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The Yarkovsky Effect - Part 4 of 7

W.K. Hartmann et al. (1999) Reviewing the Yarkovsky effect: New
light on the delivery of stone and iron meteorites from the asteroid
belt (MAPS 34, 1999, A161-A167, excerpts + summary):

Yarkovsky Effects And The Drift Toward Resonances

Early work suggested that long timescales (about 100 Ma) for Earth
delivery could arise when objects were moved by resonances onto
Mars-crossing orbits, then perturbed by Mars encounters, yet also noted
that resonances could directly send objects onto Earth-crossing orbits
in as little as 1 Ma.
As it began to be found that the timescale to move objects from their
original location in a resonance in the main belt to Earth was much
shorter, of the order 1 Ma, long CRE ages began to be seen as a problem.

The problem was made more acute when it became clear that most bodies
injected into the 3:1 and v6 main-belt resonances collide with the Sun,
with almost all the remainder ejected by Jupiter.
This has the consequence that only about 1% of the resonant asteroid
fragments eventually collide with the Earth, and almost all do so within
a few million years from their injection into resonance.
Morbidelli and Gladman (1998) found that the orbital distribution of
observed chondritic fireballs can be reconstructed assuming a constant
injection rate into the v6 and 3:1 resonances.

The typical fall times of meteorites are consistent with such a model.
However, this scenario predicts that unless there is a preexposure phase
in the main belt, most meteorites would have exposure ages of only a few
million years, which is inconsistent with observed longer ages. In
general, these ages are 100 to 1000 Ma for most iron meteorites, 5 to 50
Ma for most stone meteorites, about 7-8 Ma for a subgroup of H
chondrites clustered in CRE age.
Morbidelli and Gladman thus concluded that meteorites cannot be directly
injected into the resonances by the event that releases them from their
shielded positions in parent bodies but must have some mechanism for
delaying resonance arrival for periods up to tens of million years for
stone meteorites and up to 1000 Ma for iron meteorites, after their
release.

Thus, conventional models without Yarkovsky drift don't explain:

(a) how iron meteorites could experience hundreds of million years of
CRE, and
(b) why iron meteorites have distinctly longer CRE ages than stone
meteorites.

The Yarkovsky effect, combined with differential impact erosion,
explains these anomalies because it allows fragments of 0.1 to 100 m
scale to be created relatively far from resonances and then to drift in
the belt until they reach resonances.
Because iron meteorites are stronger and have slow drift rates, they may
escape break-up and drift for tens of millions of years to 1000 Ma
before reaching a resonance, depending on initial location.
Because stone meteorites are weaker, the meter-scale mobile bodies may
be destroyed sooner than this, giving us a shorter range of observed CRE
ages and preferentially sampling parent asteroids that are near
resonances.
Recent break-ups near resonances could create clusters of objects. The 8
Ma old group of H chondrites is consistent with a recent break-up near a
resonance.

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