Extraterrestrial Civilizations by Isaac Asimov Page B

be completely unexpected, for as they were received, astronomers saw what were clearly craters—craters that looked very much like those on the Moon.
    The craters, at least as they showed up on the
Mariner 4
pictures, seemed so many and so sharp that the natural conclusion was that there had been very little erosion. That seemed to mean not only thin air, but very little life activity. The craters shown in the photographs of
Mariner 4
seemed to be the mark of a dead world.
    Mariner 4
was designed to pass behind Mars (as viewed from Earth) after its flyby, so that its radio signals would eventually pass through the Martian atmosphere on their way to Earth. From the changes in the signals, astronomers could deduce the density of the Martian atmosphere.
    It turned out that the Martian atmosphere was even thinner than the lowest estimates. It was less than 1/100 as dense as Earth’s atmosphere. The air pressure at the surface of Mars is about equal to that of Earth’s atmosphere at a height of 32 kilometers (19 miles) above the Earth’s surface. This was another blow to the possibility of advanced life on Mars.
    In 1969, two more rocket probes,
Mariner 6
and
Mariner 7
, were sent past Mars. They had better cameras and instruments, and took more photographs. The new and much better photographs showed that there was no mistake about the craters. The Martian surface was riddled with them—as thickly, in places, as the Moon.
    The new probes, however, showed that Mars was not entirely like the Moon. There were regions in the photographs in which the Martian surface seemed flat and featureless and others where the surface seemed jumbled and broken in a way that was not characteristic of either Moon or Earth. There were still no signs of canals.
    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. Marswas 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