15.4 The Role of Catastrophes

The condensation theory accounts for the nine "characteristic" points listed at the start of this chapter. Specifically, the planets' orbits are circular (2), in the same plane (3), and in the same direction as the Sun's rotation on its axis (4) as a direct consequence of the nebula's shape and rotation. The rotation of the planets (5) and the orbits of the moon systems (6) are due to the tendency of the smaller-scale eddies to inherit the nebula's overall sense of rotation. The growth of planetesimals throughout the nebula, with each protoplanet ultimately sweeping up the material near it, accounts for point (1), which is the fact that the planets are widely spaced (even if the theory does not explain the regularity of the spacing). The heating of the nebula and the Sun's ignition resulted in the observed differentiation (7), and the debris from the accretion—fragmentation stage naturally accounts for the asteroids (8) and comets (9).

We stressed earlier that an important aspect of any solar system theory is its ability to allow for the possibility of imperfections—deviations from the otherwise well ordered scheme of things. In the condensation theory that capacity is provided by the randomness inherent in the encounters that ultimately combined the planetesimals into protoplanets. As the numbers of large bodies decreased and their masses increased, individual collisions acquired greater and greater importance. The effects of these collisions can still be seen today in many parts of the solar system—for example, the large craters on many of the moons we have studied thus far.

Having started with nine regular points to explain, we end with eight irregular solar system features that still fall within the theory's scope. It is impossible to test any of these assertions directly, but it is reasonable to suppose that some (or even all) of the following "odd" aspects of the solar system can be explained in terms of collisions late in the formative stages of the protoplanetary system. Not all astronomers believe all these explanations; however, most would accept at least some.

  1. Mercury's exceptionally large nickel—iron core may be the result of a collision between two partially differentiated protoplanets. The cores may have merged, and much of the mantle material may have been lost. (Sec. 8.7)
  2. Two large bodies could have merged to form Venus, giving it its abnormally low rotation rate. (Sec. 9.2)
  3. The Earth—Moon system may have formed from a collision between the proto-Earth and a Mars-sized object. (Sec. 8.8)
  4. A late collision with a large planetesimal may have caused Mars's curious north—south asymmetry and ejected much of the planet's atmosphere. (Sec. 10.4)
  5. The tilted rotation axis of Uranus may have been caused by a grazing collision with a sufficiently large planetesimal, or by a merger of two smaller planets. (Sec. 13.3)
  6. Uranus's moon Miranda may have been almost destroyed by a planetesimal collision, accounting for its bizarre surface terrain. (Sec. 13.6)
  7. Interactions between the jovian protoplanets and one or more planetesimals may account for the irregular moons of those planets and, in particular, Triton's retrograde motion. (Sec. 13.6)
  8. Pluto may simply be a large representative of the Kuiper belt, and the Pluto—Charon system may be the result of a collision or near-miss between two icy planetesimals before most were ejected by interactions with the jovian planets. (Sec. 13.9)