and climate could suddenly arrive unannounced from hundreds of kilometers beneath our feet.
Some of the most important recent work on global