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The Yarkovsky Effect - Part 6 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):

A 1 m radius barerock stony body with conductivity typical for ordinary
chondrites would take 20-50 Ma to move by 0.05 AU at the seasonal rate,
comparable to its disruption lifetime. In this case, the diurnal effect
is comparatively inefficient, because of the frequent spin-reorienting
collisions. We conclude that these bodies can be injected into
resonances only when starting from their vicinity. Of course, here
"starting" refers to ejection either from sizable asteroids - such as 6
Hebe, 13 Egeria, and 19 Fortuna - or from decameter-sized immediate
parent bodies, which may have themselves drifted across the belt while
shielding their interiors from cosmic-ray irradiation. On the other
hand, for meter-sized bodies with very low conductivities, the diurnal
effect becomes very efficient, and they may drift from any starting
location all the way to a resonance in a few tens of million years.
Stone meteorites created closest to resonances could drift all the way
to a resonance and be ejected before destruction. Collisional events on
asteroids very close to resonances (e.g., Hebe) can inject bodies
directly into the resonance or close enough to it to reach Earth on
short timescales <10 Ma. This could explain the 7-8 Ma peak of CRE ages
that is observed in the CRE age distribution of H chondrites (Graf and
Marti, 1995). The peak has an intriguing structure. The H5 chondrites in
the peak are observed to fall preferentially in the morning (61% - a
unique case among meteorites!); whereas the H3, H4, and H5 objects fall
preferentially in the afternoon (72%, higher than the average for all
chondrites). According to Morbidelli and Gladman (1998), the first
number is consistent with a scenario where H5 chondrites were injected
directly into the v6 resonance 7-8 Ma ago, whereas the second number is
consistent with H3/4/6 objects drifting into the resonance from a more
distant source that required 3-4 Ma of drift before reaching the
resonance. Possibly meteoroids of the latter group were ejected with
speeds that did not lead to direct injection and required an
intermediate phase of 3-4 Ma of Yarkovsky drift before hitting the
resonance.
A 20 m radius solid iron meteorite would take an estimated 500 Ma to
drift 0.2 AU to a resonance by the seasonal Yarkovsky effect. Because
the collisional lifetime is estimated to be > 1 Ga in this case, it
could easily reach a resonance before breaking up. This may be the
history of some of the iron meteorites associated with modest-sized
craters on Earth. Smaller-scale iron meteorites might have direct drift
timescales comparable to observed CRE ages if they started closer to a
resonance (possibly transported there by decameter-sized precursors) or
if their surface conductivity were lower due to porosity effects. It is
also possible that many iron meteorites, like the kilometer-sized
near-Earth asteroids, are inserted into Mars-crossing orbits through one
of many "weak" resonances active in the belt at moderate eccentricities
(Migliorini et al., 1998). Thus, a complex and coupled
dynamical/collisional history involving both Yarkovsky drift and
resonant effects can be inferred for iron fragments.

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