Production of iron nanoparticles by laser irradiation in a simulation of lunar-like space weathering

Abstract

'Space weathering' is the term applied to the darkening and reddening of planetary surface materials with time, along with the changes to the depths of absorption bands in their optical spectra. It has been invoked to explain the mismatched spectra of lunar rocks and regolith, and between those of asteroids and meteorites. The formation of nanophase iron particles on regolith grains as a result of micrometeorite impacts or irradiation by the solar wind has been proposed as the main cause of the change in the optical properties. But laboratory simulations have not revealed the presence of these particles, although nano-second-pulse laser irradiation did reproduce the optical changes. Here we report observations by transmission electron microscopy of olivine samples subjected to pulse laser irradiation. We find within the amorphous vapour-deposited rims of olivine grains nanophase iron particles similar to those observed in the rims of space-weathered lunar regolith grains. Reduction by hydrogen atoms implanted by the solar wind is therefore not necessary to form the particles. Moreover, the results support the idea that ordinary chondrites came from S-type asteroids, and thereby provides some constraints on the surface exposure ages of those asteroids.

Evidence for the effects of space weathering spectral signatures on low albedo asteroids

“…Our results are opposite the well-known reddening trend found (by numerous workers) to occur on S-type asteroids (e.g. Pieters et al 2000;Hapke 2001;Clark et al 2002;Sasaki et al 2001Sasaki et al , 2002Ishiguro et al 2007). …”

“…With higher irradiation doses, they found a reddening. It may ultimately be possible to obtain an age of the asteroid surface using a timescale of space weathering effects based on laboratory simulations (Sasaki et al 2001). Until an asteroid surface sample is returned, we will not know how accurate the laboratory experiment techniques are at simulating the effects of space weathering.…”

Peeling the astronomical onion

“…In the case of silicate grains, this will be the bare silicate surface likely terminated by -OH groups. However, silicate dust grains in the ISM are subject to space weathering and iron nanoparticle production 48 and so the silicate grain surface might be peppered with metallic atoms and clusters. As the grain cools further, the question arises as to which surface the second most abundant condensed phase species, carbon monoxide (CO), would adsorb on to, and TPD experiments provide the answer.…”

Space weathering trends among carbonaceous asteroids

“…These moderate albedo (p v ∼0.22; Mainzer et al 2011) asteroids are known to spectrally darken, redden and have increasingly suppressed absorption bands as a function of time (Belton et al 1992(Belton et al , 1994Binzel et al 1996;Chapman 1996). These spectral changes are attributed to the vapor deposition of sub-micron metallic iron (SMFe) particles onto grains during micrometeorite impacts and solar wind irradiation (Yamada et al 1999;Hapke 2001;Sasaki et al 2001;Brunetto & Strazzulla 2005). The intrinsically dark (p v ∼0.06; Mainzer et al 2011) nature of carbonaceous material and the lack of prominent absorption features at visible and near-IR wavelengths implied space weathering trends would be difficult to identify for C-complex asteroids (Hapke 2001;Moroz et al 1996).…”