Macroporosity and Grain Density of Rubble Pile Asteroid (162173) Ryugu

Grott, M.; Biele, Jens; Michel, Patrick; Sugita, Seiji; Schröder, Stefan; Sakatani, Naoya; Neumann, Wladimir; Kameda, Shingo; Michikami, Tatsuhiro; Honda, Chikatoshi

doi:10.1029/2020je006519

Cited by 34 publications

(23 citation statements)

References 71 publications

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“…They conclude that the low thermal inertia is intrinsic to the boulder and is indicative of high porosity (and low density and strength). Grott et al (2020) also confirmed this inference of high boulder porosity, and low bulk macroporosity of Ryugu, through extrapolation of the size-frequency distributions for boulders observed on Ryugu's surface. Using high-resolution optical and thermal imaging of Ryugu from the Hayabusa2 spacecraft, Sakatani et al (2021) found several boulders that are darker than the average surface and lower in thermal inertia than the MASCOT boulder.…”

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confidence: 66%

High‐Resolution Thermophysical Analysis of the OSIRIS‐REx Sample Site and Three Other Regions of Interest on Bennu

et al. 2022

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The OSIRIS‐REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft sampled asteroid (101955) Bennu on 20 October 2020 and will return the collected regolith to Earth in 2023. Before sample collection, spectral observations of four regions of interest on Bennu's surface were acquired at high spatial resolution (2–9 m per spectrometer spot) to identify the most suitable site for sampling and provide contextual information for the returned sample. In this study, we investigate thermal‐infrared (6–50 μm) observations of these four regions, including the site that OSIRIS‐REx ultimately sampled, using the Advanced Thermophysical Model with input digital terrain models derived from laser altimetry. From model‐to‐measurement comparisons, we find that the observed brightness temperatures depend strongly on small‐scale topography, local variations in thermal inertia, and the observation phase angle. Thermal inertia mapping reveals spatial variations that distinguish the different boulder types found on Bennu. A boulder bearing carbonate veins has higher thermal inertia than average, suggesting that cementation processes reduced its porosity. The thermal inertia of the site sampled is 190 ± 30 J m−2 K−1 s−1/2, which is consistent with observations of a fine‐grained regolith mixed with porous rocks. Thermophysical modeling of the site sampled predicts that the maximum temperatures experienced by the collected sample while on Bennu were 357 ± 3 and 261 ± 3 K for the surface and 50 cm depth, respectively. We predict that OSIRIS‐REx will return a sample with thermophysical properties unique from those of meteorites.

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confidence: 66%

High‐Resolution Thermophysical Analysis of the OSIRIS‐REx Sample Site and Three Other Regions of Interest on Bennu

et al. 2022

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“…It remains to be seen whether particle saturation (assuming it is present) is unique to rubble-pile asteroids such as Bennu and Ryugu, or if there is simply a lack of observations from analog features (e.g., lag deposits) on larger planetary bodies. Recently, Grott et al [68] proposed that Ryugu's PSFD is better described by a Weibull distribution of the form N(D) = N T exp(−3(D/λ) β /β),…”

Section: Comparison To Ryugumentioning

confidence: 99%

Particle Size-Frequency Distributions of the OSIRIS-REx Candidate Sample Sites on Asteroid (101955) Bennu

et al. 2021

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We manually mapped particles ranging in longest axis from 0.3 cm to 95 m on (101955) Bennu for the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission. This enabled the mission to identify candidate sample collection sites and shed light on the processes that have shaped the surface of this rubble-pile asteroid. Building on a global survey of particles, we used higher-resolution data from regional observations to calculate particle size-frequency distributions (PSFDs) and assess the viability of four candidate sites for sample collection (presence of unobstructed particles ≤ 2 cm). The four candidate sites have common characteristics: each is situated within a crater with a relative abundance of sampleable material. Their PSFDs, however, indicate that each site has experienced different geologic processing. The PSFD power-law slopes range from −3.0 ± 0.2 to −2.3 ± 0.1 across the four sites, based on images with a 0.01-m pixel scale. These values are consistent with, or shallower than, the global survey measurements. At one site, Osprey, the particle packing density appears to reach geometric saturation. We evaluate the uncertainty in these measurements and discuss their implications for other remotely sensed and mapped particles, and their importance to OSIRIS-REx sampling operations.

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“…2001). A similar analysis of rubble-pile asteroid Ryugu is presented by Grott et al (2020), who report a macro-porosity of (16 ± 3)% using the surface SFD from Michikami et al (2019). Their analysis relies on detailed modeling of the rubble characteristics, including effects from rubble size and shape distribution.…”

Section: Discussionmentioning

confidence: 75%

Internal rubble properties of asteroid (101955) Bennu

Tricarico,

Scheeres,

French

et al. 2021

Preprint

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Exploration of asteroid (101955) Bennu by the OSIRIS-REx mission has provided an in-depth look at this rubble-pile near-Earth asteroid. In particular, the measured gravity field and the detailed shape model of Bennu indicate significant heterogeneities in its interior structure, compatible with a lower density at its center. Here we combine gravity inversion methods with a statistical rubble-pile model to determine the density and size-frequency distribution (SFD) index of the rubble that constitutes Bennu. The best-fitting models indicate that the SFD of the interior is consistent with that observed on the surface, with a cumulative SFD index of approximately −2.9. The rubble bulk density is approximately 1.35 g/cm 3 , corresponding to a 12% macroporosity. We find the largest rubble particle to be approximately 145 m, whereas the largest void is approximately 10 m.

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Macroporosity and Grain Density of Rubble Pile Asteroid (162173) Ryugu

Cited by 34 publications

References 71 publications

High‐Resolution Thermophysical Analysis of the OSIRIS‐REx Sample Site and Three Other Regions of Interest on Bennu

High‐Resolution Thermophysical Analysis of the OSIRIS‐REx Sample Site and Three Other Regions of Interest on Bennu

Particle Size-Frequency Distributions of the OSIRIS-REx Candidate Sample Sites on Asteroid (101955) Bennu

Internal rubble properties of asteroid (101955) Bennu

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