Extraterrestrial Civilizations - Isaac Asimov [30]
On May 30, 1971, Mariner 9 was launched and sent on its way to Mars. This probe was not merely going to pass by Mars, it was to go into orbit about it. On November 13, 1971, it went into orbit. Mars was at that time in the midst of a planet-wide dust storm and nothing could be seen, but Mariner 9 waited. In December, 1971, the dust storm finally settled down and Mariner 9 got to work taking photographs of Mars. The entire planet was mapped in detail.
The first thing that was settled, once and for all, was that there were no canals on Mars. Lowell was wrong after all. What he had seen was an optical illusion.
Nor were the dark areas either water or vegetation. Mars seemed all desert, but here and there one found dark streaks that usually started from some small crater or other elevation. They seemed to be composed of dust particles blown by the wind and tended to collect where an elevation broke the force of the wind, on the side of the elevation away from the wind.
There were occasional light streaks, too, the difference between the two resting perhaps in the size of the particles. The possibility that the dark and light areas were differences in dust markings and that the dark areas expanded in the spring because of seasonal wind changes had been suggested a few years earlier by the American astronomer Carl Sagan (1935–). Mariner 9 proved him to be completely correct.
Only one of the hemispheres of Mars was cratered and Moonlike; the other was marked by giant volcanoes and giant canyons, and seemed geologically alive.
One feature of the Martian surface roused considerable curiosity. These were markings that wiggled their way across the Martian surface like rivers and that had branches that looked for all the world like tributaries. Then, too, both polar ice caps seemed to exist in layers. At the edge, where they are melting, they looked just like a slanted stack of thin poker chips.
It is possible to suppose that Mars’s history is one of weather cycles. It may now be in a frigid cycle, with most of the water frozen in the ice caps and in the soil. In the past, and possibly again in the future, it may be in a mild cycle, in which the ice caps melt, releasing both water and carbon dioxide, so that the atmosphere becomes thicker and the rivers grow full.
In that case, even if there is no apparent life on Mars now there may have been in the past, and there may again be in the future. As for the present, life forms could be hibernating in the frozen soil, in the form of spores.
In 1975, two probes, Viking 1 and Viking 2, the former launched on August 20, the latter on September 9, were sent to Mars. They were to land on the planet and observe it in various ways. In particular, they were to test the planet for signs of life.
They landed safely in the summer of 1976 in two widely separated places. They analyzed the Martian soil and found it to be not too different from Earth’s, but richer in iron and less rich in aluminum.
Three experiments were conducted that might detect life. All three gave results of the kind that might be expected if there were living cells in the soil.
There was, however, a fourth experiment that cast doubt upon the first three. To understand that, we will have to consider the nature of the molecules most characteristic of living organisms as we know it.
Against the background of water, there is in living organisms a rapid and never ending interplay involving complex molecules made up of anywhere from a dozen to a million atoms. These are found, in nature, only in living organisms and in the dead remnants of once living organisms.* For that reason, such complex molecules are called organic compounds.
Organic compounds have something in common—the element carbon. Carbon atoms have a unique facility for combining with each other in complex chains, both straight and branched, and in rings or collections of rings to which chains of atoms, either straight or branched, can be attached. Also attached on the outskirts of the carbon chains and rings