Vol 435|26 May 2005|doi:10.1038/nature03676

LETTERS
Origin of the cataclysmic Late Heavy Bombardment
period of the terrestrial planets
R. Gomes1,2, H. F. Levison2,3, K. Tsiganis2 & A. Morbidelli2

The petrology record on the Moon suggests that a cataclysmic
spike in the cratering rate occurred ,700 million years after theplanets formed1; this event is known as the Late Heavy Bombardment (LHB). Planetary formation theories cannot naturally
account for an intense period of planetesimal bombardment solate in Solar System history2. Several models have been proposed
to explain a late impact spike3–6, but none of them has been set
within a self-consistent framework of Solar System evolution.
Here we propose that the LHB was triggered by the rapid
migration of the giant planets, which occurred after a longquiescent period. During this burst of migration, the planetesimal
disk outside the orbits of the planets was destabilized, causing asudden massive delivery of planetesimals to the inner Solar
System. The asteroid belt was also strongly perturbed, withthese objects supplying a significant fraction of the LHB impactors
in accordance with recent geochemical evidence7,8. Our model
not only naturally explains the LHB, but also reproduces theobservational constraints of the outer Solar System9.
Previous work9 explains the current orbital architecture of theplanetary system by invoking an initially compact configuration inwhich Saturn’s orbital period was less than twice that of Jupiter. Afterthe dissipation of the gaseous circumsolar nebula, Jupiter’s andSaturn’s orbits diverged as a result of their interaction with a massive
disk of planetesimals, and thus the ratio of their orbital periods,
P S/P J, increased. When the two planets crossed their mutual 1:2
mean motion resonance (1:2 MMR, that is, P S/P J ¼ 2) their orbits
became eccentric. This abrupt transition temporarily destabilized thegiant planets, leading to a short phase of close encounters among
Saturn, Uranus and Neptune. As a result of these encounters, and of
the interactions of the ice giants with the disk, Uranus and Neptune
reached their current heliocentric distances and Jupiter and Saturnevolved to their current orbital eccentricities9. The main idea of thisLetter is that the same planetary evolution could explain the LHB,
provided that Jupiter and Saturn crossed the 1:2 MMR roughly
700 Myr after they formed. Thus, our goal is to determine if there is a
generic mechanism that could delay the migration process.
In previous studies9–12, planet migration started immediately
because planetesimals were placed close enough to the planets tobe violently unstable. Although this type of initial condition was
reasonable for the goals of those studies, it is unlikely. Planetesimaldriven migration is probably not important for planet dynamics aslong as the gaseous massive solar nebula exists. The initial conditions
for the migration simulations should represent the system thatexisted at the time the nebula dissipated. Thus, the planetesimal
disk should contain only those particles that had dynamical lifetimes
longer than the lifetime of the solar nebula. In planetary systems likethose we adopt from ref. 9, we find that they had to be beyond ,15.3
AU (Fig. 1), leading to the initial conditions illustrated in Fig. 2a.
In this configuration, the initial speed of migration would be

Figure 1 | Disk location and LHB timing. a, The histogram reports theaverage dynamical lifetime of massless test particles placed in a planetarysystem (shown as triangles) with Jupiter, Saturn and the ice giants on nearlycircular, co-planar orbits at 5.45, 8.18, 11.5 and 14.2 AU, respectively.
Initially, we placed 10 particles with e ¼ i ¼ 0 (where e is eccentricity and i isinclination) and random mean anomaly at each semimajor axis. Stable
Trojans of the planets have been removed from this computation. Eachvertical bar in the plot represents the average lifetime for those 10 particles.
We define ‘dynamical lifetime’ as the time required for a particle toencounter a planet within a Hill radius. A comparison between thehistogram and the putative lifetime of the gaseous nebula20 argues that,when the latter dissipated, the inner edge of the planetesimal disk had to beabout 1–1.5 AU beyond the outermost ice giant. b, Time at which Jupiter andSaturn crossed the 1:2 MMR, as a function of the location of theplanetesimal disk’s inner edge, as determined from our first set of migration
simulations. In all cases, the disk had a surface density equivalent to 1.9 M E
per 1 AU annulus. The outer edge of the disk was varied so that the total massof the disk was 35 M E. The disk was initially very dynamically cold, withe ¼ 0 and i , 0.58. A comparison between a and b shows that a disk thatnaturally should exist when the nebula dissipated would produce a 1:2 MMR
crossing at a time comparable to that of the LHB event.

