Extraterrestrial Civilizations - Isaac Asimov [21]
CHAPTER 3
The Inner Solar System
NEARBY WORLDS
Once Galileo began to study the sky with his telescope, he could see that the various planets expanded into tiny orbs. They appeared as mere dots of light to the unaided eye merely because of their great distance.
What’s more, Venus, being closer to the Sun than Earth is, showed phases like the Moon, as it should under such conditions if it were a dark body shining only by reflection. That was proof enough that the planets were also worlds, possibly more or less Earthlike.
Once that was established, it was taken for granted that all of them were life bearing and inhabited by intelligent creatures. Flammarion maintained this confidently, as I said in the previous chapter, as late as 1862.
The kinetic theory of gases, however, ruled out not merely the Moon as an abode of life, but any world smaller than itself. Any worlds smaller than the Moon could scarcely be expected to possess air or water. They would lack the gravitational field for it. Consider the asteroids, the first of which was discovered in 1801. They circle the Sun just outside the orbit of Mars and the largest of them is but 1,000 kilometers (620 miles) in diameter. There are anywhere from 40,000 to 100,000 of them with diameters of at least a kilometer or 2, and every last one of them lacks air or liquid water* and are therefore without life.
The same is true for the two tiny satellites of Mars, discovered in 1877. They are in all likelihood captured asteroids, and have neither air nor liquid water.
Within the orbits of the asteroids lies the “inner Solar system” and there we find four planetary bodies larger than the Moon. In addition to the Earth itself, we have Mercury, Venus, and Mars.
Of these, Mercury is the smallest, but it is 4.4 times as massive as the Moon and its diameter is 4,860 kilometers (3,020 miles), which is 1.4 times that of the Moon. Mercury’s surface gravity is 2.3 times that of the Moon and nearly 2/5 that of the Earth. Might it not manage to retain a thin atmosphere?
Not so. Mercury is also the closest of the planets to the Sun. At its nearest approach to the Sun it is at only 3/10 the distance from it that the Earth is. Any air it might have would be heated to far higher temperatures than the Earth’s atmosphere. Gas molecules on Mercury would be correspondingly speedier in their motion and harder to hold onto. Mercury, therefore, would be expected to be as airless and waterless—and as lifeless—as the Moon.
In 1974 and 1975, a rocket probe, Mariner 10, passed near Mercury’s surface on three occasions. On the third occasion, it passed within 327 kilometers (203 miles) of the surface. Mercury was mapped in detail and its surface was found to be cratered in a very Moonlike way, and its airlessness and waterlessness is confirmed. There is no perceptible doubt as to its lifelessness.
Venus looks far more hopeful. Venus’s diameter is 12,100 kilometers (7,520 miles) as compared with Earth’s 12,740 kilometers (7,920 miles). Venus’s mass is about 0.815 times that of the Earth and its surface gravity is 0.90 times that of the Earth.
Even allowing for the fact that Venus is closer to the Sun than Earth and would therefore be hotter than Earth, it would seem that Venus should have an atmosphere. Its gravitational field is strong enough for that.
And, indeed, Venus does have an atmosphere, a very pronounced one, and one that is far cloudier than ours. Venus is wrapped in a planet-girdling perpetual cloud cover, which was at once taken as adequate evidence that there was water on Venus.
The cloud cover does, unfortunately, detract from the hopeful views we can have of Venus, since it prevents us from gathering evidence as to its fitness for life. At no time could astronomers ever catch a glimpse of its surface, however good their telescopes. They could not tell how rapidly Venus might rotate on its axis, how tipped that axis might be, how extensive its oceans (if any) might be, or anything else about it. Without more evidence than the mere existence