| 4. The Supernova/Comet
Theory vs. the Superwave Theory:
Similarities, Differences, and Inherent Problems with the Former
The Firestone-West theory and the superwave theory are similar in that both involve the influx of cometary material into the solar system. The principal difference between them is that the superwave theory proposes that this cometary material enters mostly as nebular material (cometary dust and gas) accompanied by larger chunks of frozen cometary debris ranging on up to comet-sized bodies, whereas the Firestone-West theory focuses mainly on the entry of comet bodies of relatively large size (1 - 500 kilometer diameter range).
Chameleon aspect. In their initial 2005 Lawrence Berkeley Laboratory press release they proposed that a 10 kilometer diameter comet either exploded in the atmosphere or struck the North American ice sheet 12,900 years ago. In their book published in 2006, the size of the incoming comet was increased to 480 kilometers, and it was proposed to have been accompanied by four other comets ranging in size from 105 to 290 kilometers all being proposed to have ground impacts. The following year, however, the team was quoting far smaller comet sizes ranging from 2 to 3 km diameter (Mckie, 2007) to 5 km in diameter (Burns, 2007). Although previous publications by Firestone et al. (2001, 2006) associated the YD comet impact event with a supernova, their paper in the National Academy of Sciences proceedings (Firestone, et al., 2007c) makes no mention of a supernova connection. So there has been a kind of "chameleon" aspect to the progression of their theory.
Extended versus discrete duration. The energetics triggering the YD boundary firestorm in the superwave theory come primarily from the scorching effects of a ground contacting CME, the Sun becoming overly active due to the high concentrations of nebular material invading the solar system. In the Firestone-West theory, on the other hand, the energy would come mainly from the kinetic energy of the impacting comet fragments. The preexisting superwave theory predicts a temporally extended effect transpiring over several thousand years with a climax at the AL/YD boundary, whereas the Firestone-West scenario involves discrete impact events with debris presumably settling to the ground within a few years. To explain meltwater peaks and radiocarbon peaks occurring at earlier climatic boundaries, e.g., around 17,000 and 18,000 years BP, they propose an earlier wave of cometary onslaughts. Thus although comet impacts are inherently a temporally discrete phenomenon, to make their theory account for these earlier geologic events, they propose earlier bombardments associated with the passage of a more leading part of the supernova shell and presumably traveling at a higher speed, around 2500 -3200 km/s. Thus for effects to be spread out over this length of time, their proposed supernova remnant shell is inferred to be about 50 light years thick.
Age differences. Another similarity between the two theories is that in both cases the cometary material is derived from a local supernova explosion. However there are substantial differences. In the superwave theory, cometary bodies in orbit about the Sun are vaporized by the superwave cosmic rays and this vaporized debris subsequently enters the solar system to initiate a long sequence of hazardous events. Most of this cometary material is proposed to have originated from the North Polar Spur (NPS) remnant. This is a very old local supernova remnant whose shell currently immerses our solar system and whose geometrical center lies about 400 ± 200 light years from us in the constellation of Lupus. Estimates place its age at roughly 2 million years. It is expanding very slowly, moving at roughly 3 km/s, and this remnant's material has been gravitationally captured by the Sun due to the Sun's proper motion through its debris field.
In the Firestone-West theory, on the other hand, the cometary material is proposed to come from a supernova that allegedly exploded at a distance of about 200 to 250 light years from us in the direction of the Gemini constellation, the Geminga neutron star being identified as the progenitor star's remnant core (Firestone, West, Warwick-Smith, 2006). They, however, propose a much younger age for their supernova, claiming that it occurred just 41,000 years ago, or 28,000 years prior to the YDB extinction event.
Differences in entry speed. A related difference between the theories concerns the speed that each proposes for the entry of the cometary material. The superwave theory proposes that the nebular material (cometary dust and gas) enter at a relatively low velocity, at several kilometers per second for material vaporized from comets already captured in orbits about the Sun and at about 20 km/s for material vaporized from long-period comets newly entering the solar system. The former would enter from the direction of the Galactic center while the latter would approach from a direction close to the solar apex, i.e., a direction opposite to direction of the Sun's motion through the local interstellar environment.
