  Airless planetary bodies are covered by a dusty layer called regolith. The
grain size of the regolith determines the temperature and the mechanical
strength of the surface layers. Thus, knowledge of the grain size of planetary
regolith helps to prepare future landing and/or sample-return missions. In this
work, we present a method to determine the grain size of planetary regolith by
using remote measurements of the thermal inertia. We found that small bodies in
the Solar System (diameter less than ~100 km) are covered by relatively coarse
regolith grains with typical particle sizes in the millimeter to centimeter
regime, whereas large objects possess very fine regolith with grain sizes
between 10 and 100 micrometer.
