Breakup of a Long-Period Comet as the Origin of the Dinosaur Extinction
BBreakup of a Long-Period Comet as theOrigin of the Dinosaur Extinction
Amir Siraj (cid:63) & Abraham Loeb Department of Astronomy, Harvard University60 Garden Street, Cambridge, MA 02138, USA (cid:63)
Correspondence to [email protected]
The origin of the Chicxulub impactor, which is attributed asthe cause of the K/T mass extinction event, is an unsolved puz-zle.
The background impact rates of main-belt asteroids andlong-period comets have been previously dismissed as being toolow to explain the Chicxulub impact event. Here, we show thata fraction of long-period comets are tidally disrupted after passingclose to the Sun, each producing a collection of smaller fragmentsthat cross the orbit of Earth. This population could increase theimpact rate of long-period comets capable of producing Chicxulubimpact events by an order of magnitude. This new rate would beconsistent with the age of the Chicxulub impact crater, therebyproviding a satisfactory explanation for the origin of the impactor.Our hypothesis explains the composition of the largest confirmedimpact crater in Earth’s history as well as the largest one withinthe last million years . It predicts a larger proportion of impactorswith carbonaceous chondritic compositions than would be expectedfrom meteorite falls of main-belt asteroids. a r X i v : . [ a s t r o - ph . E P ] F e b trong evidence suggests that the Chicxulub impact led to the K/T massextinction event, which was the largest in the past ∼
250 Myr and broughtabout the demise of the dinosaurs.
1, 2
However, the nature of the Chicxulubimpactor is poorly understood. The latest scenario suggested postulated thatthe breakup of the Baptisina asteroid family could have led to the formationof the Chicxulub impactor. However, spectroscopic follow-up indicated thatthe Baptistina family has an S-type, rather than an Xc-type composition,making it an unlikely source of the Chicxulub impactor, which had a car-bonaceous chondritic composition,
4, 8, 9 although not ruling out entirely thepossibility due to the stochastic nature of asteroid collisions and the sub-sequent disruptive processes. Observations of the Baptisina family alsosuggested that the breakup age may be ∼
80 Myr rather than ∼
160 Myr ,further reducing the likelihood that the Baptisina breakup formed the Chicx-ulub impactor.The Chicxulub impactor could have originated from the background pop-ulations of asteroids or of comets. Main-belt asteroids (MBAs) with diam-eters D (cid:38)
10 km, capable of producing Chicxulub impact events, strike theEarth once per ∼
350 Myr.
11, 12
Based on meteorite fall statistics, one suchobject with a carbonaceous chondritic composition impacts the Earth overa characteristic timescale of ∼ . Long-period comets (LPCs) capable of producing Chicxulub-scaleimpacts strike Earth also too rarely, once per ∼ . −
11 Gyr, based on therate of Earth-crossing LPCs and the impact probability per perihelion pas-2age,
14, 15 and adopting a cumulative power-law index within the range -2.0 to-2.7.
The only cometary sample-return mission to date,
Stardust , foundthat Comet 81P/Wild 2 had carbonaceous chondritic composition, suggest-ing that such a composition could potentially be widespread in comets.
As a result, the rate of LPC impacts with carbonaceous chondritic compo-sition could be similar to the overall LPC impact rate. Within a timescaleof ∼
100 Myr, stellar encounters could boost the impactor flux by an orderof magnitude for a Myr timescale, which are insufficient in magnitude toexplain a Chicxulub impact event. We note that comets are typically morefragile and porous than asteroids.
24, 25
To find the fraction of LPCs with orbital behavior that could affect the im-pact flux at Earth, we simulated gravitational interactions between LPCs andthe Jupiter-Earth-Sun system using a semi-analytic approach. Initially, thereare N Jupiter-crossing LPCs (initial pericenter distance q (cid:46) . a ∼ AU and the distribution of pericenter distances scal-ing as q , the corresponding cross-sectional area.
23, 26
The initial inclinationdistribution is taken as uniform.
