9:45 AM - 10:00 AM
[PPS03-04] Evaluation of radiation damage on extraterrestrial organic matter by STXM-NEXAFS analysis
Keywords:Hayabusa2, Extraterrestrial organic matter, radiation damage
Recent studies on fresh particles of the Ryugu asteroid, collected by the Hayabusa2 spacecraft indicate that these particles have characteristics similar to those of CI-type chondrites, which contain abundant organic materials and hydrous minerals [1-5]. Analysis of Ryugu particles, which almost free from terrestrial weathering and contaminations, is expected to provide new insights into the origin and evolution of organic materials at primitive small body in early solar nebula .
In general, a combination of scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (STXM-NEXAFS) is a promising tool for the high spatial resolution analysis (as small as 30 nm) for extraterrestrial organic matter, such as primitive meteorites, returned samples from a comet and asteroids. For example, Ito et al. (2022) [1] reported that a unique distribution of aliphatic hydrocarbons correlated with coarse-grained phyllosilicates utilizing a STXM-NEXAFS instrument.
Although STXM-NEXAFS is known as less radiation damage technique than electron beam techniques (i.e., TEM), X-rays can also give modifications of organic materials. We, therefore, investigate changes of C-NEXAFS spectrum due to radiation dose through the STXM-NEXAFS analysis of a Ryugu particle.
The ultrathin section (25 x 25 µm2 and 150 nm thickness) of the Ryugu particle C0068-25 was extracted by a focused ion beam (FIB) apparatus at JAMSTEC (Nankoku, Japan). The sample was attached to a Cu-made Kochi grid and then placed on a STXM holder (Okazaki cell) in a facility-to-facility transfer container (FFTC) with pure nitrogen gas [6]. The STXM experiments were conducted at a beamline, BL4U, in UVSOR synchrotron facility of IMS (Okazaki, Japan). The STXM holder taken from the FFTC was mounted on the in-house air-free sample transfer system by using a N2 filled glove box. The holder in the transfer system was loaded into an analysis chamber of a STXM instrument under G1 grade N2 gas condition. The STXM chamber was evacuated and was backfilled 20 mbar of helium gas. This procedure ensures that the ultrathin section was free from terrestrial contamination or weathering during the sample preparation processes.
Figure 1 shows C K-edge NEXAFS spectra of C0068-25, obtained with spatial resolution of 150 nm. A secondary spectrum was measured after the higher spatial resolution (30 nm) scanning, N- and O-NEXAFS, for the same area and with same experimental condition of the primal spectrum.
A distinct peak at 287.3 eV (C-H bond) in the first spectrum is a unique characteristic of the Ryugu sample, but is uncommon in primitive meteorite samples. Intensity of the 287.3 eV peak significantly reduced in the secondary spectrum, and the intensity of the peak at 285.0 eV(C=C bond) increases. This suggests that high-dosed STXM analysis could alter the NEXAFS spectrum, reflecting changes in bonding structure. The X-ray dose and damage on the sample in STXM analysis essentially increase with higher spatial resolution analysis, because higher spatial resolution analysis uses smaller scanning step with smaller focusing spot of X-ray. So spatial resolution of STXM-NEXAFS analysis can be limited by the materials. An X-ray radiation damage assessment and an optimization of the experimental condition should be conducted before analyzing extraterrestrial organic materials.
Additionally, this experiment may help to understand a space weathering effect at asteroid surface by an exposure of cosmic rays. Despite space weathering, the aliphatic hydrocarbons of Ryugu particles could survive on the surface of the Ryugu asteroid. It suggests that the exposure time to cosmic rays of Ryugu particles, which transferred from the inner part to the surface of the asteroid Ryugu by gardening effect, would be short. Thus, degree of the space weathering might be inferred from this anlaysis.
References:
[1] Ito et al., 2022, Nat. Astron. 6, 1163–1171 [2] Yokoyama et al., 2022, Science 379, eabn7850 [3] Nakamura T. et al., 2022, Science 379, eabn8671 [4] Naraoka et al., 2023, Science 379, abn9033 [5] Yabuta et al., 2023, Science 379, abn9057 [6] Ito et al., 2020, Earth Planet, Space, 72, 133
In general, a combination of scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (STXM-NEXAFS) is a promising tool for the high spatial resolution analysis (as small as 30 nm) for extraterrestrial organic matter, such as primitive meteorites, returned samples from a comet and asteroids. For example, Ito et al. (2022) [1] reported that a unique distribution of aliphatic hydrocarbons correlated with coarse-grained phyllosilicates utilizing a STXM-NEXAFS instrument.
Although STXM-NEXAFS is known as less radiation damage technique than electron beam techniques (i.e., TEM), X-rays can also give modifications of organic materials. We, therefore, investigate changes of C-NEXAFS spectrum due to radiation dose through the STXM-NEXAFS analysis of a Ryugu particle.
The ultrathin section (25 x 25 µm2 and 150 nm thickness) of the Ryugu particle C0068-25 was extracted by a focused ion beam (FIB) apparatus at JAMSTEC (Nankoku, Japan). The sample was attached to a Cu-made Kochi grid and then placed on a STXM holder (Okazaki cell) in a facility-to-facility transfer container (FFTC) with pure nitrogen gas [6]. The STXM experiments were conducted at a beamline, BL4U, in UVSOR synchrotron facility of IMS (Okazaki, Japan). The STXM holder taken from the FFTC was mounted on the in-house air-free sample transfer system by using a N2 filled glove box. The holder in the transfer system was loaded into an analysis chamber of a STXM instrument under G1 grade N2 gas condition. The STXM chamber was evacuated and was backfilled 20 mbar of helium gas. This procedure ensures that the ultrathin section was free from terrestrial contamination or weathering during the sample preparation processes.
Figure 1 shows C K-edge NEXAFS spectra of C0068-25, obtained with spatial resolution of 150 nm. A secondary spectrum was measured after the higher spatial resolution (30 nm) scanning, N- and O-NEXAFS, for the same area and with same experimental condition of the primal spectrum.
A distinct peak at 287.3 eV (C-H bond) in the first spectrum is a unique characteristic of the Ryugu sample, but is uncommon in primitive meteorite samples. Intensity of the 287.3 eV peak significantly reduced in the secondary spectrum, and the intensity of the peak at 285.0 eV(C=C bond) increases. This suggests that high-dosed STXM analysis could alter the NEXAFS spectrum, reflecting changes in bonding structure. The X-ray dose and damage on the sample in STXM analysis essentially increase with higher spatial resolution analysis, because higher spatial resolution analysis uses smaller scanning step with smaller focusing spot of X-ray. So spatial resolution of STXM-NEXAFS analysis can be limited by the materials. An X-ray radiation damage assessment and an optimization of the experimental condition should be conducted before analyzing extraterrestrial organic materials.
Additionally, this experiment may help to understand a space weathering effect at asteroid surface by an exposure of cosmic rays. Despite space weathering, the aliphatic hydrocarbons of Ryugu particles could survive on the surface of the Ryugu asteroid. It suggests that the exposure time to cosmic rays of Ryugu particles, which transferred from the inner part to the surface of the asteroid Ryugu by gardening effect, would be short. Thus, degree of the space weathering might be inferred from this anlaysis.
References:
[1] Ito et al., 2022, Nat. Astron. 6, 1163–1171 [2] Yokoyama et al., 2022, Science 379, eabn7850 [3] Nakamura T. et al., 2022, Science 379, eabn8671 [4] Naraoka et al., 2023, Science 379, abn9033 [5] Yabuta et al., 2023, Science 379, abn9057 [6] Ito et al., 2020, Earth Planet, Space, 72, 133