11:45 AM - 12:00 PM
[PPS03-10] Development of laser irradiation system for space weathering simulation and measurement of Murchison meteorite spectra
Keywords:space weathering, asteroid, spectroscopy, laser irradiation
Irradiation of solar winds and bombardment of micrometeorites alter the surface materials of airless bodies, including asteroids. These space weathering processes change the spectra of surface materials, which pose challenges in obtaining insights into the un-weathered subsurface materials through remote observations. Laboratory simulations of space weathering effects have provided a well-established understanding of how space weathering alters the spectra of anhydrous silicate bodies, such as S-type asteroids and the Moon (e.g., Sasaki et al., 2001, Brunetto et al. 2015). In contrast, space weathering effects of carbonaceous chondrites remain unclear. Several previous studies have simulated the space weathering effects on C-type asteroids by irradiating carbonaceous chondrites with pulsed lasers or ions, and measuring spectral changes, showed that the spectral response varies greatly depending on many experimental conditions, such as the sample composition (Lantz et al., 2017) and irradiation energy (Nakamura et al., 2020). To solve this problem, many experiments must be performed under a wide variety of conditions, including the type and shape of the sample, and the number and energy of laser irradiations.
In order to simulate the space weathering effects under various conditions, we developed an instrument for efficient laser irradiation experiment. Specifically, the optical paths of the spectrometer and the laser were set up on separate axes to avoid interference. As a result, multi-step laser irradiation and spectral measurement can be performed simultaneously with a single vacuum draw without removing the sample or reconfiguring the instrument, thus significantly reducing the experimental time.
To validate the spectral measurement performance of the developed instrument, we conducted simultaneous experiments of multi-step laser irradiation and spectroscopic measurement using a 10 mm-sized Murchison meteorite chip. A Nd:YAG pulse laser with a wave wavelength of 1064 nm and pulse width of 10 ns was used for irradiation. The diameter of the laser beam was 0.6 mm at the sample surface. All spectra were measured at an incidence angle of 30°, emission angle of 0°, and phase angle of 30°. We irradiated the sample with three different energy conditions: 3.5, 8.4, and 12 mJ (1, 3, and 4×109 W/cm2).
The spectra became redder at wavelengths longer than 550 nm and bluer at shorter wavelengths. The shallowing of the absorption depth at 700 nm was not as obvious as in previous studies (Thompson et al., 2020), but the reddening observed at longer wavelengths was consistent with previous studies. It was found that the way in which the spectra changed in response to multi-step irradiation depended on the irradiation energy density. No significant change in spectra was observed at 3.5 mJ irradiation. At 12 mJ irradiation, reflectance decreased by about 20% after the first laser irradiation and did not change significantly after subsequent irradiations. At the intermediate energy of 8.4 mJ irradiation, exponential saturation of spectral reduction was observed, consistent with the trend obtained from the space weathering experiments on ordinary chondrites (Nakahara et al., 2024 JpGU). The results are useful in the quantitative modeling of the evolution of the surface spectra of C-type asteroids.
The system developed in this study can be used for space weathering experiments on Ryugu and Bennu grains.
References
Brunetto, R et al., 2015. Asteroids IV, 597-616.
Cho, Y. et al., 2022. Planetary and Space Science 221, 105549.
Lantz, C. et al., 2017. Icarus 285, 43-57.
Nakahara, S. et al., 2024 JpGU.
Nakamura, T. et al., 2020. Lunar and Planetary Science Conference, p. 1310.
Sasaki, S. et al., 2001. Nature 410, 555-557.
Thompson, M. et al., 2020. Icarus 346, 113775.
In order to simulate the space weathering effects under various conditions, we developed an instrument for efficient laser irradiation experiment. Specifically, the optical paths of the spectrometer and the laser were set up on separate axes to avoid interference. As a result, multi-step laser irradiation and spectral measurement can be performed simultaneously with a single vacuum draw without removing the sample or reconfiguring the instrument, thus significantly reducing the experimental time.
To validate the spectral measurement performance of the developed instrument, we conducted simultaneous experiments of multi-step laser irradiation and spectroscopic measurement using a 10 mm-sized Murchison meteorite chip. A Nd:YAG pulse laser with a wave wavelength of 1064 nm and pulse width of 10 ns was used for irradiation. The diameter of the laser beam was 0.6 mm at the sample surface. All spectra were measured at an incidence angle of 30°, emission angle of 0°, and phase angle of 30°. We irradiated the sample with three different energy conditions: 3.5, 8.4, and 12 mJ (1, 3, and 4×109 W/cm2).
The spectra became redder at wavelengths longer than 550 nm and bluer at shorter wavelengths. The shallowing of the absorption depth at 700 nm was not as obvious as in previous studies (Thompson et al., 2020), but the reddening observed at longer wavelengths was consistent with previous studies. It was found that the way in which the spectra changed in response to multi-step irradiation depended on the irradiation energy density. No significant change in spectra was observed at 3.5 mJ irradiation. At 12 mJ irradiation, reflectance decreased by about 20% after the first laser irradiation and did not change significantly after subsequent irradiations. At the intermediate energy of 8.4 mJ irradiation, exponential saturation of spectral reduction was observed, consistent with the trend obtained from the space weathering experiments on ordinary chondrites (Nakahara et al., 2024 JpGU). The results are useful in the quantitative modeling of the evolution of the surface spectra of C-type asteroids.
The system developed in this study can be used for space weathering experiments on Ryugu and Bennu grains.
References
Brunetto, R et al., 2015. Asteroids IV, 597-616.
Cho, Y. et al., 2022. Planetary and Space Science 221, 105549.
Lantz, C. et al., 2017. Icarus 285, 43-57.
Nakahara, S. et al., 2024 JpGU.
Nakamura, T. et al., 2020. Lunar and Planetary Science Conference, p. 1310.
Sasaki, S. et al., 2001. Nature 410, 555-557.
Thompson, M. et al., 2020. Icarus 346, 113775.