1

ON/MCT and GEA/OV/UFRJ, Ladeira do Pedro Antonio, 43 Centro 20.080-090, Rio de Janeiro, RJ, Brazil. 2Observatoire de la Côte d’ Azur, CNRS, BP 4229, 06304 Nice
Cedex 4, France. 3Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, Colorado, USA.

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Vol 435|26 May 2005|NATURE

dependent on the rate at which disk particles evolve onto planetcrossing orbits. The time at which Jupiter and Saturn cross their 1:2
MMR depends on: (1) their initial distance from the location of theresonance, (2) the surface density of the disk near its inner edge, and(3) the relative location of the inner edge of the disk and the outer ice
giant. On the basis of the above arguments, we initially performed aseries of eight simulations where the location of the inner edge of thedisk was set as the unique free parameter (Fig. 1). As expected, we
found a strong correlation between the location of the inner edge andthe time of the 1:2 MMR crossing. For disks with inner edges near
15.3 AU (see above), we find crossing times between 192 Myr and
880 Myr (since the beginning of the simulation).
We also performed eight simulations where we varied the initial
location of the ice giants by ,1 AU, Saturn’s location by ,0.1 AU, thetotal mass of the disk by 5 Earth masses (5M E), and its initial
dynamical state by pushing the particles’ eccentricities up to 0.1
and inclinations up to 3.58. We found that we can delay the resonantcrossing to 1.1 Gyr since the beginning of the simulation, although
longer times are clearly possible for more extreme initial conditions.
Therefore, we can conclude that the global instability caused by the
1:2 MMR crossing of Jupiter and Saturn could be responsible forthe LHB, because the estimated date of the LHB falls in the range ofthe times that we found.
Figures 2 and 3 show the evolution of one of our runs from the firstseries of eight. Initially, the giant planets migrated slowly owing toleakage of particles from the disk (Fig. 3a). This phase lasted 880 Myr,at which point Jupiter and Saturn crossed the 1:2 MMR. After the

resonance crossing event, the orbits of the ice giants became unstable
and they were scattered into the disk by Saturn. They disrupted thedisk and scattered objects all over the Solar System, including
the inner regions. The solid curve in Fig. 3b shows the amountof material that struck the Moon as a function of time. A total of
9 £ 1021 g struck the Moon after resonance crossing—roughly 50%of this material arrived in the first 3.7 Myr and 90% arrived before
29 Myr. The total mass is consistent with the estimate4 of 6 £ 1021 g,which was determined from the number and size distribution oflunar basins that formed around the time of the LHB epoch1. Such an
influx spike happened in all our runs. The amount of cometary
material delivered to the Earth is ,1.8 £ 1023 g, which is about 6% of
the current ocean mass. This is consistent with upper bounds on the
cometary contribution to the Earth’s water budget, based on D/Hratio measurement13. The average amount of material accreted by theMoon during this spike was (8.4 ^ 0.3) £ 1021 g.
The above mass delivery estimate corresponds only to the cometary contribution to the LHB, as the projectiles originated fromthe external massive, presumably icy, disk. However, our scheme
probably also produced an in flux of material from the asteroid belt.
As Jupiter and Saturn moved from 1:2 MMR towards their current
positions, secular resonances (which occur when the orbit of anasteroid processes at the same rate as a planet) swept across the entire
belt14. These resonances can drive asteroids onto orbit with eccentricities and inclinations large enough to allow them to evolve into
the inner Solar System and hit the Moon4.
We investigated the role of asteroid impactors in our LHB model

