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[PPS03-01] Quantitative evaluation of bulk density and shape of grains returned from asteroid Bennu
Keywords:Bennu, Ryugu, OSIRIS-REx, Hayabusa2, asteroid, curation
INTRODUCTION
Samples returned from asteroids preserve the intrinsic characteristics of the rocks that make up the asteroids mainly because they have not been affected by atmospheric entry or terrestrial weathering. Therefore, the characteristics of returned samples provide important information for understanding material evolution on asteroids. The physical properties of samples obtained with a non-destructive way during the curation phase prior to any sample allocation provide valuable scientific insights through statistical approaches. Remote sensing observations suggest that Ryugu and Bennu could originate from the same parent body (Tatsumi et al., 2021). On the other hand, their spectral evolutionary trends are found to be opposite (Yumoto et al., 2024). To understand the differences in material evolution on the asteroids Ryugu and Bennu, we will quantitatively evaluate the bulk density and shape of each Bennu grain through the initial description of the Bennu sample at JAXA and statistically compare it with the Ryugu particle data.
METHOD
Based on a memorandum of understanding (MOU) with NASA, Bennu samples weighing more than 0.5 wt.% (664.4 ± 0.2 mg) of the total sample weight of 121.6 g (Lauretta et al., 2024) returned from Bennu were divided into five containers (ORX-19000 to 59000) and allocated to JAXA. The Bennu samples were transported to the nitrogen-filled clean chamber at the ISAS curation facility without exposure to air. A total of 101 particles from ORX-19000 and ORX-29000 were examined individually.
Microscopic images of the particles were taken using an optical digital microscope (Keyence VHX-8000), and the thickness of the sample was obtained by focus stacking. The Maximum/minimum caliper diameters and a best-fit ellipse for each particle contour were driven from each particle image. A microbalance (A&D AD-4212D) was used to measure the weight of the container before and after loading the sample. Infrared reflection spectra at wavelengths of 2.0–13.3 μm were obtained using a microscopic infrared spectrometer (JASCO IRT-5200 / VIR-200).
The bulk density of the sample was estimated by approximating each particle to an ellipsoid based on the sample thickness and best-fit ellipse to the particle contour and applying a volume correction factor obtained by Miyazaki et al. (2023).
RESULTS & DISCUSSION
Particle sizes (max caliper diameter) were in the range of 0.79–4.25 mm and weights in the range of 0.04–11.05 mg. The bulk densities were in the range of 0.35–2.97 g cm−3 and showed a unimodal frequency distribution with a mean value of 1.87 g cm−3 and a standard deviation of 0.41 g cm−3. These are generally consistent with the bulk density of Ryugu particles collected during the first touchdown (1.81 ± 0.30 g cm−3, Miyazaki et al., 2023).
All particles were classified into eight shape types based on their triaxial ratios (Blott and Pye, 2008). The most common shape is “sub-equant block” with 61 out of 101 classified, which is also the most common shape classified for Ryugu particles (Miyazaki et al., 2023). On the other hand, Bennu samples have many “slab”-shaped grains (6/101 ~ 5.9%), which are planar and rarely observed in Ryugu grains (1/724 ~ 0.1%, Miyazaki et al., 2023). No significant correlation between shape and density was observed.
One possible reason that Bennu samples have more “slab”-shaped grains than Ryugu samples is sampling bias due to the difference in the sampling method (bullet-type and pneumatic-type). However, in the case of Ryugu grains, slab-shaped grains were rare (2/67 ~ 3.0%) even if evaluating the particles scattered by the thruster jet during the Hayabusa2 touchdown (Tachibana et al., 2022). The Ryugu particles flowed by the thruster jet should behave similarly to the Bennu grains during the pneumatic sampling. Therefore, this difference in the frequency of slab-shaped particles between Ryugu and Bennu may reflect an intrinsic difference in particle morphology on these asteroids rather than sampling bias.
Slab-shaped particles are coarse-grained and/or have relatively bright surfaces and exhibit high reflectivity in the infrared wavelengths. In the case of Ryugu samples, the brightness of the particle surface indicates the degree of space weathering (Nakato et al., 2023). In situ observations of Bennu indicate that the surface evolved brighter due to space weathering (Yumoto et al., 2024), suggesting that space weathering is potentially related to the origin of bright surfaces on the slab-shaped grains.
