Extraterrestrial Civilizations - Isaac Asimov [22]
Mars’s, on the other hand, is at once less hopeful and more hopeful.
It is less hopeful because it is distinctly smaller than Earth. Its diameter is only 6,790 kilometers (4,220 miles) and its mass is only 0.107 that of the Earth. With a mass only 1/10 that of Earth it is not exactly a large world, but on the other hand it is 8.6 times as massive as the Moon, so it is not exactly a small one, either. It is, in fact, twice as massive as Mercury.
Mars’s surface gravity is 2.27 times that of the Moon and is just about that of Mercury. Mars, however, is four times as far from the Sun as Mercury is, so that Mars is considerably the cooler of the two. Mars’s gravitational field need deal with considerably slower molecules for that reason.
It follows that although Mercury is without an atmosphere, Mars may have one—and it does. Mars’s atmosphere is a thin one, to be sure, but it is distinctly there. Mars is presumably drier than the Earth, for its atmosphere is not as cloudy as Earth’s (let alone Venus’s), but occasional clouds are seen. Dust storms are also seen, so there must be sharp winds on Mars.
The more hopeful aspect of Mars is that its atmosphere is sufficiently thin and cloud free to allow its surface to be seen (rather vaguely) from Earth. For centuries, astronomers have done their best to map what it was they saw on that distant world. (At its closest, Mars can approach as closely as 56,000,000 kilometers [34,800,000 miles] to Earth, a distance that is 146 times as far away from us as the Moon.)
The first to make out a marking that others could see as well was the Dutch astronomer Christiaan Huygens (1629–1695). In 1659, he followed the markings he could see as they moved around the planet and determined the rotation period of Mars to be only a trifle longer than that of Earth. We now know Mars rotates in 24.66 hours compared to Earth’s 24.
In 1781, the German-English astronomer William Herschel (1738–1822)* noted that Mars’s axis of rotation was tilted to the perpendicular, as Earth’s was, and almost by the same amount. Mars’s axial tilt is 25.17° as compared with Earth’s 23.45°.
This means that not only does Mars have a day-night alteration much as Earth has, but also seasons. Of course, Mars is half again as far from the Sun as we are, so that its seasons are colder than ours. Furthermore, it takes Mars longer to complete its orbit about the Sun, 687 days to our 365¼, so that the seasons on Mars are each nearly twice as long as ours.
In 1784, Herschel noted that there were ice caps about the Martian poles, as there were about Earth’s poles. There was one more point of resemblance in that the ice caps were assumed to be frozen water, and therefore to serve as proof there was water on Mars.
Mars and Venus both looked like hopeful possible abodes of life, certainly far more hopeful than the asteroids or the Moon or Mercury.
VENUS
In 1796, the French astronomer Pierre Simon de Laplace (1749–1827) speculated on the origin of the Solar system.
The Sun rotates on its axis in a counterclockwise direction when viewed from a point far above its north pole. From that same point, all the planets known to Laplace moved about the Sun in a counterclockwise direction, and all the planets whose rotations were known rotated about their axes in a counterclockwise direction. Added to that was the fact that all the satellites known to Laplace revolved about their planets in a counterclockwise direction.
Finally, all the planets had orbits that were nearly in the plane of the Sun’s equator and all the satellites had orbits that were nearly in the plane of their planet’s equator.
To account for all this, Laplace suggested that the Solar system was originally a vast cloud of dust and gas called a nebula (from the Latin word for cloud). The nebula was turning slowly in a counterclockwise direction. Its own gravitational field slowly contracted it, and as it contracted it had to spin faster and faster in accordance with