Figure 2 | The planetary orbits and the positions of the disk particles,
projected on the initial mean orbital plane. The four panels correspond tofour different snapshots taken from our reference simulation. In this run,the four giant planets were initially on nearly circular, co-planar orbits withsemimajor axes of 5.45, 8.18, 11.5 and 14.2 AU. The dynamically coldplanetesimal disk was 35M E, with an inner edge at 15.5 AU and an outer edge

at 34 AU. Each panel represents the state of the planetary system at four
different epochs: a, the beginning of planetary migration (100 Myr); b, justbefore the beginning of LHB (879 Myr); c, just after the LHB has started(882 Myr); and d, 200 Myr later, when only 3% of the initial mass of the disk
is left and the planets have achieved their final orbits.

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by the following numerical integrations. The orbits of an asteroid
belt, composed of 1,000 massless particles with semimajor axes
between 2.0 and 3.5 AU, were integrated under the gravitational
influence of the Sun, Venus, Earth, Mars, Jupiter and Saturn. Because
formation models15,16 predict that the asteroid belt was partiallydepleted and dynamically excited well before the LHB, we set theparticles’ eccentricities between 0 and 0.3 and inclinations between 08
and 308, but kept the perihelion distances, q, .1.8 AU and aphelion
distances, Q, ,4 AU. Jupiter and Saturn were forced to migrate at
rates that varied from run to run (adopted from ref. 9) by adding asuitably chosen drag-force term to their equations of motion.
We find that objects that reach Earth-crossing orbits follow one of
two general paths. Some, referred to as class 1 particles, get trapped in

Figure 3 | Planetary migration and the associated mass flux towards theinner Solar System from a representative simulation. a, The evolution of
the four giant planets. Each planet is represented by a pair of curves—the top
and bottom curves are the aphelion and perihelion distances, Q and q,
respectively. Jupiter and Saturn cross the 1:2 MMR at 880 Myr. The
subsequent interaction between the planets and the disk led to the current
planetary configuration as shown in ref. 9. b, The cumulative mass of comets(solid curve) and asteroids (dashed curve) accreted by the Moon. We haveoffset the comet curve so that the value is zero at the time of 1:2 MMR
crossing. Thus, ,5 £ 1021 g of comets was accreted before resonant crossing
and 9 £ 1021 g of cometary material would have struck the Moon during the
LHB. Although the terrestrial planets were not included in our cometary
simulations, we estimated the amount of material accreted by the Moon
directly from the mass of the planetesimal disk by combining the particles’
dynamical evolution with the analytic expressions in ref. 21. The impact
velocity of these objects ranged from 10 to 36 km s21 with an average of
21 km s21. Estimating the asteroidal flux first requires a determination of themass of the asteroid belt before resonant crossing. This value wasdetermined by first combining the percentage of asteroids remaining in thebelt at the end of a simulation (,10%, very sensitive to planet migration rateand initial asteroid distribution) with estimates of the current mass of thebelt to determine the initial asteroid belt mass (,5 £ 1023 M E). The flux
was then again determined by combining the particles’ dynamical evolution
with the analytic expressions in ref. 21. The dashed curve shows a simulation
where class 2 particles dominate. The average asteroidal impact velocity is
25 km s21.
468

