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Supplementary Information Supplementary Information SUPPLEMENTARY INFORMATION doi:10.1038/nature10967 Supplementary Information 1 The Role of Comets in the Late Heavy Bombardment The Nice model describes a plausible scenario where the Jovian planets experienced a global instability that 1led to The a reorganization Role of of Comets the outer solar in system; the planets Late moved, Heavy existing Bombardment small body reservoirs were depleted or eliminated, and new reservoirs were created in distinct locations [1, 10]. The putative trigger for this event Theis thought Nice model to be describes gravitational a plausible interactions scenario between where the giantJovian planets planets and experienced a planetesimal a global disk instability of several that tens ledof to Earth a reorganization masses residing of the just outer outside solar the system; initial planets orbits moved, of the giant existing planets. small bodyThis primordialreservoirs were disk, depleted made of orcomet-like eliminated, objects, and new was reservoirs scattered were throughout created in the distinct solar system locations by [1, planet 10]. migration.The putative trigger for this event is thoughtThe Nice to be model gravitational is compelling interactions because between it can the quantitatively giant planets explain and a planetesimal the orbits of disk the of Jovian several planets tens of[10, Earth 32], masses the capture residing of comets just outside from the the initial primordial orbits disk of the into giant several planets. different This small primordial body reservoirs disk, made in theof comet-likeouter solar objects, system was (e.g., scattered Trojans throughout of Jupiter [33]the solar and Neptune system by [34], planet the Kuipermigration. belt and scattered disk [35], theThe irregular Nice model satellites is compellingof the giant because planets [36, it can 37], quantitatively and the outer explain asteroid the belt orbits [38]). of It thehas alsoJovian been planets linked [10,in time 32], the to the capture so-called of comets Late Heavy from the Bombardment primordial disk (LHB) into of several the Moon different and small terrestrial body planets reservoirs [1]. in These the outeraccomplishments solar system are (e.g., unique Trojans among of Jupiter models [33] of outer and Neptune solar system [34], formation. the Kuiper belt and scattered disk [35], the irregularThe existence satellites of the of the terrestrial giant planets planets [36, on 37],their and current the outer orbits, asteroid as well belt as the [38]). dynamical It has also sculpting been linked of the inasteroid time to belt the by so-called sweeping Late resonances, Heavy Bombardment also provide (LHB)constraints of the for Moon the Nice and model. terrestrial These planets issues [1]. have These been accomplishmentsexplored in considerable are unique detail among by [11,models 12, of 17, outer 32, 39]. solar Their system results formation. provide an important context for our approachThe existence because of they the terrestrial show that planets solutions on for their the current Nice model orbits, can as reproducewell as the current dynamical conditions, sculpting provided of the asteroidthat the belt giant by planets sweeping migrate resonances, from their also original provide orbits constraints to their for current the Nice orbits model. in less These than issues 1 My. have been exploredA potential in considerable problem withdetail the by original [11, 12, Nice 17,model, 32, 39]. however, Their results concerns provide its use an of important comets as a context key component for our approachof the LHB because in the they inner show solar that system. solutions This scenario for the Nice may model be discordant can reproduce with the current ancient conditions, crater size-frequency provided thatdistributions the giant(SFDs) planetsfound migrate on from Mercury, their the original Moon, orbits and to Mars. their For current example, orbits [20] in argue less than they 1 closely My. resemble theA SFDs potential of asteroids problem within the the main original belt, Nice and model, not the however, crater SFDs concerns found its useon the of comets outer planetas a key satellites component1. If ofcomets the LHB and in asteroids the inner indeed solar system. have contrasting This scenario SFDs, may one be can discordant potentially with use the this ancient difference crater to size-frequency test the Nice distributionsmodel as well (SFDs) as set limits found on on the Mercury, size of the LHB-eraMoon, and comet Mars. impactor For example, population [20] argueand/or they the closelyinitial primordial resemble thedisk SFDs population of asteroids just before in the the main LHB. belt, and not the crater SFDs found on the outer planet satellites1. If cometsFor and this asteroids calculation, indeed we assumedhave contrasting that comets SFDs, contribute