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The Yarkovsky Effect - Part 3 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):

Recent Studies Of Related Effects: The "Seasonal" Yarkovsky Effect

Rubincam (1995a) considered a new variant of the Yarkovsky effect: a
component of the force associated with thermal re-radiation acting along
the polar axis for asteroids with nonzero obliquity. It arises because
the hemisphere experiencing "autumn" is hotter (and radiates more
thermal energy) than the hemisphere experiencing "spring."
Rubincam (1995b) and Farinella et al. (1998) called this the "seasonal"
Yarkovsky effect, to distinguish it from the "diurnal" effect associated
with "low-obliquity" rotation.
The seasonal Yarkovsky effect is maximized at 90° obliquity and vanishes
at zero obliquity. Unlike the diurnal effect, it does not depend on spin
rate and it does not ever produce increasing semimajor axis; the
seasonal effect produces only inward drift.
For a regolith-free basaltic asteroid of 20 m in diameter, Rubincam
found the time to reduce the semimajor axis from 3 to 1 AU is about 800
Ma, and a still longer time for an iron meteorite of this size. However,
chondrites have much shorter observed CRE ages, often <20 Ma, and he
therefore concluded that the seasonal Yarkovsky effect is not important
for chondrites. Rubincam (1995a) also devoted one paragraph to the fact
that asteroids do not have to go from 3 to 1 AU by this effect, but only
from mid-belt to a resonance. He cited a characteristic time of the
order 100 Ma for a 10 m scale body to drift a few tenths of an
astronomical unit to a resonance. Although Rubincam thus opened the door
to the possibility of Yarkovsky drift delivering small bodies to
resonances, he mainly emphasized longer timescales associated with drift
over relatively large distances by fairly large bodies (10 to 100 m in
diameter; see also Rubincam, 1998).
Farinella et al. (1998) recognized shorter timescales associated with
smaller (1 m) bodies starting closer to resonances. They computed drift
rates for a variety of parameters, including both stone and metal
bodies, various sizes, diurnal (low obliquity) and seasonal (high
obliquity) cases, fast and slow rotation, and regolith vs. nonregolith
surfaces. An important point to note in summary is that the dominant
effect on any given object could alternate between diurnal and seasonal
if collisions alter the rotation state. When the seasonal effect
dominates (obliquity near 90°), the drift would be slowly inward toward
the Sun; but if collisions place the body in various diurnally-dominated
states (obliquity near 0° or 180°), it could random walk inward and
outward, with the rate of this random walk higher for longer spin
periods.

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