23, 26
We then follow the orbital perturbationprescription for a restricted three-body scattering. At the initial closest ap-proach to Jupiter, calculated by selecting a random phase angle in Jupiter’sorbit and computing the minimum distance between Jupiter and the LPC’sorbit b J , the change in semi-major axis a resulting from the three-body inter-action is computed as ∆(1 /a ) = (4 M J v J √ a (cos γ + K cos δ ) /M / (cid:12) b J √ G (1 + K )), where M J is the mass of Jupiter, M (cid:12) is the mass of the Sun, v J is the3eliocentric orbital speed of Jupiter, G is the gravitational constant, γ is theangle between the velocity vectors of Jupiter and the LPC, δ is the anglebetween the normal in the orbital plane to the approach of the LPC at thetime of its closest approach to Jupiter and the velocity vector of Jupiter, and K ≡ ( GM J a/M (cid:12) b J ). The new inclination is approximated by the numeri-cally derived fitting function, ≈ arccos [cos i − .
38 sin i Q − / ( b J /a )], where Q ≡ ( q/b J ). The updated eccentricity is calculated through conservationof the Tisserand parameter, T = (1 /a ) + 2 (cid:112) a (1 − e ) cos i , across the en-counter. If the LPC crosses the orbit of Earth, defined as q (cid:46) a > × AU or e ≥ ∼ − perJupiter-crossing orbit.We find that for N = 10 particles, ∼
20% of Earth-crossing events, de-fined as perihelia within the orbital radius of the Earth q (cid:46) q (cid:46) r (cid:12) (2 ρ (cid:12) /ρ obj ) / , where r (cid:12) is the radius of the Sun, ρ (cid:12) is the mean mass den-sity of the Sun, and ρ obj ∼ . − is the mean density of the LPC,since they were captured into highly eccentric orbits by interacting with theSun-Jupiter system. This is consistent with previous estimates of the sun-grazing LPC population. If the LPC is solely bound by gravity, then it4s tidally disrupted. This is consistent with comets being the most fragilebodies in the Solar system, being mostly formed by weakly bound aggre-gates.
24, 30–32
Some comets may be highly heterogeneous rubble piles as aresult of impact gardening and collisional processes,
25, 33 with some pieceshaving relatively higher strengths, as was proposed to explain the origin ofrare H/L chondrites.
31, 34
The characteristic change in v ∞ for the fragmentsis, ∆ v ∞ ∼ √ v ∆ v , where v ∼ (cid:112) GM (cid:12) /d (cid:12) ,R and ∆ v ∼ (cid:112) Gm/R , where d (cid:12) ,R is the Sun’s Roche radius, m is the mass of the progenitor, and R is theradius of the progenitor. The change in v ∞ , ∆ v ∞ , is comparable to the orig-inal v ∞ for an LPC. The time between disruption and crossing the Earth’sorbit is ∼ ( d ⊕ / (cid:112) GM (cid:12) /d ⊕ ) ∼ τ , where d ⊕ ∼ τ is the tidal disruption encounter timescale, τ ≡ (cid:112) d (cid:12) /GM (cid:12) .This is consistent with the conversion of R ∼
30 km LPCs into fragmentswith effective radii of R ∼ . as wellas the formation of the Gomul and Gipul crater chains. Data from Gomuland Gipul, as well other crater chains on Callisto and Ganymede, indicatethat the fragments typically vary in size only by a factor of order unity ,due to the gravitationally bound rubble pile fragmentation model, althoughsome second-order disruption effects are possible. We note that the canonicalequation z b = z (cid:63) − H (cid:2) ln 1 + ( l/ H ) (cid:112) f p − (cid:3) for the parameters consid-ered here is only consistent with z b <
0, implying that despite experiencingdisruption during atmospheric entry, the comet fragment does not suffer5n airburst, which was the fate of the Tunguska impactor,
40, 41 but insteadforms a crater, as observed. In the equation above, z b is the altitude at whichthe airburst occurs, z (cid:63) is the altitude at which the comet begins to disrupt, H is the scale height of the atmosphere, l = L sin( θ ) (cid:112) ρ obj / ( C D ρ a ( z (cid:63) ) isthe dispersion length scale, f p = ( L ( z ) /L ) is the pancake factor, L = 2 R is the impactor diameter, ρ obj is the impactor density, ρ a is the atmosphericdensity, θ is the impact angle, and C D is a drag coefficient.We now consider the effect that tidal disruption of a fraction of LPCs hason the impact rate of cometary bodies capable of producing Chicxulub. Wefirst note that D (cid:38)
10 km progenitors, as considered here, are not thermallydisrupted at large distances like smaller comets. We adopt the size distribu-tion of Kuiper belt objects (KBOs) as a proxy for large LPCs or Oort cloudobjects, due to their shared histories.
