Experimental evidence for physical alteration of fine regolith on near-Sun asteroids

The surfaces of airless bodies evolve through processes such as (micro)meteoroid impacts and thermal fatigue, which modify physical properties such as grain size, porosity, and mineral texture. These properties provide clues to surface evolution and are necessary for interpreting telescopic observations. Near-Sun asteroids (NSAs), which reach heliocentric distances of 0.2 au, are notable targets, as their surfaces are likely modified by intense solar radiation. However, experimental studies of how radiative heating modifies the physical properties of carbonaceous asteroid surfaces remain scarce compared with studies of thermally induced compositional and spectral changes. We investigate these modifications experimentally. Near-Sun conditions at heliocentric distances 0.09<r<0.19 au are simulated using the Space and High-Irradiance Near-Sun Simulator. CM-type carbonaceous chondrite simulants are irradiated under vacuum. Brightness is monitored in situ with a video camera during heating, and morphological changes are examined using a stage microscope and a scanning electron microscope (SEM). The irradiated samples show initial darkening followed by brightening, consolidation, and morphological modifications from sub-micrometer to millimeter scales. The observed brightness modifications are more consistent with changes in the absorption properties of the sample than with variations in surface roughness. SEM imaging of the heated samples shows structures similar to those formed by sintering, which are completely absent in unheated samples. These results suggest that sintering is a likely candidate for the mechanism responsible for the observed consolidation and morphological changes. This work provides experimental evidence to help constrain the surface evolution of NSAs in general, and supports sintering as a possible mechanism proposed for (3200) Phaethon in particular.