Experimental investigation of solar radiation effects on Mercury’s surface regolith

Introduction

Mercury’s surface undergoes extreme diurnal and annual temperature variations driven by high-intensity insolation,  radiative loss due to its lack of atmosphere, its spin-orbit resonance, and eccentric orbit. These thermal variations have proven important for the regolith production [1], caused by thermal cracking on atmosphereless planetary bodies. In this study, we aim to examine the solar radiation effects on Mercury’s surface regolith.

 

Measurements

We simulate the power density of the solar radiation impinging on a planetary surface at Mercury’s perihelion distance of 0.31 au using the Space and High-Irradiance Near-Sun Simulator (SHINeS) [2] at Luleå University of Technology to study the spectral response of terrestrial analogue materials. The analogues used include basalt, gabbro, andesite, anorthosite, diorite, and boninite, and they were selected for their close geochemical properties to Mercury’s surface. Notably, boninite is currently considered one of the closest terrestrial hermean analogues. The samples were ground and sieved to <75 μm to simulate the fine-grain regolith of Mercury [3]. The reflectance spectra in the visible to near-infrared (VIS/NIR) range (0.44 - 2.5 μm) were acquired pre- and post-irradiation to evaluate spectral changes.

 

Results

The experimental results (Figure 1) show a decrease in reflectance in all samples across the VIS and NIR spectra, except gabbro, which increases brightness in the NIR above 1470 nm. Darkening and reddening were more pronounced in samples with higher Fe content, such as boninite and basalt, while gabbro, despite its moderate Fe content, remained spectrally stable. We attribute this discrepancy to the dependence of solar radiation absorption on the samples’ albedo. The less reflective samples absorb more energy.

Figure 1: The reflectance spectra in the visible and near-infrared regions for the six samples  - basalt (BAS), andesite (AND), anorthosite (ANO), boninite (BON), diorite (DIO), and gabbro (GAB) – before (blue line) and after (red line) irradiation at 1.40 W/cm². Most samples exhibit a reduction in brightness in the VIS and NIR  spectra. Gabbro presents an increase of brightness in the NIR above 1470 nm. In the NIR, boninite shows a significant drop in brightness along with changes in its absorption band depths [Latsia et al. 2025, in review].

References

[1] Molaro J. and Byrne, S. 2012. Rates of temperature change of airless landscapes and implications for thermal stress weathering. Journal of Geophysical Research: Planets, 117(E10).

[2] Tsirvoulis, G., Granvik, M. and Toliou, A., 2022. SHINeS: Space and High-Irradiance Near-Sun Simulator. Planetary and Space Science, 217, p.105490.

[3] Domingue, D.L., Chapman, C.R., Killen, R.M., Zurbuchen, T.H., Gilbert, J.A., Sarantos, M., Benna, M., Slavin, J.A., Schriver, D., Trávníček, P.M. and Orlando, T.M., 2014. Mercury’s weather-beaten surface: Understanding Mercury in the context of lunar and asteroidal space weathering studies. Space Science Reviews, 181, pp.121-214.

[4] Hunt, G.R. and Ashley, R.P., 1979. Spectra of altered rocks in the visible and near infrared. Economic Geology, 74(7), pp.1613-1629.

[5] Maturilli, A., Helbert, J., John, J.M.S., Head III, J.W., Vaughan, W.M., D'Amore, M., Gottschalk, M. and Ferrari, S., 2014. Komatiites as Mercury surface analogues: Spectral measurements at PEL. Earth and Planetary Science Letters, 398, pp.58-65