KBOs with radii ranging from R ∼ −
10 km and R ∼
30 km can be described with a power-law index of q ∼
46, 47 N ( > R ) ∝ R − q .The size distribution for LPCs, which have been observed up to radii of R ∼
10 km, is consistent with the extrapolation of the q ∼
42, 48 R ∼ . R ∼
30 km are primarily bound by gravity, as indicated by modeling con-sistent with the observed size-density relationship
49, 50 and as implied by thelocation of the break in the size distribution.
46, 47, 51
Most asteroids with sizesof D (cid:38)
10 km are not considered strengthless, meaning that if they passedwithin the Sun’s Roche limit, they most likely would not produce fragments6f the necessary size to explain Chicxulub. Since the mass of an LPC scales as R and the abundance of LPCs scalesas R − q , the overall enhancement of the time-averaged flux of cometary im-pactors capable of producing Chicxulub impact events resulting from thebreakup and immediate crossing of the ∼ ∼
10 in radius is, ∼ . × (30 km / . − q ) ≈ ∼ −
730 Myr. Irrespective of composition, the total impact rate ofLPC fragments that could cause Chicxulub impact events is comparable tothe total impact rate of MBAs that trigger events. We note that in order tobe in agreement with the lack of an observed increase in the Earth’s dust ac-cretion rate across the K/T event over timescales of ∼ q (cid:38) −
3, which can be testedthrough detailed modeling of such tidal disruption events.The carbonaceous chondritic composition fraction of LPCs might be com-parable to unity, since the first cometary target of a sample return missionComet 81P/Wild 2 indicated a carbonaceous chondritic composition. How-ever, the tiny aggregate particles collected had very low tensile strengths,potentially complicating the understanding of cometary structure in gen-eral. Adopting the assumption that the carbonaceous chondritic compo-7ition fraction of LPCs might be comparable to unity, the impact rate oftidally-disrupted LPCs is consistent with the Chicxulub impact event be-ing the largest mass extinction event in the last ∼
250 Myr, and is signif-icantly larger than the impact rate of MBAs that could cause Chicxulubimpact events. In particular, the probability that the Chicxulub impactorwas an LPC fragment is larger than the probability that it was an MBA ifthe carbonaceous chondritic composition fraction of the LPC progenitors is (cid:38) − (cid:38) − Stardust will constrain the frac-tion of comets with carbonaceous chondritic compositions and thereby serveas important test for our hypothesis. In addition, measurements of the sizedistribution of Oort cloud objects will improve the precision of our model.Since comets with D (cid:46)
10 km are thermally disrupted at large distancesfrom the Sun and also the size distribution of comets with D (cid:38)
60 km isdescribed by a power law with a cumulative power-law index steeper than -3, our model only applies to the progenitor size range of 10 km (cid:46) D (cid:46)
60 km,thereby not affecting the overall crater size distribution.Our hypothesis predicts that other Chicxulub-size craters on Earth aremore likely to correspond to an impactor with a carbonaceous chondritc com-position than expected from the carbonaceous chondritc composition fraction8
10 15 20 25 30
Progenitor radius [km] C h i c x u l u b i m p a c t r a t e [ G y r ] T i d a l l y d i s r u p t e d l o n g - p e r i o d c o m e t s Intact long-period cometsMain-belt asteroidsObserved Chicxulub impact
Figure 1: The impact rate of tidally disrupted LPCs with energies compara-ble to that of the Chicxulub impactor, with the impact rates of intact LPCsand MBAs for reference, in addition to the range of rates that would ex-plain the observed Chicxulub impact, including 95% Poisson errors. MostLPCs and ∼
10% of MBAs are assumed to have a carbonaceous chondriticcomposition (see text for details). 9f MBAs. We note that meteorite fall statistics should still reflect the com-positions of asteroids, as canonically assumed. For small LPCs that passwithin the Sun’s Roche radius, the ablated mass is ∼ ( R L (cid:12) τ / d (cid:12) Q ), where L (cid:12) is the luminosity of the Sun, d (cid:12) ,R is the Roche radius of the Sun, τ is theencounter timescale, and Q is the energy per unit mass necessary to vaporizethe material. Adopting Q ∼ × erg g − , the initial mass is comparableto the ablated mass for object radii of R ∼ ∼ g, which is orders ofmagnitude above the preatmospheric entry masses of objects that dominatethe meteorite flux at the Earth’s surface. This magnitude of ablation indi-cates that mass loss is negligible for the progenitor size range considered here.In addition, the heating due to solar irradiation, ∼ K over ∼ s, doesnot exceed the expected heating from the impact itself, so no additionalsignatures of thermal processing would be expected. Shoemaker-Levy 9, 2015TB145, and the Encke complex are all examples of large fragments resultingfrom tidal disruption.
