Broca's Brain - Carl Sagan [88]
The ultimate objective of comparative planetology is, I suppose, something like a vast computer program into which we put a few input parameters—perhaps the initial mass, composition, angular momentum and population of neighboring impacting objects—and out comes the time evolution of the planet. We are very far from having such a deep understanding of planetary evolution at the present time, but we are much closer than would have been thought possible only a few decades ago.
Every new set of discoveries raises a host of questions which we were never before wise enough even to ask. I will mention just a few of them. It is now becoming possible to compare the compositions of asteroids with the compositions of meteorites on Earth (see Chapter 15). Asteroids seem to divide neatly into silicate-rich and organic-matter-rich objects. One immediate consequence appears to be that the asteroid Ceres is apparently undifferentiated, while the less massive asteroid Vesta is differentiated. But our present understanding is that planetary differentiation occurs above a certain critical mass. Could Vesta be the remnant of a much larger parent body now gone from the solar system? The initial radar glimpse of the craters of Venus shows them to be extremely shallow. Yet there is no liquid water to erode the Venus surface, and the lower atmosphere of Venus seems to be so slow-moving that dust may not be able to fill the craters. Could the source of the filling of the craters of Venus be a slow molasseslike collapse of a very slightly molten surface?
The most popular theory on the generation of planetary magnetic fields invokes rotation-driven convection currents in a conducting planetary core. Mercury, which rotates once every fifty-nine days, was expected in this scheme to have no detectable magnetic field. Yet such a field is manifestly there, and a serious reappraisal of theories of planetary magnetism is in order. Only Saturn and Uranus have rings. Why? There is on Mars an exquisite array of longitudinal sand dunes nestling against the interior ramparts of a large eroded crater. There is in the Great Sand Dunes National Monument near Alamosa, Colorado, a very similar set of sand dunes nestling in the curve of the Sangre de Cristo mountains. The Martian and the terrestrial sand dunes have the same total extent, the same dune-to-dune spacing and the same dune heights. Yet the Martian atmospheric pressure is 1/200 that on Earth, the winds necessary to initiate the saltation of sand grains are ten times that for Earth, and the particle-size distribution may be different on the two planets. How, then, can the dune fields produced by windblown sand be so similar? What are the sources of the decameter radio emission on Jupiter, each less than 100 kilometers across, fixed on the Jovian surface, which intermittently radiate to space?
Mariner 9 observations imply that the winds on Mars at least occasionally exceed half the local speed of sound. Are the winds ever much larger? What is the nature of a transonic meteorology? There are pyramids on Mars about 3 kilometers across at the base and 1 kilometer high. They are unlikely to have been constructed by Martian pharaohs. The rate of sandblasting by wind-transported grains on Mars is at least 10,000 times that on Earth because of the greater speeds necessary to move particles in the thinner Martian atmosphere. Could the facets of the Martian pyramids have been eroded by millions of years of such sandblasting from more than one prevailing wind direction?
The moons in the outer solar system are almost certainly not replicas of our own, rather dull satellite.