The Firestone-West theory, on the other hand, proposes that the cometary bodies responsible for the YD boundary catastrophe had entered at a very high velocity, thousands of kilometers per second, due to the remnant's outward expansion. For the iron rich grains which they say entered the solar system and impacted the Earth around 34 kyrs b2k, they estimate a speed of about 10,000 km/s; 200 to 250 light years divided by 7,000 years (LBNL press release, 2005). For the comet bearing remnant shell passing around 13 kyrs b2k, they imply a speed of around 2100 to 2700 km/s; 200 to 250 light years divided by 28,000 years.
Problem 1: Supernova direction does not align with the radiant for long-period comets. As we shall see below, there are several reasons why such high speeds are problematic for the Firestone-West theory. But the direction of entry also poses a problem. They propose that this material was entering the solar system from the direction of Geminga, a neutron star that currently lies in the constellation Gemini and which they claim to be the core remnant of their proposed 41 kyr b2k supernova. However, if a barrage of comets had passed through the solar system from that direction, one would expect that some of these would have been gravitationally captured into highly eccentric orbits aligned in that direction. Instead, long-period comets, which comprise most of the comets entering the solar system, have a radiant that deviates by about 50° from the Geminga direction. The Geminid meteor shower does not stand as very good evidence of a past disturbance from this direction since this shower, which originates from the Kuiper belt, makes up a tiny fraction of the cometary debris that presently enters the solar system. Moreover its proximity to this direction may be regarded as mainly a chance association.
Problem 2: Insufficient material to form a cometary barrage. Another problem with the Firestone-West model is that the supernova shell would provide an insufficient amount of material to form a cometary barrage. Let us say that the supernova progenitor was a 10 solar mass star and that given enough time that 70% of its mass would be able to condense into cometary masses. This amounts to a dispersal of about 1034 grams of comet forming material. Spread this out over a 250 light-year diameter shell, and you come up with only 20 nanograms per square centimeter! Figuring how much material would intersect an area the size of the Earth's orbit around the Sun and you calculate an amount sufficient to condense into a two kilometer diameter comet.
But, Firestone, West, and Warwick-Smith (2006) imply that this shell of supernova cometary debris was passing through our solar system for approximately 6000 years since, as mentioned above, they attempt to explain geologic anomalies as recent as 12,950 years b2k and as early as 18,000 to 19,000 years b2k. So to get some reasonable coverage for these earlier events we should spread this mass out over this period of time. As a result, we find that there is a chance that a one kilometer diameter comet might have intersected the Earth's orbit every 500 years. Factor into that the probability that the Earth might have a single direct hit and you are down to one chance in a hundred million per millennium. Certainly, these probabilities are far too low to explain a single event, let alone a whole series of events. They certainly would not appreciably raise the probability of Earth being hit by a comet over probabilities that already exists from comets currently entering our solar system. So it seems that their theory that Earth was struck by a barrage of comets originating from a passing supernova shell does not have much merit.
They might try to modify their theory by saying that the solar system must have been struck by a clump of supernova debris, e.g., a 100 million-to-1 enhancement over the amounts estimated above on the basis of the uniform distribution assumption. However, this would drastically reduce the likelihood that a supernova explosion this close to the solar system would have affected us. Supernovae within 300 light years of the Sun are estimated to occur about once every million years. The chance that such a supernova would occur and have a clump cross our solar system now drops to once every hundred billion years, or essentially not likely in the Earth's 4.5 billion year history.
Claiming that the supernova remnant swept up dust and gas present in the interstellar medium and thereby added to its total mass would not help much either since there isn't much gas to sweep up. The Sun resides within a very low density 300 light year radius cavity called the "Local Bubble" which contains a gas density of only 10-3 particles per cm3, or about 10-27 g/cm3. Over a 250 light year distance this would present a column density of about 200 nanograms per square centimeter. So if the supernova shell were to sweep up 100% of this material and accelerate it to its speed of 103 104 km/s, it would only increase the remnant's total column density by 10 fold. The downside is that a ten fold increase in remnant mass would decelerate the remnant to a speed ten fold lower. As a result, it would take some 200,000 years to reach the solar system and hence should still be somewhere out in space coming towards us. The idea of killing off the Pleistocene megafauna would certainly be out of the question.