Erika Kurahashi

Simulation of space weathering by nanosecond pulse laser heating: dependence on mineral composition, weathering trend of asteroids and discovery of nanophase iron particles

Abstract The spectral mismatch between asteroids and ordinary chondrites, is explained by a so-called “space weathering” process where impacts of interplanetary dust altered the optical properties of asteroid surfaces. To simulate the heating by micrometeorite impacts, pellet samples of olivine, pyroxene, and anorthite are irradiated by a pulse laser beam (1064nm) with a pulse duration of 6–8 nanoseconds, which is comparable with a micron-sized particle impact. After the laser irradiation, bidirectional reflectance spectra between 250 and 2600nm of samples are measured by step of 10nm. Laser-irradiated samples show significant reddening: the reduction of spectra is much larger in the visible region than in the near-infrared region. Changes of pyroxene spectra are much smaller than those of olivine. Some asteroid spectra such as 349 Dembowska and 446 Aeternitas can be reproduced by the mixing of spectra of irradiated samples. To clarify the microscopic process and cause of reflectance change, we observed the irradiated samples using transmission electron microscopy (TEM). In rim regions of irradiated olivine grains from pellet samples, nanophase iron particles (several to 30 nm in size) are widely spread. In contrast, no trace of structural change is found on irradiated olivine crystal samples. This suggests that the presence of regoligh-like surface would be essential for the effective space weathering on asteroids. Very small asteroids without regolith could be hardly weathered.

Laboratory simulation of space weathering: Changes of optical properties and TEM/ESR confirmation of nanophase metallic iron

Abstract Reflectance spectra of S-type asteroids are different from those of ordinary chondrites. This spectral mismatch is explained by space weathering processes, where high-velocity dust particle impacts change the optical properties of the uppermost regolith surface of asteroids. S-type asteroids exhibit more overall depletion and reddening of spectra, and more weakening of absorption bands relative to ordinary chondrites. Nanophase metallic iron particles, which are formed through vapor deposition from dust impact heating, are considered as the most essential cause of space weathering. In this study, we describe the spectral changes of olivine and pyroxene using nanosecond pulse laser irradiation and the presence of nanophase metallic iron particles in laser-irradiated materials by transmission electron microscopy (TEM) and electron spin resonance (ESR). The irradiated spectra of the samples show a reduction of the overall spectra (250–2600 nm) and a reddening with weakened absorption bands. Nanophase metallic iron particles were found not only in laser-irradiated olivine samples, but also in laser-irradiated pyroxene samples by TEM. Strong ESR signals, which derive from nanophase iron particles, are observed in the irradiated olivine samples. Moreover, ESR intensities increase with the space weathering degree simulated as laser irradiation time. One possible application of space weathering is the estimation of the relative age of asteroids using the relation between optical effects and quantities of produced nanophase iron particles.

The mystery of 506.5 nm feature of reflectance spectra of Vesta and Vestoids: Evidence for space weathering?

Although asteroid 4 Vesta and Vestoids have been believed to be the source of a group of basaltic meteorites called HEDs, there have been detailed spectral analyses on the spectral redness and the 506.5 nm absorption band, suggesting controversy on their space weathering processes and origins. In order to evaluate a possibility that such an apparent inconsistency may be explained by the space weathering, the 506.5 nm spectral feature and reddening trend are examined for Vesta and Vestoids, HED meteorites, lunar soils, and laser irradiated pyroxene samples in this paper. Our results indicate that all fresh HED meteorites have the 506.5 nm band at different wavelengths according to their classes, lunar soils seem to lose the 506.5 nm band as they mature, and pulse laser irradiation on the pyroxene sample seems to reduce the 506.5 nm band. Therefore, absence of the 506.5 nm band on some Vestoids can be due to space weathering although the relationship between the visible redness and presence/absence of the 506.5 nm band of Vesta and Vestoids is inconsistent with the assumed HED-lunar space weathering trend based on the above laboratory results. Other possible explanations are that some Vestoids experienced shock heavy enough to erase the 506.5 nm band and that pyroxenes on some Vestoids are not similar to those in HED meteorites. Even if the latter case is true and some Vestoids are not made of HED materials, HED meteorites could still come from Vesta unless we assume all Vestoids have to be fragments of Vesta.

Space weathering on Mercury

Space weathering is a process where formation of nanophase iron particles causes darkening of overall reflectance, spectral reddening, and weakening of absorption bands on atmosphereless bodies such as the moon and asteroids. Using pulse laser irradiation, formation of nanophase iron particles by micrometeorite impact heating is simulated. Although Mercurian surface is poor in iron and rich in anorthite, microscopic process of nanophase iron particle formation can take place on Mercury. On the other hand, growth of nanophase iron particles through Ostwald ripening or repetitive dust impacts would moderate the weathering degree. Future MESSENGER and BepiColombo mission will unveil space weathering on Mercury through multispectral imaging observations.

Laboratory simulation of space weathering: ESR measurements of nanophase metallic iron in laser-irradiated materials

S-type asteroids are believed to be parent bodies of ordinary chondrites. However, the reflectance spectra of S-type asteroids are different from those of ordinary chondrites. This spectral mismatch is strongly considered as a result of space weathering, where high-velocity dust particle impacts should change the optical properties of the uppermost regolith surface of asteroids. To simulate space weathering by impact heating of dust particles, we irradiated nanosecond pulse laser beam onto planetary surface materials, whose pulse duration and energy rate are comparable with those of real dust impacts. The laser-irradiated samples show optical changes similar to that by space weathering, and contain nanophase metallic iron particles considered as the essential cause of space weathering. After laser-irradiations, we observed the samples by an Electron Spin Resonance (ESR) to perform quantitative analysis of nanophase metallic iron particles. We report the first description that the quantities of nanophase metallic iron particles in olivine samples increase at higher space weathering degree.

Space weathering: spectral change and formation of nanophase iron due to pulse laser irradiation simulating impact heating of interplantary dust flux

The spectral mismatch between S-type asteroids and ordinary chondrites, is explained bythe so-called “space weathering” process where impacts of interplanetary dust should change the optical properties of asteroid surfaces. To simulate the heating by dust impacts, comminuted olivine and pyroxene samples were irradiated by a nanosecond pulse laser. Laser-irradiated samples show significant depletion and reddening of the reflectance. Some asteroid spectra such as 349 Dembowska and 446 Aeternitas are reproduced by mixing of irradiated spectra. Changes of pyroxene spectra are much smaller than those of olivine. This is consistent with the statistical data of asteroids where olivine-rich asteroids have more reddened spectra. If energy efficiencies for changing the optical properties are the same between dust impact and laser irradiation, olivine-rich surfaces should be darkened and reddened in 108 yrs. According to TEM analysis, nanophase particles (several to 30 nm in size) probably of iron are widely spread in the rim region of irradiated olivine grains. In contrast, no trace of structural change was observed on irradiated olivine crystal samples. The presence of a regolith-like surface would be essential for effective space weathering on asteroids.