| Institution(s) supplémentaire(s) | (1) Department of Earth, Atmospheric, and Planetary Sciences, Purdue University; (2) NASA Headquarters, Mary W. Jackson Building; (3) Northern Arizona University; (4) ARES, NASA Johnson Space Center |
Abstract | Space weathering (SW) refers to continuous processes like solar wind irradiation and micrometeoroid (MM) bombardment that cause the alteration of airless surfaces across the solar system (Pieters and Noble 2016). The effects of SW vary depending on the initial planetary surface composition (Lantz et al. 2017). The microstructural, chemical and spectral effects of SW are fairly well-understood for the Moon and S-type asteroids but are far less-constrained for Mercury and other highly reduced parent bodies like E-type asteroids. Mercury has a unique surface composition (low Fe/high volatile content wrt. the Moon). Its proximity to the Sun results in a more intense solar wind flux, and the high-flux and velocity nature of its MM impactors population (Cintala et al. 1992) create an extreme SW environment, which require further investigation through laboratory experiments.
In this work, we considered several forsteritic olivine pressed-powder pellets samples with different FeO contents but similar grain size (45-125 mic): SC-001 (San Carlos olivine, Fo90-91), F-T-004 (0.53 wt.% Fe) and F-S-002 (0.05 wt.% Fe). All samples were mixed with powdered graphite (5 wt.%) to reproduce the high amount of carbon on Mercury, as seen in the LRM. Here we focus on pulsed-laser irradiation of the samples to simulate the short duration high-temperature events associated with MM impacts on the surface of Mercury. Each sample has been irradiated by a Nd-YAG laser (1064 nm, 6 ns pulse duration, energy of 48 mJ/pulse) with 1 and 5 pulses under ultra-high vacuum. See (Thompson et al. 2021) for more details.
The SC-001 sample has a higher near-infrared (0.65-2.5 mic) reflectance and a deeper 1 mic band after 1 laser pulse than originally, but these parameters decrease to their lowest value after 5 pulses. F-T-004 initially has a blue spectral slope but becomes redder and brighter with the continued laser irradiation dose. The same trend is observed for F-S-002, with more significant reddening and brightening after irradiation.
Analyses with scanning electron microscopy revealed two main textures in the irradiated samples: one is carbon-rich and fluffy, the other is a melt containing vesicles. Further analyses with energy dispersive X-ray spectroscopy in the transmission electron microscope showed the carbon-rich texture is composed of several C-globule-like deposits distinct from the original graphite, and the melt contains a uniform, amorphous material, rich in Si and poor in Mg and O.
Our results confirm the effects of SW on optical, morphological, microstructural and chemical properties of airless surfaces are intimately dependent on their composition. By considering low-Fe samples, we reproduced results from previous graphite-free SW studies (e.g., reddening of NIR spectra, presence of nanoparticles in the melt layer (Sasaki and Kurahashi 2004; Trang et al. 2018)) and we also observed new features like carbon-rich fluffy textures due to the presence of graphite. |
