Intersection with the debris of a large (50-100 km) short-period comet during the Upper Palaeolithic provides a satisfactory explanation for the catastrophe of celestial origin which has been postulated to have occurred around 12900 BP, and which pre-saged a return to ice age conditions of duration 1300 years. The Taurid Complex appears to be the debris of this erstwhile comet; it includes at least 19 of the brightest near-Earth objects. Sub-kilometre bodies in meteor streams may present the greatest regional impact hazard on timescales of human concern
Bill, will you help the Tusk out a bit and provide a response, which I can post, to the claim below by Dr. Mark Boslough? There are several ways to approach his statement, but I am interested in your take.
“There’s no plausible mechanism to get airbursts over an entire continent,” said Boslough.
Sent from my iPhone George A. Howard
Response from Bill Napier that day:
You asked me to comment on Mark Boslough’s claim that “There’s no plausible mechanism to get airbursts over an entire continent.” As I’ve already demonstrated in the refereed literature that there is such a mechanism, I’m not sure what I can add! However, let me try to pinpoint where I believe Boslough is going wrong. I have in hand an abstract of a talk he gave a couple of years ago and I don’t suppose his stance has changed much since then:
“The YDB impact hypothesis of Firestone et al. (2007) is so extremely improbable it can be considered statistically impossible in addition to being physically impossible. Comets make up only about 1% of the population of Earth-crossing objects. Broken comets are a vanishingly small fraction, and only exist as Earth-sized clusters for a very short period of time.”
~The Bos, Geol. Soc. America annual meeting (21-23 Nov 2010), Denver.
It’s true that comets currently make up a minority of the population of Earth-crossing objects (1% is extreme but let that pass), but that’s only at the 1 km level or thereabouts. As you go to larger objects, the balance shifts profoundly. For example, there are no 10 km asteroids currently in Earth crossing orbits, but we do have large cometary Earth-crossers, e.g Halley at 11 km, Swift-Tuttle at 27 km and so on. Asteroids of 10 km or more can’t be shifted out of the main belt at a sufficient rate to account for the large terrestrial impact craters: the transfer rate is an order of magnitude too slow. The action lies with the big comets.
Large populations of them have been discovered on the fringes of the planetary system in recent years, thanks to deep, wide-angle surveys. Their number is still uncertain, and their orbital dynamics is still being worked out, but it is recognised that from time to time rare, giant comets will feed into short-period orbits from these populations, weaving between the giant planets in unstable orbits which may lead to their entering our neighbourhood on relatively short timescales. They do this for the most part by feeding through the Jupiter family of comets, that is short-period comets whose orbits are strongly influenced by that giant planet.
Chiron, for example, which is over 200 km in diameter and orbiting beyond Saturn, has probably dipped in and out of our neighbourhood several times in its past. The half-life for doing so is about 0.2 million years, each episode lasting a few thousand years. It probably has several thousand times the mass of the entire near-Earth asteroid system. There are several known bodies in this size range and similarly unstable orbits, and the sample is likely incomplete. Large-scale orbital computations have shown that they have the propensity to become Earth-crossers on timescales (each) of order a million years.
It follows from this that a giant comet residing in a short-period, Earth-crossing orbit is not uncommon on geological timescales. There is nothing anomalous about an erstwhile giant comet having been around say over the last 100,000 years.
This is all by way of background because we know that in fact two such comets have indeed been around in the recent past. One is the progenitor of the Kreutz sungrazers, which was probably 100 km across and began to break up 1700 years ago. The other is the progenitor of comet Encke and the Taurids, which was probably of similar size but much greater age, at least 20,000 and perhaps up to 100,000 years. Kreutz was high inclination and its debris never came our way. Encke is in the ecliptic and we’re still immersed in the debris, the Taurid complex.
Which leads to the question: what do we expect from a 100-200 km comet in a short-period Earth-crossing ecliptic orbit?
“Broken comets are a vanishingly small fraction, and only exist as Earth-sized clusters for a very short period of time.”
This one sentence contains two profound misconceptions. First, hierarchic disintegration is now generally recognised as the major route whereby comets die. It’s a common process. Second, ‘Earth-sized clusters’ have nothing to do with it. We are dealing with concentrations of fragments having, say, 10,000 times the cross-sectional area of the Earth. For a 100 km comet to undergo disintegration in our neighbourhood gives us a hugely enhanced impact hazard. Fragments totalling even a 1,000th the size of Chiron would have a mass of 10**18 g. Passage through such a debris field would yield about 10** 14 g of material impinging on the Earth. This is likely to be in the form of dust, pebbles, all the way up to super-Tunguska objects. The overall energy amounts to something like 5000 Tunguskas, striking a hemisphere of the Earth over a period of a few hours as we pass through. What are the odds that we would in fact pass through such debris in the course of a short-period, giant comet’s disintegration? This communication is already quite long enough, but detailed numerical modelling based on lifetimes, drift, shepherding resonances and the like reveal that one or two such encounters are reasonably probable events over the active lifetime of the Taurid progenitor (papers in preparation; see also my 2010 MNRAS paper).
In a nutshell, Mark Boslough’s cometary model is irrelevant. It has nothing to do with the actual circumstances which prevailed in our environment over the Holocene and earlier. It takes no account of, and indeed shows no awareness of, modern developments in cometary dynamics. Any competent referee would reject it.
I could say more, but perhaps that’s enough to be going on with.