Simulating Micrometeoroid Bombardment on Mercury in the Laboratory

Introduction

The surfaces of airless bodies across the solar system are continually altered due to their exposure to interplanetary space [1]. This process, known as space weathering (SW), is driven by solar wind irradiation and micrometeoroid bombardment. SW alters the microstructural, chemical, and spectral characteristics of grains on the surfaces of airless bodies across the solar system. The effects of SW vary with heliocentric distance (e.g., solar wind flux decreases further from the sun) and they are also linked to the initial surface composition of the body [2]. The effects of SW are well-understood for the Moon and S-type asteroids: darkening and reddening of spectra, and attenuated absorption bands in the visible-near infrared wavelengths. These spectral effects are driven by the production of metallic Fe nanoparticles (npFe) via both solar wind irradiation and micrometeoroid bombardment. However, for highly reduced bodies like Mercury, the microstructural, chemical, and spectral effects of SW are far less constrained. Due to its proximity to the Sun, Mercury experiences a more intense solar wind flux, as well as higher flux and velocity of micrometeoroid impactors [3]. It also has a unique surface composition: low Fe (<2 wt.%) [4] and high volatile content with regions particularly rich in graphite, up to 4 wt.% in the low reflectance material (LRM) [5]. To better understand the effects of this harsh SW environment at Mercury, laboratory experiments are crucial.

Here, we present analyses of the microstructural, chemical, and spectral characteristics of Mercury analog samples subjected to pulsed laser irradiation to simulate the short duration, high temperature events associated with micrometeoroid impacts.

Samples and methods

We prepared samples of forsteritic olivine at NASA’s Johnson Space Center (JSC), with various FeO contents representative of those at the surface of Mercury: F-T-004 (0.53 wt.% Fe) and F-S-002 (0.05 wt.% Fe), and SC-001 (San Carlos olivine, Fo_90-91) as a standard sample comparable to previous experiments. The powdered samples (45–125 µm grain size) were mixed with graphite (5 wt.%) to simulate the high-carbon content of LRM. The mixtures were pressed into pellets at Northern Arizona University and irradiated with an Nd-YAG (λ=1064 nm) pulsed laser under ultra-high vacuum with 1 and then 5 pulses of ~6 ns (48 mJ/pulse). For more details on the samples preparation, see [6].

Infrared (0.65–2.5 µm) reflectance spectra of the samples were acquired using a Nicolet IS50 FTIR spectrometer. The surface morphology of the samples was analyzed by scanning election microscopy (SEM) using a FEI Nova NanoSEM200 at Purdue University. Finally, electron-transparent thin sections of the samples were prepared with a FEI Helios NanoLab 660 focused ion beam (FIB) for analysis with the 200 keV JEOL 2500 transmission electron microscope (TEM) at JSC.

Results

Near-infrared spectroscopy

The reflectance spectra show that the SC-001 sample becomes brighter and the 1 µm absorption band is deeper after 1 laser pulse, but after 5 total pulses the spectrum is darker and the band depth is lower than the unirradiated sample (Fig. 1a).

The unirradiated F-T-004 sample has a slight blue spectral slope and no 1 µm absorption band. With increasing laser pulses, the reflectance increases and the spectral slope becomes more red (Fig. 1b).

The F-S-002 sample is similar to F-T-004 (higher reflectance and spectral slope after irradiation), but the increase in reflectance is more significant (Fig. 1c).

Microstructural and chemical analyses

Analyses of the F-S-002 and F-T-004 irradiated sample surface morphologies using SEM identified two primary textures (Fig. 2a). The first feature is fluffy and rich in carbon and the second is a vesiculated melt deposit. Both textures are distributed across the irradiated sample surfaces. TEM analyses of these textures show the carbon-rich region is made of globule-like deposits, which may be melt products due to irradiation, while the melt region is amorphous and contains small (<5 nm) nanoparticles (Fig. 2b).

Energy dispersive X-ray spectroscopy in the TEM revealed that the melt texture is enriched in Si and is depleted in Mg and O compared to the underlying material.

Conclusion

The analyses of laboratory experiments indicate that SW affects the optical, morphological, microstructural and chemical properties of Mercury analogs. Our initial analyses suggest that spectral changes are highly correlated to their initial composition and are affected by even minor variations in Fe content. In particular, the use of laser irradiated low-Fe, C-rich samples produced characteristics consistent with lunar-style SW (e.g., reddening and darkening of infrared reflectance spectra, nanoparticles in the melt layer [7,8]) and new features (e.g., carbon-rich fluffy textures).

The effects of solar wind irradiation on the microstructural, chemical, and spectral characteristics of Mercury analogs will be investigated using H and He ions-irradiated samples. Other sample compositions (e.g., including sulfur) will also be considered.

References

[1] Pieters, C.M., and Noble, S.M., Space weathering on airless bodies, J. Geophys. Res-Planet., 121, 1865-1884, 2016.

[2] Lantz, C., et al., Ion irradiation of carbonaceous chondrites: A new view of space weathering on primitive asteroids, Icarus, 285, 43-57, 2017.

[3] Cintala, M.J., Impact-induced thermal effects in the lunar and mercurian regoliths, J. Geophys. Res.-Planet., 97, 947-973, 1992.

[4] Nittler, L.R., et al., The Major-Element Composition of Mercury’s Surface from MESSENGER X-ray Spectrometry, Science, 333, 1847-1850, 2011.

[5] Klima, R.L., et al., Global Distribution and Spectral Properties of Low-Reflectance Material on Mercury, Geophys. Res. Letters, 45, 2945-2953, 2018.

[6] Thompson, M.S., et al., Understanding the Space Weathering of Mercury Through Laboratory Experiments, LPSC LII, abstract 2548, 2021.

[7] Sasaki, S., and Kurahashi, E., Space weathering on Mercury, Adv. Space Res., 33, 2152-2155, 2004.

[8] Trang, D., et al., Space Weathering of Graphite: Application to Mercury, LPSC XLIX, Abstract 2083, 2018.