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Title - Ryugu’s nucleosynthetic heritage from the outskirts of the Solar System
By - Timo Hopp et al, 2022
Summary - .
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By - Masatoshi Hirabayashi et al, Aug 2020
Summary - Their oblate shapes with equatorial ridges may have evolved due to their rotational conditions at present and in the past.
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Title - An artificial impact on the asteroid (162173) Ryugu formed a crater in the gravity-dominated regime
By - Arakawa et al, Mar 2020 (full paper available with me)
Summary - The impact produced an artificial crater with a diameter >10 meters, which has a semicircular shape. Ejecta curtain grew for over 8 minutes and it never fully detached from the surface. The crater formed in the gravity-dominated regime; in other words, crater growth was limited by gravity not surface strength.
Supplementary paper (PDF) - Contains useful info on crater morphology, impact cratering experiments, crater formation time, ejecta detachment and deposition, crater size scaling law, effects of boulder size on the crater scaling law etc. 2 videos are also provided in main link.
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Title - Size of particles ejected from an artificial impact crater on asteroid 162173 Ryugu
By - Wada et al, Mar 2021
Summary - SCI produced an artificial impact crater with a diameter of 14.5 m. Results suggest that the subsurface layer of Ryugu is composed of relatively small particles compared to the uppermost layer.
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Title - The Asteroid 162173 Ryugu: a Cometary Origin
By - Hitoshi Miura et al, Jan 2022
Summary - This study indicates that spinning top-shaped, rubble-pile objects with high organic content, such as Ryugu and Bennu, are comet–asteroid transition objects (CATs). CATs are small objects that were once active comets but have become extinct and apparently indistinguishable from asteroids. The entire study is based on numerical simulation of the sublimation of water ice from a porous cometary nucleus.
News - Phys.org, Nagoya, MSN, Preprint version 1, My Twitter thread on it
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Title - Spectrally blue hydrated parent body of asteroid (162173) Ryugu
By - Eri Tatsumi et al, Oct 2021
Summary - Spectrally blue Ryugu’s parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C), suggesting that Ryugu’s parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2–2.5 Ma. Paper contains illustration of hydrothermal history of Ryugu’s parent body.
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Title - The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes
By - S.Sugita et al, Mar 2019
Summary - We estimate that the impact craters penetrating the top 10 meters of Ryugu’s surface have existed for 10-100 million years. In contrast, the low number density of small craters (~10 m in diameter) suggests a very young resurfacing age (≲ 1 million years) for the top 1-meter layer. Ryugu possesses the average spectrum of a Cb-type asteroid. Ryugu’s geometric albedo at 0.55 µm is 4.5 ± 0.2%, among the lowest in the Solar System. The high boulder abundance and the spectral properties of the boulders are consistent with dehydrated surface materials. The scenario of partial dehydration of Ryugu's parent-body due to internal heating suggests that asteroids formed from materials that condensed at ≤150 K (the H2O condensation temperature under typical solar nebula conditions) must have either formed sufficiently early to contain high concentrations of radiogenic species, such as 26Al, or formed near the Sun, where they experienced other heating mechanisms.
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Title - First asteroid gas sample delivered by the Hayabusa2 mission: A treasure box from Ryugu
By - Ryuji Okazaki et al, 2022
Summary - .
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By - Pieter Tack et al, 2022
Summary - .
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By - Martin Jutzi et al, Nov 2022 (Sabina Raducan, Yun Zhang, Patrick Michel & Masahiko Arakawa)
Summary - Strength + gravity regime. Bern Smoothed-Particle-Hydrodynamics (SPH) impact code. Our numerical simulations are able to reproduce the outcome of the impact on Ryugu, including the effects of boulders originally located near the impact point. Masahiko Arakawa et al, 2020 concluded that the impact might have taken place in the gravity-dominated regime and therefore the surface cohesion on Ryugu must be very small, less than 1 Pa. It is unclear whether traditional impact models and crater scaling-laws are applicable to the low-strength low-gravity cratering environment.
Links - My Twitter thread on it
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Title - Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu
By - Patrick Michel et al, 2020
Summary -
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Title - Uracil in the carbonaceous asteroid (162173) Ryugu
By - Yasuhiro Oba et al, 2023
Summary - This paper reports the detection of uracil, one of the 4 nucleobases in ribonucleic acid, in aqueous extracts from Ryugu samples. In addition, nicotinic acid (niacin, a B3 vitamer), its derivatives, and imidazoles were detected in search for nitrogen heterocyclic molecules.
News - Science News
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Title - Insight into multi-step geological evolution of C-type asteroids from Ryugu particles
By - Yasuhiro Oba et al, 2023
Summary - The Ryugu particles we studied are similar to CI (Ivuna-type) chondrites but with some important differences, such as the presence of Na–Mg phosphates and Na-rich phases and the lack of ferrihydrite and gypsum. Ryugu particles experienced several steps of aqueous alteration, metasomatism and brecciation under variable conditions. These materials represent mixed lithologies and formed at different locations within their parent asteroid. The evidence presented here demonstrates that the C-type asteroid Ryugu experienced a complex geologic evolution shortly after its formation.
News - Science News
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By - Martin Jutzi, Sabina Raducan, Yun Zhang, Patrick Michel, Masahiko Arakawa, 2022
Summary - The DART impact probably occurred in the transitional cratering regime, between strength and gravity (strength + gravity regime). However, our results also show that the cratering efficiency can be strongly affected by the presence of a very small amount of cohesion.
