The best known crater in the world is Meteor Crater in northern Arizona. Twelve hundred meters wide, 170 meters deep, and 5 kilometers in circumference, Meteor Crater is the surviving scar from an iron meteorite thumping into the plains thereabouts around 50,000 years ago in a cataclysmic explosion that liberated energy equivalent to 20 million tons (Mt) of TNT. To put that number into perspective, consider that the explosion that formed Meteor crater was equivalent to almost 2,000 times the power of the bombs dropped on Hiroshima and Nagasaki.
The outrageous suggestion that I am going to make is that the Taurid Complex was producing phenomenal meteor storms between 4,500 and 5,000 years ago, accompanied by multiple Tunguska-class atmospheric detonations, and that Stonehenge I was designed to allow the (awestruck, terrified) culture of southern England to make observations of the phenomena and to perhaps predict their recurrence. Peter Lancaster Brown, in his book on megalithic sites, wrote that "Eclipses, comets and meteorites are astronomical phenomena widely observed by the ancients. But probably only eclipses were predictable. . ." In other words, he assumed, along with many others, that Stonehenge could only have been built for eclipse prediction, comet returns not being foreseeable until Edmond Halley came along, meteorite falls also being inherenlty unpredictable. I am going to differ from that opinion here, and in passing note that when scientists write "probably or "it is likely," in fact, they mean "there is a vague possibility"--of fault of which I am undoubtedly guilty in this book.
We have see in Chapter 7 that Comet Encke is associated with the Taurid Complex. If we follow the orbit of P/Encke backward, we find that it intersected that of the Earth around 1,800 and 5,000 years ago (at its ascending node) and 2,100 and 4,700 years ago (at its descending node). This is useful because it tells us that objects with orbits like that take around 7,000 years to complete a full precession cycle, and during that time there are four epochs in which close approaches to the Earth occur, that is, parent objects can produce four meteor showers. However, we should remember that P/Encke suddenly brightened a couple of centuries ago, not having been seen previously (even though it would have been expected to have been a naked-eye comet, extrapolating its current dimming backward and returning every three years), so it seems that at least for some centuries it was of asteroidal brightness. With this in mind, we should not necessarily think of it as being the sole parent of the complex. Other large objects have also contributed, and the times at which the overall products intersect the Earth will be different. Also, P/Encke is not positioned centrally in the trail of debris detected by the Infra-Red Astronomy Satellite in 1983.
It would be useful if we could carry out precise dynamical evolution studies of the Taurid meteoroid stream by using individual meteor orbits, but because meteors are only observed in the atmosphere for a second or so, the orbits that we determine for them are always imprecise. This is especially true for the radar-detected daytime showers (about which more is stated later), but, in any case, the meteors that we have been able to observe in the present epoch are on the periphery of the stream and so do not represent the orbit of the core. Back in the early 1950s, however, Fred Whipple, in collaboration with Salah El-Din Hamid of the Helwan Observatory in Egypt, carried out a detailed study of the evolution of the nighttime meteors for which Harvard University astronomers had determined good orbits during the previous two decades. They found that there was evidence of an exceptional event, which they saw as a break-up of a proto-Encke's comet around 4,700 years ago. Due to the necessary approximations applied in Whipple and Hamid's modeling, that age should be considered to be good to within perhaps plus or minus 500 years. They also found evidence of another break-up about 1,400 years ago, except that they found the orbit of the parent object to be different from that of P/Encke. They suggested that this was a fragment of the original; that is, they modeled the origin of the Taurids a a hierarchical disintegration of a large comet, proceeding over may thousands of years. Similarly, work by myself, with David Asher and Victor Clube and by Poulat Babadzhanov and Yuri Obrubov, has shown that about 5,000 years is required for the characteristics of the four main meteor showers observed now to have been attained. Note that this does not correspond to the time when P/Encke had a node near 1 AU, but that is not the problem; the point is that it does indicate that sort of cometary break-up occurred 5,000 years ago, which is precisely when Stonehenge I was being built, and the complex of material released into the stream would soon have precessed so as to intersect the Earth.
In the intervening five millennia, we might expect that three distinct periods of exceptional meteor activity would have occurred; one about 5,000 years ago (because by chance the complex formed when it had a node near 1 AU), once in the first half of the present millennium (and there are good reasons to expect it to be split into two periods, as in the eleventh and fifteenth centuries), and once between. These episodes would not necessarily be equally spaced. Clube and Napier have discussed extensively the evidence for various medieval records describing catastrophes associated with the great meteor showers of the eleventh and fifteenth centuries, and there is similar mythical and legendary evidence of Armageddon-type events over the past few millennia, in particular in the last few centuries B.C. and the fifth and 6th centuries A.D. Note that the analysis of Whipple and Hamid indicates a break-up that corresponds with the latter (that is, around 1,400 years ago). Clube and Napier have argued that the Dark Ages commencing around then were caused by the conflagrations resulting from interesections between the core and the Earth.
