Colour and Time: The Evolution of Crater Rays on the Moon and the Asteroid 4Vesta

The solid surfaces of airless bodies continuously evolve due to bombardment by objects from space and solar wind.  We see evidence of this evolution in the differences in colour between surfaces known to be younger (such as freshly excavated crater rays) and older surfaces.  In a recent publication [1], we presented a new complete catalogue of the Moon’s rayed craters with diameters of 5 km and greater between ±50 degrees of the equator. In ongoing work, we are creating a catalogue of the rayed craters with diameters 2 km and greater on the asteroid 4Vesta. We use these catalogues and a model of impact gardening to examine how quickly the surfaces of large rocky bodies like the Moon and smaller rocky bodies like the asteroid 4Vesta evolve over timescales of years to billions of years.

Here, we present the results of the quantitative analysis of the maturity and composition of the lunar rayed crater population through the lense of diverse remote sensing datasets. Perhaps unsurprisingly, we find that the most charismatic rays have the least nanophase iron (also denoted ‘npFe’; i.e., they are the least mature). More compelling, however, is that the most charismatic rays include diverse and distinguishable mineralogical contrasts, for example, rays in both plagioclase, olivine, and FeO abundances. Further, regardless of whether the mineralogical contrast is high or low (i.e., a dark or bright ray), maturity is suppressed. As rays degrade, they appear more thermophysically and mineralogically homogenous; however, faint thermophysical and mineralogical contrasts can persist longer than it takes regolith to saturate with nanophase iron and disappear into the optically mature background. We demonstrate that comparative analysis of rayed crater populations can help us distinguish the timescale for various space weathering thresholds, such as the destruction of a thermophysical ray, the saturation of nanophase iron, and the homogenisation of mineralogical contrasts.

[1] Ghent, R. R., Costello, E. S., & Parker, A. H. (2024). The Population of Young Craters on the Moon: New Catalog and Spatial and Temporal Analysis. The Planetary Science Journal, 5(4), 89.