Samples returned from asteroids preserve the intrinsic characteristics of the rocks that make up the asteroids mainly because they have not been affected by atmospheric entry or terrestrial weathering. Therefore, the characteristics of returned samples provide important information for understanding material evolution on asteroids. The physical properties of samples obtained with a non-destructive way during the curation phase prior to any sample allocation provide valuable scientific insights through statistical approaches. Remote sensing observations suggest that Ryugu and Bennu could originate from the same parent body (Tatsumi et al., 2021). On the other hand, their spectral evolutionary trends are found to be opposite (Yumoto et al., 2024). To understand the differences in material evolution on the asteroids Ryugu and Bennu, we will quantitatively evaluate the bulk density and shape of each Bennu grain through the initial description of the Bennu sample at JAXA and statistically compare it with the Ryugu particle data.
METHOD
Based on a memorandum of understanding (MOU) with NASA, Bennu samples weighing more than 0.5 wt.% (664.4 ± 0.2 mg) of the total sample weight of 121.6 g (Lauretta et al., 2024) returned from Bennu were divided into five containers (ORX-19000 to 59000) and allocated to JAXA. The Bennu samples were transported to the nitrogen-filled clean chamber at the ISAS curation facility without exposure to air. A total of 101 particles from ORX-19000 and ORX-29000 were examined individually.
Microscopic images of the particles were taken using an optical digital microscope (Keyence VHX-8000), and the thickness of the sample was obtained by focus stacking. The Maximum/minimum caliper diameters and a best-fit ellipse for each particle contour were driven from each particle image. A microbalance (A&D AD-4212D) was used to measure the weight of the container before and after loading the sample. Infrared reflection spectra at wavelengths of 2.0–13.3 μm were obtained using a microscopic infrared spectrometer (JASCO IRT-5200 / VIR-200).
The bulk density of the sample was estimated by approximating each particle to an ellipsoid based on the sample thickness and best-fit ellipse to the particle contour and applying a volume correction factor obtained by Miyazaki et al. (2023).
RESULTS & DISCUSSION
Particle sizes (max caliper diameter) were in the range of 0.79–4.25 mm and weights in the range of 0.04–11.05 mg. The bulk densities were in the range of 0.35–2.97 g cm−3 and showed a unimodal frequency distribution with a mean value of 1.87 g cm−3 and a standard deviation of 0.41 g cm−3. These are generally consistent with the bulk density of Ryugu particles collected during the first touchdown (1.81 ± 0.30 g cm−3, Miyazaki et al., 2023).
All particles were classified into eight shape types based on their triaxial ratios (Blott and Pye, 2008). The most common shape is “sub-equant block” with 61 out of 101 classified, which is also the most common shape classified for Ryugu particles (Miyazaki et al., 2023). On the other hand, Bennu samples have many “slab”-shaped grains (6/101 ~ 5.9%), which are planar and rarely observed in Ryugu grains (1/724 ~ 0.1%, Miyazaki et al., 2023). No significant correlation between shape and density was observed.
One possible reason that Bennu samples have more “slab”-shaped grains than Ryugu samples is sampling bias due to the difference in the sampling method (bullet-type and pneumatic-type). However, in the case of Ryugu grains, slab-shaped grains were rare (2/67 ~ 3.0%) even if evaluating the particles scattered by the thruster jet during the Hayabusa2 touchdown (Tachibana et al., 2022). The Ryugu particles flowed by the thruster jet should behave similarly to the Bennu grains during the pneumatic sampling. Therefore, this difference in the frequency of slab-shaped particles between Ryugu and Bennu may reflect an intrinsic difference in particle morphology on these asteroids rather than sampling bias.
Slab-shaped particles are coarse-grained and/or have relatively bright surfaces and exhibit high reflectivity in the infrared wavelengths. In the case of Ryugu samples, the brightness of the particle surface indicates the degree of space weathering (Nakato et al., 2023). In situ observations of Bennu indicate that the surface evolved brighter due to space weathering (Yumoto et al., 2024), suggesting that space weathering is potentially related to the origin of bright surfaces on the slab-shaped grains.