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The Yarkovsky Effect - Part 2 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):

The "classic" or "diurnal" Yarkovsky effect

The "classic" Yarkovsky effect is a change in orbital elements, notably
semimajor axis, of a rotating asteroidal body in the range of
centimeters up to tens of meters, due to asymmetries between the
longitudes of sunlight absorption and the longitudes of maximum thermal
re-radiation.
Because the "afternoon" temperature tends to be higher than in the
"morning" quadrant, the thermal radiation produces a nonradial force on
the body. This "clasic" effect, due to rotation, has been called the
"diurnal" Yarkovsky effect to distinguish it from other effects
discovered later.
The diurnal effect is maximized at zero (or 180°) obliquity of the spin
axis and vanishes at 90° obliquity.

An interesting quality of the classic Yarkovsky effect is that it
depends on the sense of rotation - prograde or retrograde:

- Prograde rotation causes a drift away from the Sun;
- Retrograde causes a drift toward the Sun.

The effect is much stronger on stone (especially dark stones) than on
iron meteorites, because iron meteorites have much larger thermal
conductivity, tending to erase the asymmetry mentioned above.
Peterson (1976) concluded that a rotating 1 m diameter black stony
sphere required 30 Ma to move from 3 AU to 1 AU, whereas a similar iron
body took roughly 1600 Ma in his calculation. For larger stony bodies,
Peterson noted that the timescale is proportional to size; a 10 m black
stone would thus take 300 Ma to move from 3 AU to 1 AU.
Peterson noted that the timescales he calculated for stone and iron
meteorites were similar to the actually observed cosmic-ray exposure
(CRE) ages for these bodies; thus he suggested that direct drift from
the belt to Earth by the Yarkovsky effect might be the dominant
mechanism for the delivery of meteorites to Earth.
Later workers realized that Jovian resonances are a major, fast-acting
agent in moving fragments from certain parts of the belt directly into
planet-crossing orbits. Thus, we now see that the importance of the
Yarkovsky effect may be to deliver small fragments from their points of
collisional origin in the belt to nearby resonances.
What matters in determining CRE timescales is not the time to drift from
the belt to 1 AU, but the time to drift from a source region (a
collision site) in the belt to the nearest resonance that may deliver
the object to Earth.
Instead of moving 2 AU to reach an Earth-crossing orbit, the object
might typically need to move only 0.2 AU in orbital semimajor axis to
reach a resonance, and even less if the parent body already lies in the
vicinity of one of the main resonances.
The actual time for Yarkovsky drift to deliver a body to a resonance
will involve collisional spin-up and pole reorientations resulting into
random walk effects.

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