Far beyond the orbit of Neptune, nearly halfway to the nearest stars, our solar system is surrounded by a vast spherical reservoir of comets known as the Oort cloud. In the classical view, first proposed by the Dutch astronomer Jan Oort in 1950, these comets remain in the distant reservoir until a passing star perturbs the cloud, diverting some of the comets toward the inner solar system. Once close to the Sun, the comets may careen through the solar system for thousands of years until they are ejected into interstellar space or until they collide with another body such as a planet.
This scenario has largely been accepted for several decades now as the most likely explanation for the orbital habits of certain comets--except that a passing nearby star is no longer seen as the primary perturber of the Oort cloud. Some recent developments suggest other explanations. The resulting debate has implications not just for the celestial mechanics of comets, but for theories about the mass extinctions of species that shape life on earth.
For those unfamiliar with the theories, a brief history lesson should provide some background for the present debate. As the story goes, every 26 million years or so the fossil record seems to record the extinction of an abnormally great number of species. The cycle can be traced back through the past 250 million years and just happens to include the extinction event at the end of the Cretaceous period, 65 million years ago, that marked the end of the dinosaurs. This finding alone is remarkable, but it became even more so when several groups of scientists independently proposed that the cause of the extinctions was nothing less than a periodic bombardment of projectiles from space.
The intellectual climate was ripe for the idea: Only a few years earlier the discovery of a deposit of iridium in the rocks of the Cretaceous-Tertiary boundary suggested to some scientists that an extraterrestrial impactor had wiped out the dinosaurs. Especially intriguing was the finding that the record of terrestrial impacts seemed to have a 28- to 32-million-year cycle. The apparent convergence of the extinctions and the impacts on a near 30-million-year cycle kindled a cosmic question: What mechanism could drive a cycle of extinctions and impacts with such an enormously long period?
Among the more intriguing responses was a theory that linked the extinction cycle to another well-known cycle: the periodic oscillation of the solar system, back and forth across the plane of the Milky Way galaxy, about once every 30 million to 35 million years. The extinctions appeared to be occurring just when the solar system was crossing the densest part of the galactic disk. According to the proponents of the theory, the Oort cloud was being greatly perturbed by something in the galactic midplane, and this caused a catastrophic rain of comets on the inner solar system (including the earth) every 30 million years.
Some thought that the perturbing something was the gravitational effect of giant molecular clouds situated in the midplane of the galactic disk. Others disagreed with this notion, arguing that the effects of the giant molecular clouds should be about as strong in the midplane as they are above and below the plane at the galactic latitudes traversed by the solar system.
At just about the time this issue was being debated, several Oort-cloud experts proposed that the cumulative effects of the local matter in the plane perpendicular to the galactic disk--the so-called disk tides--were far more significant than the intermittent gravitational effects of passing stars or giant molecular clouds. This threw a small wrench into the oscillating-solar-system theory because it wasn't clear just how the disk tides would modulate the flux of comets at different heights above or below the galactic midplane. Some scientists were unperturbed by the absence of a precise understanding, however, taking it on faith that the strength of the disk tides would be sufficient to give the Oort cloud a good kick every 30 million years.
And so the matter stood until 1995, when John Matese and Patrick Whitman of the University of Southwestern Louisiana and their colleagues Mauri Valtonen of Finland and Kimmo Innanen of Canada attempted to assess the quantitative effects of the disk tides. Their numerical models of Oort-cloud dynamics suggested that as the solar system oscillates through the galactic plane, the disk tides modulate the comet flux from the Oort cloud by a ratio of about 4 to 1, with the greatest effect in the midplane of the galaxy (Icarus 1995, 116:255). The results brought new life to the theory by providing a mechanism for the 30-million-year galactic clock.
Gravitational tides of the Milky Way may pull comets free of the Oort cloud as the solar system oscillates in the galactic plane.
It was enough to convince some scientists that there might be something to the theory after all. Notable among these is Gene Shoemaker of the U.S. Geological Survey, who at one time believed that the periodicity was a "statistical fluke." The work of Matese and his colleagues convinced him that the "impact surges are real... and that [the comet flux is] controlled by the fluctuating galactic tidal forces." The Matese study, he said, "is a landmark contribution in understanding the history of bombardment of the earth."
Recently, Matese and his colleague Daniel Whitmire have taken their studies of the Oort-perturbing effects of the galaxy a step further (The Astrophysical Journal Letters, November 20, 1996). Their analysis of a selected group of comet orbits indicates that the entire galaxy, including the distant matter at its central core, plays a role in jostling some comets free of the cloud. Unlike the disk tides, these distant-matter tides exert their effects within the plane of the galactic disk. Whereas the disk tides might account for about two-thirds of all Oort-cloud comets that we observe, the distant-matter tides may be responsible for nearly another one-third. (Perturbing effects of nearby stars and giant molecular clouds account for a small remainder.) The distant-matter tides turn out to be significantly more important than anyone would have imagined, including the authors. "We didn't expect to detect an effect from distant galactic matter at all," said Matese. "It was a completely serendipitous discovery."
Not everyone agrees that Matese and Whitmire have made an adequate case for the existence of distant-matter tides. Paul Weissman of the Jet Propulsion Laboratory in Pasadena, California points out that "the effects of the [distant-matter] tides only show up when a small subset of the comet data are used." Matese responds that the subset was selected so that only "high-quality classes of comets with well-determined orbits" would be included in the analysis.
As for the role of the distant-matter tides in the 30-million-year cycle of Oort-cloud perturbations, Matese admits that it is too soon to say. "We don't know whether it will increase or decrease the 4-to-1 modulation effect of the disk tides, but it probably won't be a large change."
Either way, Weissman doesn't think it will make a difference for the oscillating-solar-system theory of cosmic impacts and periodic extinctions. "If you consider that [Oort-cloud] comets only account for 25 percent of the impacts on the earth at present, and the fact that the comet flux is currently at the maximum point of the 4-to-1 modulation, then the [disk tide] is only modulating the frequency of one-quarter of all impacts." He adds, "Going through the galactic plane is not like going over a speed bump. There is a broad distribution of matter, and the solar system oscillates slowly through it. The modulation effect is gradual; it shouldn't produce a dramatic spike in the comet flux." Weissman believes that asteroids, which account for most of the other 75 percent of the impact craters on the earth, play a greater role in the impact extinctions.
Matese counters that although Oort-cloud comets may only account for 25 percent of the terrestrial impacts, they are disproportionately the larger impact craters: "Impactors that make craters greater than 100 kilometers in diameter are the ones that play the key role in the extinction events." He notes that comet impacts appear to be potentially responsible for the four largest terrestrial-impact craters known: Chesapeake Bay on the East Coast of the United States and Popagai in Siberia (both dated at 35 million years), Chicxulub on the Yucatán peninsula (65 million years old and suspected of being produced by the impactor that may have killed the dinosaurs) and Manicouagan in Quebec (dated at 210 million years). In turn, Weissman points out that "it is very difficult to determine what type of object caused a particular crater because the impactor is vaporized in the impact. Others have claimed that Chicxulub was caused by an asteroid, not a comet."
The supporting or confuting evidence for these ideas should be in the rocks and perhaps in the orbital dynamics of distant comets. For the time being it appears that the Oort comet cloud and the galaxy will continue to be entangled with the dinosaur extinctions and extraterrestrial impactors.