As discussed, there are four main meteor showers associated with the Taurid Complex, two nighttime showers presently occurring in October-November and two daytime showers in May-July. We have also seen that it could take up to 7,000 years for the orbit to completely precess around and produce the four distinct showers in fact. due to the initial orientations, 5,000 years has been just sufficient. One possibility therefore would be that the parent comet arrived in its orbit about 5,000 years ago and immediately broke up, producing the complex except with later disintegrations (like that at 1,400 years ago) later adding to it. However, it could also be that the parent comet arrived rather father back in time, and the fragmentation 5,000 years ago was long after the arrival date. One way in which that possibility could be investigated would be to investigate whether there are other showers from the same complex. The point is that the precession rate of P/Encke and the stream branches producing the four main showers implies a loop in 7,000 years. Meteroids released earlier might have slightly smaller or larger orbits, however, and precess at a different rate. In fact, studies of meteor showers have shown that there are two other sets of four showers, usually termed the Chi Orionid and the Piscid streams, that are indeed related to the more powerful Taurids. Studies of the differential precession rates show that these required about 20,000 years to have attained their present situations; that is, the {cometary progenitor} arrived in the inner solar system about 20,000 years ago. The fact that these other showers are much weaker than the Taurids also implies that there must have been a later set of disintegration events, as deduced by Whipple and Hamid; If the comet had simply decayed gradually as astronomers tend to model cometary behavior (obviously being in error to do so), the Chi Orionids and the Piscids would be stronger showers than the Taurids.
If a comet had arrived in the inner solar system 20,000 years ago, episodically liberating large amounts of dust, then surely we would have other evidence to back up this contention? Two sources of evidence spring to mind. First of all, the meteoroids and dust striking the Earth could affect the climate through dust-veiling of the atmosphere and would eventually settle out and leave dust deposits in the polar ice caps. In 1983, Paul LaViolette submitted his Ph.D. thesis to Portland State University, presenting the results of his study of late Pleistocene polar ice, the period from 20,000 to 14,000 years ago. This is the period when the Last Ice Age was terminating, with the occurrence of abrupt climatic alterations. He found that there is a much higher concentration of iridium and nickel, apparenly derived from interplanetary dust, in the ice cores at those levels than would be produced by the present influx of dust to the Earth. Further, he found that there were five distinct episodes of dust deposition during those 6,000 years, which correlates rather nicely with the expected precession rate: Four in 7,000 years is the present rate, but earlier the orbit would likely have been larger, giving a shorter precession period. During those five epochs of heightened dust influx to the Earth, the collection rate of such particles was about a hundred times as high as they are presently. This is obviously a major series of events, which would be expected to affect the terrestrial environment in one way or another.
Such dust is gradually slowed down in the Earth's atmosphere unless the particles are large enough to ablate, in which case their evaporated products will eventually settle out. The Moon possesses no such atmospheric shield, however, so dust and meteroids alike strike the lunar surface without being decelerated. The second source of evidence for the interplanetery dust flux being higher in the past than it is now comes from the tiny craters and pits produced by their impacts. Studies of lunar rock regurned during the Apollo program allowed the ages of such microcraters to be determined in the following way. Rocks exposed on the lunar surface are continually bombarded by charged particles from the Sun (solar cosmic rays). These penetrate the rocks and leave tracks in the crystalline structure which can be counted, with the density of tracks being an indicator of the exposure age of the rock. The same method is used for determining the length of time between a meterorite's release from its parent and its arrival at the Earth. In the case of a microcrater, however, the clock is reset: As the dust grain impacts and excavates the tiny pit, it melts the rock in the base of the crater, obliterating any earlier tracks. Thus, by counting the cosmic ray tracks in that melted rock, the time since the formation of the microcrater can be determined.
One of the major players in this work was Herb Zook of NASA-Johnson Space Center in Houston. In collaboration with others, he set about determining the age distribution of these microcraters, but he came up with a surprising result: He found that the impact rate from 20,000 years ago was about ten times as high as that expected from the present population of dust. Zook is a fine scientist, so he took a look at the assumptions upon which his analysis was based, and in particular he noted that he had assumed that the solar cosmic ray flux was the same 20,000 years ago as it is now, implying that the rate of track production has been invariant over that time. There were two possible interpretations of the data, which he discussed at length:
1. The Sun was much more active 20,000 years ago, with more solar flares occurring, these producing a higher flux of solar cosmic rays;
2. There have been one or more great episodic enhancements in the interplaetary dust population over the past 20,000 years, likely resulting from one or two very large comets arriving in the inner solar system.
Zook decided that item 1 was the more likely answer. I believe that Zook made the wrong choice. For one thing, there is no other evidence to support a conjecture that the Sun was much more active so (astronomically) recently. I interpret the lunar microcrater ages as being evidence in support of the idea that a giant comet broke up in the inner solar system, starting around 20,000 years ago. One can also appeal to other direct measurements of the expoxure ages of interplanetary dust grains collected in the upper atmosphere: The same cosmic ray track method renders an exposure age for these of 10,000 to 30,000 years.