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Title - Carbonate record of temporal change in oxygen fugacity and gaseous species in asteroid Ryugu
By - Wataru Fujiya et al, July 2023
Summary -
News - Phys.org
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By - M. Hamm et al, 2022
Summary - The low thermal inertia of the boulders on Ryugu is one of the major surprises of the Hayabusa2 mission. Based on thermal inertia estimates prior to the arrival of Hayabusa2, a granular surface with mm to cm-sized regolith was expected to dominate the surface of Ryugu. Bennu’s surface showed a similar thermal inertia & was expected to be dominated by such regolith as well but is dominated by boulders, just like in Ryugu's case. The low thermal inertia of the boulders on Ryugu can only be explained by high intrinsic porosity. The porosity estimate of 46.7 % for the observed boulder is considerably higher than those of CM and CI chondrites, i.e., 22% for CM and 34.9% for CI. It is also higher than the porous, pristine fragments of the ungrouped C2 Tagish Lake meteorite (40%). Such high porosity indicates a low strength of the boulder material on both asteroids. Therefore, fragments of asteroids like Bennu and Ryugu would probably not survive the fall through Earth’s atmosphere and, representative, macroscopic samples of either asteroid might not be found in our meteorite collections.
Evidence strongly hints at absence of a thick dust layer on Ryugu. The dust layer is understood to be very thin.
Hint 1 : On Ryugu, the shape of the diurnal temperature variation and the cooling rate at night, hint at the presence of only a thin dust layer. Consequently, the low thermal inertia of the boulders can only be explained by high intrinsic porosity & weak strength.
Hint 2 : Another strong hint for the absence of a thick dust layer is the high spectral contrast in the mid-IR emissivity. A dust-dominated surface should show characteristics similar to powdered samples. Yet, the thin-section meteorite spectra are a better match to Ryugu’s emissivity. The contrast of the spectra of powdered samples is by an order of magnitude smaller than the MARA estimates, such that powders seem to be a poor analogue to the boulders on Ryugu.
The spectra of Bennu and the emissivity estimate of Ryugu both show an emissivity drop, which is believed to be caused by a fine dust layer on large rocks but it is unclear how the volume scattering effects emerge on such highly porous surfaces, that might be very similar to sintered dust particles. [ it seems that the large rocks are mimicking the behaviour of fine dust particles ]
The group-level similarity of the mid-IR emissivity to highly aqueously altered CM and CI chondrites strongly supports the presence of phyllosilicates in Ryugu’s surface material. This is in line with near-IR observations of the surface, which indicates the presence of Mg-rich phyllosilicates. The narrowness and weakness of the reflectance minimum were interpreted as an indication for partial dehydration of Ryugu’s surface and the absence of other bands in that wavelength range implies that there are no minerals bearing molecular water, present at the surface of Ryugu.
In contrast, Bennu shows a broader 2.74 µm feature, also indicating the presence of hydrated minerals but perhaps a slightly lesser degree of alteration. The estimated mid-IR emissivity of Ryugu shows similarities to the spectra derived on Bennu by the OSIRIS-REx Thermal Emission Spectrometer (OTES). Although the spectrum of Bennu has less contrast, its general appearance is very similar to that of Ryugu and the emissivity within the MARA bands follows the same trend as the one of CMs, CIs and Ryugu. The position of emissivity minima and maxima in the mid-IR indicate the presence of phyllosilicates on Bennu. This leaves us with the question, why, if the near-IR spectra of Bennu and Ryugu are different due to the partial dehydration postulated for Ryugu, the mid-IR spectra are similar to the same aqueously altered carbonaceous chondrite groups.
Thermogravimetric measurements show that dehydration of phyllosilicates, i.e. the loss of H2O, and dehydroxylation, i.e. the loss of OH, occurs in steps between 200 and 800 °C, depending on the phyllosilicate. The loss of the H2O and OH groups between the silicate layers leads to the formation of amorphous phyllosilicates. At high temperatures (>700 °C), olivine crystallises changing the structure of the silicates. Some dehydrated phyllosilicate spectra retain a weak 2.72 µm feature, as observed in laboratory measurements of moderately dehydrated (above 300 °C but below 700 °C) samples of CI1 chondrite Ivuna. These spectra showed similarity to Ryugu’s near-IR spectrum, and indicate that the surface of Ryugu experienced heating within that range.
Our results show that strong aqueous alteration occurred in Ryugu’s parent body prior to any dehydration. The fact the mid-IR emissivity of both Bennu and Ryugu are very similar to the most aqueously altered meteorites in a spectral region that is sensitive to the presence of phyllosilicate, supports the idea that both bodies experienced a high degree of aqueous alteration.
Neither Bennu nor Ryugu can be exclusively attributed to either CI or CM on the basis of spacecraft measurements. However, while individual CM chondrites match the emissivity presented here well, the best overall match is obtained by CI chondrites, whereas Bennu’s emissivity is slightly better matched by the most aqueously altered CM chondrites. Ryugu could therefore consist of material that was originally more aqueously altered than Bennu.
Recent work found that both asteroids might have originated in the same parent body and formed from sections of the parent body that experienced different extents of heating. However, differences in spectra of bright, presumably exogenic boulders, which impacted with the respective parent bodies, seem to point towards 2 distinct parent bodies. The temperature limit of <700 °C is consistent with recent studies of potential parent bodies that can explain the observed global distribution of porous rocks on Ryugu, indicating a parent body accreting after 2–2.5 Ma after the formation of calcium-aluminium-rich inclusions. Our results are also consistent with a recent investigation of the 0.7 µm feature in regions of Ryugu that were best shielded against solar heating and space weather. That study found signs of extensive aqueous alteration of the material similar to CM chondrites and subsequent heating below 400 °C.
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