Broca's Brain - Carl Sagan [87]
Despite the expectation of almost all planetary scientists, Mars turns out to be covered with thousands of sinuous tributaried channels probably several billion years old. Whether formed by running water or running CO2, many such channels probably could not be carved under present atmospheric conditions; they require much higher pressures and probably higher polar temperatures. Thus the channels—as well as the polar laminated terrain on Mars—may bear witness to at least one, and perhaps many, previous epochs of much more clement conditions, implying major climatic variations during the history of the planet. We do not know if such variations are internally or externally caused. If internally, it will be of interest to see whether the Earth might, through the activities of man, experience a Martian degree of climatic excursions—something much greater than the Earth seems to have experienced at least recently. If the Martian climatic variations are externally produced—for example, by variations in solar luminosity—then a correlation of Martian and terrestrial paleoclimatology would appear extremely promising.
Mariner 9 arrived at Mars in the midst of a great global dust storm, and the Mariner 9 data permit an observational test of whether such storms heat or cool a planetary surface. Any theory with pretensions to predicting the climatic consequences of increased aerosols in the Earth’s atmosphere had better be able to provide the correct answer for the global dust storm observed by Mariner 9. Drawing upon our Mariner 9 experience, James Pollack of NASA Ames Research Center, Brian Toon of Cornell and I have calculated the effects of single and multiple volcanic explosions on the Earth’s climate and have been able to reproduce, within experimental error, the observed climatic effects after major explosions on our planet. The perspective of planetary astronomy, which permits us to view a planet as a whole, seems to be very good training for studies of the Earth. As another example of this feedback from planetary studies on terrestrial observations, one of the major groups studying the effect on the Earth’s ozonosphere of the use of halocarbon propellants from aerosol cans is headed by M. B. McElroy at Harvard University—a group that cut its teeth for this problem on the aeronomy of the atmosphere of Venus.
We now know from space-vehicle observations something of the surface density of impact craters of different sizes for Mercury, the Moon, Mars and its satellites; radar studies are beginning to provide such information for Venus, and although it is heavily eroded by running water and tectonic activity, we have some information about craters on the surface of the Earth. If the population of objects producing such impacts were the same for all these planets, it might then be possible to establish both an absolute and a relative chronology of cratered surfaces. But we do not yet know whether the populations of impacting objects are common—all derived from the asteroid belt, for example—or local; for example, the sweeping up of rings of debris involved in the final stages of planetary accretion.
The heavily cratered lunar highlands speak to us of an early epoch in the history of the solar system when cratering was much more common than it is today; the present population of interplanetary debris fails by a large factor to account for the abundance of the highland craters. On the other hand, the lunar maria have a much lower crater abundance, which can be explained by the present population of interplanetary debris, largely asteroids and possibly dead comets. It is possible to determine, for planetary surfaces that are not so heavily cratered, something of the absolute age, a great deal about the relative age,