There is contemporary evidence for the zodiacal dust clouds having been boosted by the Taurid Complex. Earlier I wrote about the dust team at the University of Kent. This was a active detector, which counted the impacts on the spacecraft as it flew through the dust cloud surrounding Comet Halley. However, McDonnell also had a passive experiment on board NASA's Long Duration Exposure Facility satellite (LDEF), which was in Earth-orbit from 1984 to 1990. That experiment--actually a series of detectors, on different faces of the LDEF__consisted of thin sheets of metal foil, which would be cratered by very small dust particles and punctured by larger ones. When LDEF was returned to Earth and dismantled, researches at the University of Kent began counting the crater pits and perforations, using high-powered microscopes and other techniques. The result was a surprise: They had expected the numbers of impacts on opposite faces--in particular, the face always pointing north of the ecliptic and the one directed southward--to have equal numbers of strikes, because one might expect there to be as much dust passing its ascending node as its descending node. This would have provided a useful bench mark for the interpretation of the date from the other faces. The north and south faces did not play ball, however. There were many more dust impacts on the north face than on the south. Neil McBride and Andrew Taylor, two young researchers at Kent, started to scratch their heads and look at plausible reasons for this being the case. They knew that another dust impact detector on LDEF, an active counter that had worked only for the first 9 months of its lifetime in orbit, had produced evidence of enhanced dust impact rates during meteor showers-- and that was not expected, because the conventional wisdom says that small dust grains are absent from meteroid streems--so McBride and Taylor looked at whether discrete meteor shower effects could produce the north-south asymmetry. They were having great difficulties with this when I pointed out to them that the subtle effect they were looking for might be accommodated by the Taurid Complex, in particular because it is symmetric about Jupiter's orbital plane rather than the ecliptic. Of course, I had in mind the interpretation given here of the lunar microcraters, and also Fred Whipple's suggestion back in 1967 that the Taurids and Comet Encke are powering the zodiacal dust cloud. As the reader has likely anticipated, McBride and Taylor found that the LDEF data are fitted rather nicely by a model using a main source based on the Taurids.
Let us return to the larger products of such a comet disintegration. From a catastrophist viewpoint, the association of huge atmospheric detonations similar to Tunguska with intense meteor showers does not appear ludicrous at all, and yet such ideas have been dismissed with vigor from mainstream science. The culprit--or maybe we should imply say the catalyst--was Immanuel Velikovsky. In a series of books published around the middle of this century, the most notorious of which is the misnamed Worlds in Collision, Velikovsky put credence in the various historical records and mythical depictions (including many in the Bible) of a huge comet crossing the sky with attendant calamites and came up with the absurd idea that the "comet" was in fact Venus having a near-miss of the Earth. Anyone with knowlege of collegelevel physics should be able to work out for themselves that Velikovsky's idea is in breach of various laws of physics and hence is untenable. Nevertheless, a breed of Velikovsky disciples emerged, similar to alien-contact enthusiasts, and they proved to be the bane of astronomers, with occasional resurrections occurring even today.
The real problem for science is that astronomers, in America in particular, became to entrenched and vehement in their criticism of Velikovsky's astronomical nonsense that their mindsets also became instilled with not only a rejection of, but also a nonconsideration of, the possibility that the myths and records of past civilizations might contain important information about what was happening in the sky in pre-modern times. In fact, the similarity between the legends of disparate human cultures are startling similar. In scientific publications I have pointed out that Australian Aborigines and New Zealand Maoris have oral traditions of strange rocks falling from the sky, causing awful fires and many deaths, and this scenario is common in the myths of other peoples. On one hand, astronomers have prided themselves in instructing geologists that impact catastrophes were responsible in part for the shaping of the planet, but on the other hand, they have been blind to the fact that they have made a uniformitarian assumption when it comes to their own science: That the sky as it is now, is as it ever was, at least while humans have walked the Earth. There is ample evidence not only from historical records of various forms, but also from the analysis of data from this century (such as Whipple's modeling of the Taurid meteors), that around 5,000 years ago the sky did not appear as quiescent as it does now, and that since that time there have been other disruptions of the heavens, producing conflagrations here below.
Is there anything to link the ancient Egyptians with such calamitous events occurring in the sky that have been described earlier? Well, one example is from the Egyptian hieroglyphics; the symbols for thunderbolt and meteorite are the same and contain a star. That is, it seems that the Egyptians associated meteors and meteorites with explosions above their heads, which is certaily indictive of a tumult taking place in the sky of a type different to our experiences today. Various European cultures also associted certain gods with both thunder and meteorites--for example, Thor and Zeus.We can either shrug such things off as the imaginings of ignorant primitive peoples--who were nevertheless able to construct magnificent structures such as Stonehenge and the pyramids and orient them to various celestial phenomena--or start taking a rather different view, perhaps more enlightened, and interpret their relics in light of an alternative perspective of what might have been happening in their skies.