Figure 37: The reflectivity o f an aggregate o f organic particles 10~5 cm in diameter as a function o f porosity. For comparison, the nuclei of active comets have albedos p ~ 0.02-0.04, while dark spots on comet Borrelly have p ~ 0.008 (Nelson et al., 2004). Brownlee particles and particles collected in
statsospheric collections by Narlikar et al. (2003) are o f probable cometary origin and have porosity o f 75% or more.
With this model, it has been found experimentally that the thickness o f the crust on the surface of a comet increases proportionately to the square root o f the insolation time, while the gas production rate proceeds in inverse proportion to the thickness o f the crust (Ibadinov et al., 1991). Laboratory and numerical work (Ibadinov, 1993 and references therein) shows that the rate of fading o f short-period comets, and its dependence on perihelion distance, are well reproduced by a nucleus of graphite particles embedded in water ice, with 80 percent porosity and thermal conductivity 0.05 W^K"1. In this case, for a comet in a Halley-type orbit, the crust grows at ~5 cm y r 1. The pressure o f the escaping water vapour is insufficient to break such a crust (loc. cit. Kiihrt and Keller, 1996).
The volume o f space out to which a body of albedo p is detectable varies as p m and so, if there is an expectation that 400 dormant comets with canonical p = 0.04 should by now
m*1.45-ik
Values of k from bottom curve to top * k=0.1 - k=0 * k=0.2 * k=0.3 - k=0.4 4 — i - i - i i * - i - i « » i i ■ i I 1 I I
albedos o f this order are the dark spots on Comet Borrelly, which have p -0.008 (Nelson et al., 2004). If the predicted population of dark Haileys had this albedo, ~40 would by now have been discovered. There is clearly a strong selection effect against the
discovery o f astronomical objects in the solar system with albedos p « 0.008 (Jewitt and Femdndez, 2001).
5.4 Current impact hazard
The above solution to the ‘missing comet’ problem leads to the supposition that there exists a large population o f extremely dark comets in Earth-crossing orbits. They are » undetectable with current NEO search programmes but are impact hazards nonetheless.
Levison et al. (2002) computed that, without disintegration, there would be a population ofAr~3xl04 dormant Halley-type comets with diameters D > 2.4 km and perihelia q < 1
AU. For P -6 0 yr and mean impact speed -6 0 kms'1, then from (5.11) the mean interval between impacts o f such bodies is -0.67 My, with impact energies fcl.5 * 106Mt (Jeffers et al., 2001). This leads to the conclusion that impacts o f at least 1.5* 107 Mt energy are
expected at mean intervals »2.3 My. This interval will o f course be increased somewhat if we take account o f the revised estimates o f comet fluxes from the LINEAR data (Neslusan, 2007).
But these rates are well in excess of those expected from the NEO system currently being mapped out (mainly S-type asteroids: Morbidelli et al., 2002). Hughes (2003) has argued that there is no room for a significant cometary contribution, active or dormant, on the grounds that the impact rate from the near-Earth asteroid population is adequate to produce the known rate at which terrestrial craters are produced. This argument depends on a scaling relation between the diameter of the impactor and the crater which it forms,
estimate that a terrestrial impact rate of about one Jupiter family comet (active or dormant) per My. The expected impact rate is also significantly higher than has been inferred from lunar cratering data (Neukum and Ivanov, 1994).
Current detection and deflection strategies involve the assumption that decades or centuries of warning will be available following the discovery o f a threatening asteroid. However if the major impact hazard indeed comes from this essentially undetectable population, the warning time o f an impact is likely to be at most a few days. A typical Halley-type dormant comet spends 99 % of its time beyond the orbit o f Mars and so a full mapping o f this population is beyond current technology.
If the Halley-type population is derived in large part by capture o f comets from the long- 'period system (Bailey and Emel’yanenko, 1998), then perturbations of the Oort cloud (as discussed in Ch. 4) may yield an upsurge in the dark Hailey population, and ultimately in the flux o f impactors on the Earth. As we have pointed out earlier the Oort cloud is demonstrably sensitive to Galactic perturbers o f various sorts - stars, nebulae and tides (Byl 1986; Napier and Staniucha, 1982 etc.). Nurmi et al. (2001) confirm that the flux of comets from the Oort cloud, and hence the impact rate, may fluctuate by an order of magnitude arising from the motion of the sun with respect to the Galactic midplane. Since we are at present passing through the plane o f the Galaxy, it is expected that the current impact rate is several times higher than that deduced from the lunar cratering record, which is time-averaged over one or two Gy.
The significance for the panspermia thesis is that the dark Haileys are a present-day link in the chain between the Galactic disturbances discussed in Chapter 4, huge impacts that dislodge microbiota from the Earth. The idea that passages of the Sun through the spiral arms o f the Galaxy might induce terrestrial disturbances has been discussed by a number o f authors (e.g. McCrea, 1975). Napier and Clube (1979) specifically proposed that bombardment episodes might occur during such passages, leading to mass extinctions. Leitch and Vasisht (1998) have shown that, indeed, the Sun was passing through a spiral arm during the Cretaceous-Tertiary and Permo-Triassic extinctions o f 65 and 250 million
with mass extinctions, comet bombardment episodes (e.g. the K-T impact event) would have the effect o f directly contributing to panspermia by transferring life-bearing rocks containing even plant seeds (Tepfer and Leach, 2007) and surface dust from the Earth to nearby nascent planetary systems (Napier, 2004; Wallis and Wickramasinghe, 2004). It is also not impossible that new genera are introduced via impacting comet material. In this connection, the sudden appearance of diatoms in the fossil record at precisely the time o f the K-T impact is worthy o f note (Hoover et al., 1986).