the periapse secular resonance with Saturn (which affects eccentricities) and are driven directly onto Earth-crossing orbits. Other
particles, referred to as class 2, stay in the asteroid belt, but are
dynamically excited by resonant sweeping onto unstable orbits.
These objects slowly leak out of the asteroid belt and can evolve
into the inner Solar System. The two classes produce impact spikes
with different temporal behaviours. Roughly 50% of class 1 particles
arrive in the first 10 Myr, while 90% arrive within ,30 Myr. Conversely, the median arrival time for class 2 particles is ,50 Myr and
90% arrive within ,150 Myr. Class 2 particles dominated in our runs
(Fig. 3). However, a preliminary investigation into this issue showsthat this result is probably sensitive to the exact evolution of the giant
planets and the dynamical state of the asteroid belt. Thus, the best we
can conclude is that the impact spike due to asteroids is between these
two extremes.
We find that (3–8) £ 1021 g of asteroids hit the Moon during oursimulations (Fig. 3). This amount is comparable to the amount ofcomets. So, our model predicts that the LHB impactors should havebeen a mixture of comets and asteroids. Unfortunately, we cannot say
with any certainty the exact ratio of comets to asteroids in our model
because, although the amount of cometary material is fairly wellconstrained (probably better than a factor of 2), the amount ofasteroidal material is not well known (and could be outside the range
reported above), because we do not have good estimates of the mass
distribution in the asteroid belt before the LHB. It should also be
noted that this ratio is probably a function of impactor size, because
comets and asteroids probably have different size distributions. Thisratio probably also varied with time. Within the first ,30 Myr comets
dominated according to these simulations, but the last impactors
were asteroidal. This is consistent with recent cosmochemical findings suggesting that some of the Moon’s basins were formed byasteroids7,8.
Our results support a cataclysmic model for the lunar LHB.
Although many aspects of the LHB are not well known1, oursimulations reproduce two of the main characteristics attributed tothis episode: (1) the 700 Myr delay between the LHB and terrestrial
planet formation, and (2) the overall intensity of lunar impacts. Ourmodel predicts a sharp increase in the impact rate at the beginning of
the LHB. Unfortunately, the available lunar data are not yet capableof addressing this prediction.
Our model also has the advantage of supplying impactors that area mixture of comets and asteroids. Our model predicts that theasteroid belt was depleted by a factor of ,10 during the LHB. This
depletion does not contradict collisional evolution models17,18. On
the contrary, the late secular resonance sweeping could explain why
we do not see a large number of asteroid families that were produced
during the LHB18. Our model predicts that the LHB lasted frombetween ,10 Myr and ,150 Myr. Correspondingly, the drop-off in
impact rates could be quite fast (with 50% of the impacts occurringin the first 3.7 Myr and 90% in 29 Myr) or moderately slow (with
50% of the impacts occurring in the first 50 Myr and 90% in 150 Myr)
We are unable to pinpoint more exact values because the durationand the drop-off of the LHB depends on the relative contributions ofclass 1 asteroids, class 2 asteroids, and comets, which in turn are very
sensitive to the pre-LHB orbital structure of the asteroid belt.
Most importantly, our scheme for the LHB is the result of a genericmigration-delaying mechanism, followed by an instability, which isitself induced by a deterministic mechanism of orbital excitation of
the planets9.This revised planetary migration scheme naturally
accounts for the currently observed planetary orbits9, the LHB, thepresent orbital distribution of the main-belt asteroids and the origin
of Jupiter’s Trojans19.
Received 6 December 2004; accepted 18 April 2005.
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Hartmann, W. K., Ryder, G., Dones, L. & Grinspoon, D. in Origin of the Earth andMoon (eds Canup, R. & Righter, K.) 493–-512 (Univ. Arizona Press, Tucson,
2000).

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Acknowledgements R.G. thanks Conselho Nacional de Desenvolvimento
Cientı́fico e Tecnológico for support for his sabbatical year in the OCA
observatory in Nice. K.T. was supported by an EC Marie Curie Individual
Fellowship. A.M. and H.F.L. thank the CNRS and the NSF for funding
collaboration between the OCA and the SWRI groups. H.F.L. was supported by
NASA’s Origins and PG&G programmes.
Author Information Reprints and permissions information is available at
npg.nature.com/reprintsandpermissions. The authors declare no competing
financial interests. Correspondence and requests for materials should be
addressed to A.M. (morby@obs-nice.fr).

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