  Knowledge of the surface thermal inertia of an asteroid can provide insight
into surface structure: porous material has a lower thermal inertia than rock.
We develop a means to estimate thermal inertia values of asteroids and use it
to show that thermal inertia appears to increase with spin period in the case
of main-belt asteroids (MBAs). Similar behavior is found on the basis of
thermophysical modeling for near-Earth objects (NEOs). We interpret our results
in terms of rapidly increasing material density and thermal conductivity with
depth, and provide evidence that thermal inertia increases by factors of 10
(MBAs) to 20 (NEOs) within a depth of just 10 cm. Our results are consistent
with a very general picture of rapidly changing material properties in the
topmost regolith layers of asteroids and have important implications for
calculations of the Yarkovsky effect, including its perturbation of the orbits
of potentially hazardous objects and those of asteroid family members after the
break-up event. Evidence of a rapid increase of thermal inertia with depth is
also an important result for studies of the ejecta-enhanced momentum transfer
of impacting vehicles ("kinetic impactors") in planetary defense.
