On the difference between internally or externally mixed darkening agent on regolith reflectance

The regolith on the surface of atmosphereless Solar System objects, such as asteroids, moons, and some planets, is affected by space weathering. On iron-bearing minerals such as olivine, space weathering introduces nano- and micron-sized free iron inclusions in the thin surface layer of the host material. These iron inclusions will darken the overall reflectance in the UV-Vis-NIR range, and depending on the inclusion size, can also introduce a positive slope in the reflectance spectra.

Iron is not the only possible darkening agent in regolith. For example, Mercury is relatively dark with Bond albedo around 9 % and reflectance spectra from the MASCS instrument on NASA MESSENGER mission in UV-Vis-NIR typically between 2 and 8 % [1]. However, the MESSENGER mission did not detect iron on the surface[2], and carbon, in the form of graphite, is one possible darkening agent for the Mercury surface[3].

Laboratory analogues to regolith materials are an important step in understanding the composition of the regolith, and among other properties, reflectance spectra of the analogue material should match the one of the targets. With iron and space weathering, the experimental methods such as laser pulses simulate the weathering mechanism, thus creating the iron inclusions in the surface layer of the particles. With graphite, however, analogue materials have usually been external mixtures. Incaminato et al.[4] touch this issue and note about the problem in creating internal mixtures with graphite and a host material. If the host material needs to be melted to get graphite in, the graphite can react with oxygen in high temperatures by forming CO and CO₂ and evaporate. The solution to this problem could be to melt and mix the materials in an oxygen-free atmosphere, but to our knowledge, this has not been tested.

As a motivation for conducting internal mixtures with graphite in oxygen-free environment, we point out that the scattering properties of materials change as they are embedded in a host material instead of air or free space. There are two mechanisms behind this. First, the wavelength of light is lower in a material with a refractive index (the real part) above 1. Therefore, the effective size of the inclusion (the physical size in relation to the wavelength) will be larger. This can be especially important with small inclusions with sizes in the scale of some tens to some hundreds of nanometers. Second, the contrast between the refractive indices of the inclusion and the host decreases. This will decrease the scattering cross section of the inclusion.

The final difference in the reflectance of mixtures where the darkening agents are mixed externally or internally depend on the size of the inclusions, the size of the host particles, and their complex refractive indices, including extinction, but we can simulate the effect. In our preliminary tests with small (radius of 10 nm) iron or graphite inclusions either outside or inside a single 100-μm-radius olivine host particle, we can see how the cases with internal mixture produce lower scattering cross sections and higher absorption cross sections compared with external mixtures. In the albedo and reflectance spectra consisting of these particles, this would result in lower values for the internal mixtures.

We will expand the cases studied with larger inclusions and conduct radiate transfer simulations to quantify the final effect in the reflectance of a macroscopic surface. These results should underline the importance of mimicking properly the probable internal mixture of graphite with, e.g., Mercury analogue materials[5].