A debate has raged over whether bombardment has occurred randomly or periodically. Originally, astronomers were inclined to the idea that asteroids and comets were stray bodies, taken out of the Asteroid Belt and Oort Cloud by chance events. Some astronomers dispute this stray bolide hypothesis or 'stochastic catastrophism' (Steel et al. 1994, 473), favouring instead a coordinated or coherent catastrophism.
Stochastic catastrophism, although demanding random strikes, does not preclude the possibility of bombardment episodes, which might also occur randomly. There are theoretical grounds, and some empirical evidence, for conjecturing that bombardment tends to occur as episodic showers ('storms' is a more apt description), roughly every 30 million years, each shower lasting a few million years (Clube and Napier 1982; Hut et al. 1987; Bailey et al. 1990, 412-15; Rampino 2002). Admittedly, the empirical evidence is questionable (e.g. Grieve and Shoemaker 1994). However, linear time-series analysis of craters of various sizes revealed spectral peaks at 30 million years for craters with 5-km diameters or less, 35 million years for 35-km diameters or less, and 36 million years for 90-km diameters or less (Rampino and Stothers 1998; see also Rampino 2002).
Several mechanisms might explain episodic storms of space debris. The Nemesis hypothesis argues that the Sun might have a companion star on a highly eccentric orbit that perturbs the Oort Cloud at perihelion passage (Davis et al. 1984; Whitmire and Jackson 1984). The solar companion was named Nemesis after the Greek goddess who relentlessly persecutes the excessively rich. It was also dubbed the 'Death Star', presumably after the Empire's space station in Star Wars (Weissman 1984). The Planet X hypothesis proposes that an undiscovered 10th planet orbits in the region beyond Pluto, and produces comet showers near the Earth with a very stable frequency (Whitmire and Matese 1985; Matese and Whitmire 1986). Another hypothesis focuses on the up-and-down motion of the Solar System about the Galactic plane. The period of oscillation about the Galactic plane is roughly 67 million years, with estimates of the period varying between 52-74 million years (see Innanen et al. 1978; Bahcall and Bahcall 1985). Because of this bobbing motion, the Solar System passes through the Galactic plane, where interplanetary matter tends to be denser, every 33 million years, and reaches its maximum distance (about 80-100 parsecs) from the Galactic plane every 33 million, too. Now, there is an approximate correspondence between Galactic plane crossings by the Solar System and the boundaries between the geological periods (Innanen et al. 1978). The two phenomena could be related causally if the vertical motion of the Solar System about the Galactic plane were to cause comet showers (Rampino and Stothers 1984a, 1984b; Rampino 2002). This is a reasonable scenario because most medium-sized molecular clouds are concentrated near the Galactic plane. Thus, the vertical oscillation of the Sun through the Galactic plane, which has a half-period of 33 million years, would modulate the rate at which the Sun would encounter stars and molecular clouds. The modulation may involve the perturbation of the Oort Cloud and inner cometary reservoir leading to comet showers lasting several million years. It is also possible that the Solar System periodically passes through dense nebulae as it orbits the Milky Way, which would again perturb comets in the Oort Cloud (Clube 1978; Clube and Napier 1982, 1984; Napier and Clube 1979).
Other astronomers propose what might be termed a 'harmonized catastrophism' to supersede stochastic catastrophism. Two main schools advocate this new view. One school, named coherent catastrophism by its creators (Steel 1991, 1995; Steel et al. 1994), contends that large comets disintegrate to produce clusters of fragments, ranging in size from microns, metres, tens and hundreds of metres, to kilometres. Such clusters will form a train of debris with a characteristic orbit. If the node of the orbit (the point at which it crosses the ecliptic) is near 1 AU, and if the cluster passes its node when the Earth is near, then it repeatedly crosses the Earth's orbit. The outcome is cluster-object impacts at certain times of the year, every few years, depending on the relationship between the Earth's and the cluster's orbital periods. However, an impact occurs only when precession has brought the node to 1 AU, so only on time-scales of every few thousand years. One cluster - the Taurid complex - is presently active, and has been for the last 20,000 years. It has produced episodes of atmospheric detonation, which the proponents of coherent catastrophism believe that these may have had material consequences for the biosphere and for civilization. However, a caveat seems appropriate here, for not all astronomers accept this brand of cosmic catastrophism. They demur chiefly because Duncan Steel's coherent cata-strophism includes unusual views about the nature of comets and meteorites, and erroneously suggests that coherent clusters of streams of cometary debris somehow dominate the terrestrial impact rate, which does not square with the average rates deduced by Shoemaker and others (e.g. Chapman 1996).
The second, and equally controversial, school advocating non-random bombardment might be called coordinated catastrophism. It sees the Earth, Sun, and Solar System as coupled non-linear systems (Shaw 1994). This idea seems immensely powerful. It leads to a new picture of Earth history that outlaws happenstance and instates chaotic dynamics as its centrepiece; a picture that shows a grand coordinated theme played out over aeons, and that portrays gradual and catastrophic change in the living and non-living worlds as different expressions of the same non-linear processes. A vital ingredient of this new view is that comets, asteroids, and meteoroids fly around the Solar System in critically self-organized, as opposed to random, regimes. Evidence suggesting that cratering on planets and satellites has distinct patterns in space and in time supports this view. On Earth, this pattern is accounted for by the early stage of heavy bombardment (more than 4 billion years ago), during which colossal impacts, such as the one responsible for creating the Moon, led to an uneven distribution of terrestrial mass in the form of a meridional 'keel' of high density rock. This 'keel' has subsequently influenced the incoming flight paths of space debris. The result is that impacts have tended to centre around three geographical nodes (cratering nodes) - one in north-central North America, one in north-eastern Europe, and one in Australia - at least during the Phanerozoic aeon.
In conclusion, the bombardment of the Earth by space debris is not a rare event, but is a widespread and, geologically speaking, common happening. It is a fundamental cosmic and geological process, the understanding of which is leading to new insights into the history of the Earth. As well as producing craters, impacts may cause climatic change, mass extinctions (p. 115), and superfloods (p. 86). More contentiously, they may also trigger geophysical processes such as magnetic reversals, continental drift, and volcanism. The bombardment hypothesis has overturned the comforting worldview of Isaac Newton, wherein the planets endlessly revolve about the Sun, and the Solar System operates, in an orderly fashion, smoothly and gracefully in the manner of a clockwork machine. It offers instead a violent Cosmos and Solar System, in which catastrophic collisions between cosmic bodies are commonplace. This change of worldview is profound and somewhat disquieting, for the:
new results [of astronomy] promise to revolutionise our perceptions, not only of the earth sciences and biological evolution, but also the early history of mankind and its immediate future. The circumterrestrial environment is hazardous, the ordered universe of Newton and his successors an illusion. There would seem to be little that can be done to avert an eventual catastrophe: at present there may be about a million Tunguska-sized missiles orbiting in the inner Solar System, none of which have been charted; and the in situ destruction of a swarm of missiles, or the prevention of a stratospheric dusting, seems to be well beyond the capacity of twentieth-century technology.
(Clube and Napier 1986b, 246)
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