32, 54, 55
Additionally, the observation that the largestparticles in most observed meteoroid streams are cm-sized is not surprising,since larger particles are naturally more rare than smaller particles.Indeed, Vredefort, the only confirmed crater on Earth larger than Chicxu-lub (by a factor of ∼ may correspond to an impactor with a car-bonaceous chondritic composition. Additionally, since LPC fragment Chicx-ulub impactors should strike Earth once every ∼ −
730 Myr, fragmentsan order of magnitude smaller in radius, if produced by the same progeni-10ors, would strike Earth no more frequently than once per ∼ . − .
73 Myrand if a significant fraction of the progenitors have a carbonaceous chondritccomposition, the most recent such crater should reflect such a composition.Indeed, the Zhamanshin crater, the largest confirmed impact crater on Earthformed in the last ∼ Myr (an order of magnitude smaller in radius thanChicxulub), shows evidence that the impactor may have had a carbona-ceous chondritc composition, providing support to our model. Additionally,the likely existence of a well-separated reservoir of carbonaceous chondriticmaterial beyond the orbit of Jupiter in the solar protoplanetary disk lendsfurther support to our model. Our model is in no conflict with the Moon’scratering rate, since it only applies in the size range around Chicxulub-scaleimpactors. The cross-sectional area of the Moon is an order of magnitudesmaller than Earth, implying that a Chicxulub size impactor would be veryrare (once per few Gyr), and thereby implying that such an LPC impactevent may have not happened for the Moon. References Alvarez, L. W., Alvarez, W., Asaro, F. & Michel, H. V. Extraterrestrial Causefor the Cretaceous-Tertiary Extinction.
Science , 1095–1108 (1980). Schulte, P. et al.
The Chicxulub Asteroid Impact and Mass Extinction at theCretaceous-Paleogene Boundary.
Science , 1214 (2010). Bottke, W. F., Vokrouhlick´y, D. & Nesvorn´y, D. An asteroid breakup 160Myrago as the probable source of the K/T impactor.
Nature , 48–53 (2007). Reddy, V. et al.
Composition of 298 Baptistina: Implications for the K/Timpactor link.
Meteoritics and Planetary Science , 1917–1927 (2009). Masiero, J. R. et al.
Main Belt Asteroids with WISE/NEOWISE. I. PreliminaryAlbedos and Diameters.
ApJ , 68 (2011). . Mougel, B., Moynier, F., G¨opel, C. & Koeberl, C. Chromium isotope evidencein ejecta deposits for the nature of Paleoproterozoic impactors.
Earth and Plan-etary Science Letters , 105–111 (2017). . Magna, T. et al.
Zhamanshin astrobleme provides evidence for carbonaceouschondrite and post-impact exchange between ejecta and Earth’s atmosphere.
Nature Communications , 227 (2017). Kyte, F. T. A meteorite from the Cretaceous/Tertiary boundary.
Nature ,237–239 (1998). Trinquier, A., Birck, J.-L. & Jean All`egre, C. The nature of the KT impactor.A Cr reappraisal.
Earth and Planetary Science Letters , 780–788 (2006). Eugster, O., Herzog, G. F., Marti, K. & Caffee, M. W.
Irradiation Records,Cosmic-Ray Exposure Ages, and Transfer Times of Meteorites , 829 (2006). Bottke, W. F. et al.
Debiased Orbital and Absolute Magnitude Distribution ofthe Near-Earth Objects.
Icarus , 399–433 (2002). Granvik, M. et al.
Debiased orbit and absolute-magnitude distributions fornear-Earth objects.