one can potentially< 10% of the use ancient this difference lunar crater to test populations the Nice modelproduced as well by as the set LHB. limits Thus, on the their size signature of the LHB-era would comet presumably impactor be population mixed in and and/or lost theamong initial the primordial crater size diskfrequency population distribution just before produced the LHB. by asteroids. The number of D>20 km craters found on the most ancient 4 2 4 2 lunarFor surfaces this calculation, is 2-3 10 we− assumedkm− , while thatcomets those on contribute terrains near< 10% Nectaris of the basin ancient are lunar 1 10 crater− km populations− [28]. The × × producedsurface area by the of the LHB. Moon Thus, is 3 their.8 10 signature7 km2, while would recently presumably calculated be mixed impact in andprobabilities lost among between the crater the Moon size × 8 frequencyand test bodiesdistribution scattered produced out of by the asteroids. primordial The comet number disk of duringD>20 the km LHB craters are found (8 3) on the10− most(e.g., ancient [32]). 4 2 ± ×4 2 lunarNote surfaces that the is lunar 2-3 impact10− km probabilities− , while those above on may terrains change near as new Nectaris variants basin of the are Nice 1 10 model− km are− explored[28]. The in × × surfaceadditional area detail. of the MoonIf we assume is 3.8 no107 cometskm2, while disrupt recently en route calculated to the inner impact solar probabilities system, the between number the of cometsMoon × 9 8 10 andcapable test bodies of making scatteredD>20 out km of lunar the primordial craters in comet the disk disk had during to be the limited LHB to are (83 103) to10− 2(e.g.,10 [32]).. We ∼ ×± × ∼ × Notecall thesethat the values lunarNneed impact. probabilities above may change as new variants of the Nice model are explored in additionalThe number detail. Ifof we comets assume in no the comets primordial disrupt disk en at route the to time the of inner the solar LHB system, can be the roughly number estimated of comets by capable of making D>20 km lunar craters in the disk had to be limited to 3 109 to 2 1010. We assuming that the Nice model made the scattered disk, the likely source of most∼ × Jupiter-family∼ × comets. As call these values Nneed. 1As an aside, we point out that recent work suggests that, when all factors are taken into account, comets hitting the Moon mayThe actually number have ofa main comets belt-like in the SFD primordial (J. Richardson, disk personal at the communication). time of the LHB The distinguishing can be roughly characteristic estimated would by be assumingthat the power that law the slopes Nice of model the cometary made SFDthe scattered are more extreme disk, the than likely those of source a main of belt-like mostJupiter-family SFD; that is, the steepcomets. branches As of the cometary SFD are steeper than the main belt and the shallow branches are shallower. Further modeling is needed to determine1As an aside, whether we point such aout SFD that can recent easily work be “hidden” suggests that, when when covered all overfactors by are a younger taken into main account, belt-like comets SFD. hitting the Moon may actually have a main belt-like SFD (J. Richardson, personal communication). The distinguishing characteristic would be that the power law slopes of the cometary SFD are more extreme than those of a main belt-like SFD; that is, the steep branches of the cometary SFD are steeper than the main belt and the shallow branches are shallower. Further modeling is needed to determine whether such a SFD can easily be “hidden” when covered over by a younger main belt-like SFD. WWW.NATURE.COM/NATURE | 1 RESEARCH SUPPLEMENTARY INFORMATION estimated by [16], the number of kilometer-sized comets in the scattered disk is 1 109, with perhaps a ∼ × factor of 5 uncertainty. The efficiency of placing primordial disk objects into the scattered disk has been 3 10 estimated from numerical simulations to be 4 10− [40]. Together, they yield a range of 5 10 to ∼ × ∼ × 1 1012. ∼ × The comet size needed to make a D>20 km lunar crater is poorly known, but a reasonable estimate 3 is D>2 km, based on standard cometary bulk densities of 0.6 g cm− [41], lunar impact velocities of 1 25 km s− [32], and Pi-group crater-scaling law relationships [30]. The size distribution of the primordial disk just before the LHB for 1 to 2 km diameter projectiles is unknown, but a conservative estimate is that the ratio of D>1 km comets to D>2 km comets is 2 to 4. Thus, this suggests the number of comets capable of making D>20 km lunar craters in the primordial disk just before the LHB ranged between 1 1010 to 5 1011. We call this value N . ∼ × ∼ × estimate For LHB-era asteroid impactors to dominate comet impactors, N N . Interestingly, the estimate ≤ need ranges of Nestimate and Nneed slightly overlap, suggesting a potential solution set is possible.
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