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

The surfaces of airless bodies are directly exposed to the space environment and evolve through various processes such as (micro-)meteoroid impacts and thermal fatigue. These processes can modify physical properties such as the grain size, porosity, and mineral texture. Studying the physical properties can therefore provide clues as to the evolution of airless surfaces, and understanding them is also necessary for interpreting telescopic observations of the bodies. Near-Sun asteroids (NSAs), which approach the Sun at heliocentric distances of łesssim0.2,au, are notable targets in this regard, 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 changes in mineral composition and the resulting spectral alterations. This study experimentally investigates how intense radiative heating at near-Sun distances modifies the physical properties of carbonaceous asteroid surfaces. Near-Sun conditions at heliocentric distances of 0.09<r<0.19,au were simulated in the laboratory using the Space and High-Irradiance Near-Sun Simulator. CM-type carbonaceous chondrite simulants were irradiated under vacuum. Brightness changes were monitored in situ with a video camera during heating, and morphological changes were 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 rather than 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 the sintering hypothesis as a possible mechanism proposed for (3200) Phaethon in particular.