Icarus , 181–207 (2018). . Burbine, T. H., McCoy, T. J., Meibom, A., Gladman, B. & Keil, K.
MeteoriticParent Bodies: Their Number and Identification , 653–667 (2002). Francis, P. J. The Demographics of Long-Period Comets.
ApJ , 1348–1361(2005). astro-ph/0509074 . Weissman, P. R. The cometary impactor flux at the Earth. In Valsecchi, G. B.,Vokrouhlick´y, D. & Milani, A. (eds.)
Near Earth Objects, our Celestial Neigh-bors: Opportunity and Risk , vol. 236 of
IAU Symposium , 441–450 (2007). Toth, I. Connections between asteroids and cometary nuclei. In Lazzaro, D.,Ferraz-Mello, S. & Fern´andez, J. A. (eds.)
Asteroids, Comets, Meteors , vol. 229of
IAU Symposium , 67–96 (2006). Fern´andez, J. A. & Morbidelli, A. The population of faint Jupiter family cometsnear the Earth.
Icarus , 211–222 (2006). Tancredi, G., Fern´andez, J. A., Rickman, H. & Licandro, J. Nuclear magnitudesand the size distribution of Jupiter family comets.
Icarus , 527–549 (2006). Nakamura, T. et al.
Chondrulelike Objects in Short-Period Comet 81P/Wild 2.
Science , 1664 (2008). Zolensky, M. et al.
Comparing Wild 2 particles to chondrites and IDPs.
Mete-oritics and Planetary Science , 261–272 (2008). Cody, G. D. et al.
Cosmochemistry Special Feature: Establishing a molecularrelationship between chondritic and cometary organic solids.
Proceedings of theNational Academy of Science , 19171–19176 (2011). Bridges, J. C., Changela, H. G., Nayakshin, S., Starkey, N. A. & Franchi, I. A.Chondrule fragments from Comet Wild2: Evidence for high temperature pro-cessing in the outer Solar System.
Earth and Planetary Science Letters ,186–194 (2012). Vokrouhlick´y, D., Nesvorn´y, D. & Dones, L. Origin and Evolution of Long-period Comets. AJ , 181 (2019). . Blum, J., Schr¨apler, R., Davidsson, B. J. R. & Trigo-Rodr´ıguez, J. M. ThePhysics of Protoplanetesimal Dust Agglomerates. I. Mechanical Properties andRelations to Primitive Bodies in the Solar System.
ApJ , 1768–1781 (2006). Trigo-Rodriguez, J. M. & Blum, J. Tensile strength as an indicator of the degreeof primitiveness of undifferentiated bodies.
P&SS , 243–249 (2009). Fouchard, M., Rickman, H., Froeschl´e, C. & Valsecchi, G. B. Distribution oflong-period comets: comparison between simulations and observations.
Astron-omy & Astrophysics , A24 (2017). Valtonen, M. & Karttunen, H.
The Three-Body Problem (2006). Weissman, P. R. & Lowry, S. C. Structure and density of cometary nuclei.
Meteoritics and Planetary Science , 1033–1047 (2008). Bailey, M. E., Chambers, J. E. & Hahn, G. Origin of sungrazers - A frequentcometary end-state.
Astronomy & Astrophysics , 315–322 (1992). Brownlee, D. et al.
Comet 81P/Wild 2 Under a Microscope.
Science , 1711(2006). Trigo-Rodr´ıguez, J. M. & Llorca, J. The strength of cometary meteoroids: cluesto the structure and evolution of comets.
MNRAS , 655–660 (2006). Trigo-Rodr´ıguez, J. M., Rimola, A. & Martins, Z. Aqueous processing of organiccompounds in carbonaceous asteroids. In
EGU General Assembly ConferenceAbstracts , EGU General Assembly Conference Abstracts, 10230 (2015). Beitz, E., Blum, J., Parisi, M. G. & Trigo-Rodriguez, J. The Collisional Evolu-tion of Undifferentiated Asteroids and the Formation of Chondritic Meteoroids.
ApJ , 12 (2016). . Trigo-Rodr´ıguez, J. M. & Williams, I. P. Dynamic Sources of ContemporaryHazard from Meteoroids and Small Asteroids. In Trigo-Rodr´ıguez, J. M., Grit-sevich, M. & Palme, H. (eds.)
