Mon. Not. R. Astron. Soc. 366, 977–982 (2006) doi:10.1111/j.1365-2966.2005.09851.xEvidence for cometary bombardment episodesW. M. Napier!Cardiff Centre for Astrobiology, Cardiff University, 2 North Road, Cardiff CF10 3DFAccepted 2005 November 9. Received 2005 October 3; in original form 2005 July 13ABSTRACTEvidence is found that large terrestrial impacts tend to cluster in discrete episodes, with char-acteristic separations 25–30 Myr and durations of about 1–2 Myr. The largest impactors arestrongly concentrated within such events, and the Cretaceous–Tertiary extinctions occurredwithin one of them. The evidence also indicates the presence of a weak periodicity, whichmight be ∼24, ∼35 or ∼42 Myr depending on which peaks are taken as harmonics. The peri-odicity is most easily explained as a result of the action of the Galactic tide on the Oort cometcloud. The two longer period solutions are consistent with Galactic density estimates and withthe current passage of the Solar system through the plane of the Galaxy. Other episodes maybe a result of sporadic encounters with spiral arms, nebulae or stars.Key words: comets: general – Earth – Oort Cloud – Galaxy: disc.1 I N T RO D U C T I O NImpact cratering on the Earth is now well established as a catas-trophic geological process. That there might be a pattern of episod-icity, or even periodicity, in the rate of arrival of large impactors hasbeen suggested from time to time. Napier & Clube (1979) proposedthat Galactic disturbances of the Oort Cloud might generate bom-bardment episodes, which in turn could generate mass extinctions oflife and geological disturbances. Periodicity of such episodes mightbe expected by virtue of spiral arm penetrations (ibid.; Leitch &Vasisht 1998) and the variation of the Galactic tides which perturbthe orbits of long-period comets (Napier 1987; Matese et al. 1995;Nurmi, Valtonen & Zheng 2001), but several other phenomena couldyield sporadic bombardment episodes: the breakup of a very largecomet (Clube & Napier 1984; Bailey et al. 1994); the passage of astar through a dense inner Oort Cloud (Hills 1984; Hut et al. 1987);or a close encounter with a massive nebula (Napier & Staniucha1982; Clube & Napier 1982).About 170 confirmed impact structures were known by the endof 2004, 40 of which have been dated with precision σ!10 Myr,are less than 250 Myr old and have diameters"3 km (Earth ImpactDatabase 2005). 35 of these 40 have ages with formal σ!5 Myr,18 with σ!1 Myr. The data base of well-dated craters has abouttripled in the last 20 yr and has now reached the stage where strongepisodicity and periodicity, as predicted by the above hypotheses,might now be detected with some degree of statistical confidence. Inthis paper I describe the results of such an analysis. I find that thereis evidence that large impacts do not occur at random but, rather, areconcentrated into strong discrete episodes of bombardment. Bothrandom and periodic components seem to be present, the latter con-sistent with a solar oscillation about the Galactic plane. The random!E-mail: [email protected] interfere with the analysis and do not allow the periodicity tobe specified unambiguously: it may be one of ∼(24, 35, 42) Myr. Italso appears that we are immersed in a bombardment episode now.Episodicity or periodicity in the rate of impacts, if present, hasa bearing on the likely source of impactors and the mechanismswhich might be responsible for terrestrial phenomena such as massextinctions. A one-off stray asteroid yields a ‘sudden death’ set ofexpectations; an episode of cometary bombardment will involveboth prolonged multiple impacts and atmospheric dusting and soyield another. Further, given the existence of surges, assessments ofthe current celestial hazard based on Poissonian assumptions aboutcratering on old surfaces may not reflect current circumstances.2 L A R G E B O L I D E STable 1 lists the 40 terrestrial impact craters known to epoch 2004November, satisfying the following criteria:(i) diameter"3 km;(ii) age!250 Myr;(iii) ages quoted to precision σ!10 Myr. These have been culledfrom the Earth Impact Database (2005).Six of these 40 craters are less than 5 Myr old. One, Kara-Kul inTajikistan, has an age given only as an upper limit of <5 Myr, which Ihave replaced by 2.5 ± 2.5 Myr. All but five have σ< 5 Myr. Theirage distribution is plotted in Fig. 1, along with a cubic spline fit.The fit clearly shows a decline in the known craters with increasingage, presumably because of loss with time through erosion andsedimentation.3 C L U S T E R I N GAssuming the true cratering rate has not changed secularly over thelast 250 Myr, it is clear from the steep decline in Fig. 1 that discoveryof impact craters is highly incomplete, even amongst the largestC"2005 The Author. Journal compilationC"2005 RAS978 W. M. NapierTable 1. Impact craters used in this paper.N Crater D (km) Age (Myr) σ (Myr)01 Gusev 3 49.0 0.202 Zapadnaya 3 165.0 5.003 Steinheim 4 15.0 1.004 Chiyli 6 46.0 7.005 Wetumpka 7 81.0 5.006 Wanapetei 8 37.2 1.207 Bigach 8 5.0 3.008 Mien 9 121.0 2.309 Ragozinka 9 46.0 3.010 Karla 10 5.0 1.011 Bosumtwi 10 1.1 0.012 Marquez 13 58.0 2.013 Deep Bay 13 99.0 4.014 Zhamanshin 14 0.9 0.115 Logoisk 15 42.3 1.116 El’gygytyn 18 3.5 0.517 Dellen 19 89.0 2.718 Gosses Bluff 22 142.5 0.819 Rochechouart 23 214.0 8.020 Lappajarvi 23 73.3 5.321 Ries 24 15.1 0.122 Boltysh 24 65.17 0.6423 Haughton 24 23.0 1.024 Kamensk 25 49.0 0.225 Steen River 25 91.0 7.026 Mistastin 28 36.4 4.027 Manson 35 73.8 0.328 Carswell 39 115.0 10.029 Mjolnir 40 142.0 2.630 Araghuaina 40 244.40 3.2531 Montagnais 45 50.50 0.7632 Kara-Kul 52 2.5 2.533 Tookoonooka 55 128.0 5.034 Kara 65 70.3 2.235 Morokweng 70 145.0 0.836 Puch.-Kat. 80 167.0 3.037 Chesapeake 90 35.5 0.338 Manicougan 100 214.0 1.039 Popigai 100 35.7 0.240 Chicxulub 170 64.98 0.05ones. If all recent craters #3 km across have been discovered – anunlikely assumption – completeness has declined to ∼40 per centwithin 100 Myr and ∼10 per cent within 200 Myr (much highersurvival rates have been claimed by Hughes 2000). Thus, to bedetected at all in the geological record, a comet shower would haveto be relatively strong; and it might now be represented, if at all,by only two or three surviving impact structures of similar age,scattered over the Earth. A periodic modulation of amplitude say2:1 to 5 : 1 in the cratering rate as a result of variable Galactictides might