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in the eccentricities of Jupiter and Saturn when they pass through a 1:2
mean motion resonance during their orbital migration
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under the gravi-
tational influence of a swarm of planetesimals. Such a resonance passage
would have destabilized the planetesimal disk beyond the orbits of the large
planets, causing a sudden massive delivery of cometary planetesimals to
the inner solar system. In this scenario, the asteroid belt is also destabi-
lized because of sweeping gravitational resonances; together, these cause
a major spike in the intensity of cometary as well as asteroid impacts on
the inner planets. The relative intensity of comets versus asteroids in the
projectile population of the LHB is not well determined in currently pub-
lished dynamical simulations. Because the impact signature of the crater
record in the inner solar system is asteroidal, we conclude that either comets
played a minor role or their impact record was erased by later-impacting
asteroids.
Another important implication of our results comes from the fact that
we have compared the SFD of 4 Gyr-old projectiles with that of the
current MBAs, and found them strikingly similar. This could mean that
there has been almost no collisional evolution in the main asteroid belt
over the last
4 Gyr, ever since the LHB. This is seemingly weird, but
recent numerical models of the collisional evolution of MBAs support this
fact,
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,
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revealing the rather stationary SFD of MBAs. Therefore, it is
probably safe to regard the SFD of the current MBAs as a fossil of the LHB
projectiles.
From the comparison between the SFDs of younger crater projectiles
and the current NEAs, it seems that NEAs have been the impacting source
of the newer craters since LHB ceased. Although most NEAs are con-
sidered to have originated in MBAs from a dynamical point of view,
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we have a greater number of smaller objects among the NEA popula-
tion than the MBA population, judging from the slope difference between
Figs. 2(a)-(d). This evidence, in addition to the wide variety of ages of the
young crater population, leads us to the conclusion that there has been a
size-dependent, long-term transport process that conveys MBAs (preferen-
tially smaller ones) to the inner solar system. A plausible candidate for this
kind of mechanism is the Yarkovsky effect, a (generally slow) dynamical
effect caused by the thermal time lag of asteroids when they absorb and re-
emit solar radiation. Since the Yarkovsky effect works much more effectively
on smaller asteroids, it is perfectly eligible to selectively transport small
asteroids from the main belt to the terrestrial planet region over a long
timespan.
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