Assessment and Mitigation of Asteroid ImpactHazards: Proceedings of the 2015 Barcelona Asteroid Day , vol. 46, 11 (2017). Hahn, J. M. & Rettig, T. W. Tidal disruption of strengthless rubble piles - adimensional analysis.
Planet. Space Sci. , 1677–1682 (1998). Walsh, K. J. Rubble Pile Asteroids.
Annual Review of Astronomy and Astro-physics , 593–624 (2018). . Schenk, P. M., Asphaug, E., McKinnon, W. B., Melosh, H. J. & Weissman, P. R.Cometary Nuclei and Tidal Disruption: The Geologic Record of Crater Chainson Callisto and Ganymede.
Icarus , 249–274 (1996). Collins, G. S., Melosh, H. J. & Marcus, R. A. Earth Impact Effects Program:A Web-based computer program for calculating the regional environmental con-sequences of a meteoroid impact on Earth.
Meteoritics and Planetary Science , 817 (2005). Boslough, M. B. E. & Crawford, D. A. Low-Altitude Airbursts and the ImpactThreat.
International Journal of Impact Engineering , 1441–1448 (2008). Kresak, L. The Tunguska Object: a Fragment of Comet Encke?
Bulletin of theAstronomical Institutes of Czechoslovakia , 129 (1978). Asher, D. J. & Steel, D. I. On the possible relation between the Tunguska bolideand comet Encke.
P&SS , 205–211 (1998). Fern´andez, J. A. & Sosa, A. Magnitude and size distribution of long-periodcomets in Earth-crossing or approaching orbits.
MNRAS , 1674–1690 (2012). . Morbidelli, A. & Brown, M. E.
The kuiper belt and the primordial evolution ofthe solar system , 175 (2004). Stern, S. A. The evolution of comets in the Oort cloud and Kuiper belt.
Nature , 639–642 (2003). Kenyon, S. J., Bromley, B. C., O’Brien, D. P. & Davis, D. R.
Formation andCollisional Evolution of Kuiper Belt Objects , 293 (2008). Fraser, W. C. & Kavelaars, J. J. The Size Distribution of Kuiper Belt Objectsfor D gsim 10 km. AJ , 72–82 (2009). . Schlichting, H. E., Fuentes, C. I. & Trilling, D. E. Initial Planetesimal Sizesand the Size Distribution of Small Kuiper Belt Objects. AJ , 36 (2013). . Boe, B. et al.
The orbit and size-frequency distribution of long period cometsobserved by Pan-STARRS1.
Icarus , 252–272 (2019). . Brown, M. E. The Density of Mid-sized Kuiper Belt Object 2002 UX25 and theFormation of the Dwarf Planets.
ApJL , L34 (2013). . Wahlberg Jansson, K. & Johansen, A. Formation of pebble-pile planetesimals.
Astronomy & Astrophysics , A47 (2014). . Kenyon, S. J. & Bromley, B. C. The Size Distribution of Kuiper Belt Objects. AJ , 1916–1926 (2004). astro-ph/0406556 . Zubovas, K., Nayakshin, S. & Markoff, S. Sgr A* flares: tidal disruption ofasteroids and planets?
MNRAS , 1315–1324 (2012). . Zolensky, M., Bland, P., Brown, P. & Halliday, I.
Flux of Extraterrestrial Ma-terials , 869 (2006). Babadzhanov, P. B., Williams, I. P. & Kokhirova, G. I. Near-Earth Objects inthe Taurid complex.
MNRAS , 1436–1442 (2008). M¨uller, T. G. et al.
Large Halloween asteroid at lunar distance.
A&A , A63(2017). . Jenniskens, P. On the dynamics of meteoroid streams.
Earth, Planets, andSpace , 555–567 (1998). Ivanov, B. A. Numerical Modeling of the Largest Terrestrial Meteorite Craters.
Solar System Research , 381–409 (2005). Schulz, T. et al.
The Zhamanshin impact structure, Kazakhstan: A comparativegeochemical study of target rocks and impact glasses.
GCA , 209–229 (2020). Spitzer, F. et al.
Isotopic evolution of the inner Solar System inferred frommolybdenum isotopes in meteorites. arXiv e-prints arXiv:2006.13528 (2020). ..