Surface thermal properties of near-Earth asteroids

Asteroids up to a few tens of meters in diameter may spin very fast,
completing an entire rotation within a few minutes.
It is not clear whether small and fast rotators are able to keep dust and
small gravel particles (regolith) on their surface, or rather if these materials are
ejected in outer space due to the fast rotation.

To undertand surface properties of asteroid, we developed a model for constraining the
thermal conductivity of small, fast-rotating near-Earth asteroids.
Our approach is based on the comparison between the measured Yarkovsky
drift and a value predicet by using a theoretical model, that depends on
the orbital, physical and thermal parameters of the object.
The necessary parameters are either deduced from statistical distribution
derived for near-Earth asteroids population or determined from observations with associated uncertainty.
With this information available, we performed Monte Carlo simulations,
and produced a probability density distribution for the thermal conductivity.
The results may suggest whether regolith is likely present on these objects.

Besides, since measurements of the Yarkovsky effect became common thanks to modern
astrometry, our model can be applyed to a large sample of near-Earth asteroids.
This may permit to understand global properties of the NEO population, and investigate if
thermal inertia and surface properties depend on the size or the rotation period of the
object.

Asteroid 2011 PT. Asteroid (499998) 2011 PT is a small object with 35 meters
diameter, that rotates with a period of 11 minutes. Applying our model, we found that the measured
Yarkovsky drift can only be achieved when the thermal conductivity K of the surface is low.
The resulting probability density function for the conductivity is bimodal, with two most likely
values being around 0.0001 and 0.005 W m^{−1} K^{−1} (see Figure 1, top
panel).
Moreover, the probability of K being smaller than 0.1 W m^{−1} K^{−1} is at least 95 per cent.
Translating the results in terms of thermal inertia, we constrained it to be either
11^{+7}_{-5} or 88^{+90}_{-45} J m^{−2}
K^{−1} s^{−1/2} (see Figure 1, bottom panel)
This result was unexpected, and it could indicate that the surface of 2011 PT is covered with a
thermal insulating layer, composed of a regolith-like material similar to lunar dust.