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Potiszil, C.; Yamanaka, M.; Sakaguchi, C.; Ota, T.; Kitagawa, H.; Kunihiro, T.; Tanaka, R.; Kobayashi, K.; Nakamura, E. The Evolution of Ryugu. Encyclopedia. Available online: https://encyclopedia.pub/entry/46898 (accessed on 22 June 2024).
Potiszil C, Yamanaka M, Sakaguchi C, Ota T, Kitagawa H, Kunihiro T, et al. The Evolution of Ryugu. Encyclopedia. Available at: https://encyclopedia.pub/entry/46898. Accessed June 22, 2024.
Potiszil, Christian, Masahiro Yamanaka, Chie Sakaguchi, Tsutomu Ota, Hiroshi Kitagawa, Tak Kunihiro, Ryoji Tanaka, Katsura Kobayashi, Eizo Nakamura. "The Evolution of Ryugu" Encyclopedia, https://encyclopedia.pub/entry/46898 (accessed June 22, 2024).
Potiszil, C., Yamanaka, M., Sakaguchi, C., Ota, T., Kitagawa, H., Kunihiro, T., Tanaka, R., Kobayashi, K., & Nakamura, E. (2023, July 18). The Evolution of Ryugu. In Encyclopedia. https://encyclopedia.pub/entry/46898
Potiszil, Christian, et al. "The Evolution of Ryugu." Encyclopedia. Web. 18 July, 2023.
The Evolution of Ryugu
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The asteroid 1999 JU3, which would later be named Ryugu, was classified as a Cg-type asteroid in 2001, based on its strong UV absorption feature shortward of 0.55 um and its flat to slightly reddish slope longward of 0.55 um. Cg-type asteroids are part of the C-complex of asteroids, which were suggested to be “primitive” in nature and potentially the parent bodies for carbonaceous chondrites. The linking of carbonaceous chondrites and C-complex asteroids relates to several interpretations concerning features in the near infrared spectra of C-complex asteroids. The features were interpreted as arising from secondary alteration minerals, including goethite, hematite, jarosite and phyllosilicates, that are the products of aqueous alteration and which are found in carbonaceous chondrites.

Ryugu Hayabusa2 organic matter origin of life prebiotic chemistry asteroid comet astrobiology amino acid sample return

1. Ryugu (1999 JU3): Remote Sensing Observations and Predictions

Many carbonaceous chondrite meteorites contain significant amounts of OM [1], which made 1999 JU3 of interest to potential sample return missions that would like to investigate the origin and evolution of OM in our solar system. Additionally, 1999 JU3 represented a particularly accessible asteroid for a mission that would like to rendezvous with a C-complex asteroid [2]. Accordingly, 1999 JU3 was selected as the target of the Hayabusa2 mission [3].
In the time between the selection of the Hayabusa2 mission target and the rendezvous of the Hayabusa2 space craft with Ryugu, several ground-based investigations were undertaken to try and predict what the asteroid may be composed of. The first of these studies suggested that Ryugu was most similar to a heated CM2 chondrite composition, based on comparisons of the reflectance spectra of Ryugu, obtained from the Very Large Telescope in Chile to that obtained from two heated Murchison (CM2) samples (heated to 900 °C and 1000 °C) [4]. In contrast, a subsequent study concluded that Ryugu was actually more similar to an unheated specimen of the Mighei (CM2) meteorite [5].
As the Hayabusa2 spacecraft approached Ryugu, it was able to record spectra of the asteroid’s surface. As a result of the remote sensing investigation, the asteroid was reclassified to a Cb-type asteroid, which is similar to Cg-type asteroids but has a flat or linear slope and lacks the strong UV absorption feature of the Cg-type asteroids. Furthermore, the composition of Ryugu was estimated to be most similar to a moderately dehydrated carbonaceous chondrite, due to the absence of a ubiquitous 0.7 um absorption band (associated with Fe-rich serpentine, a hydrated mineral) and one of the lowest albedos of any solar system object [6].
Accordingly, a scenario was envisioned in which the asteroid Ryugu was formed through the catastrophic collision of a much larger asteroid, and which was subsequently gravitationally reaccumulated to form a rubble pile asteroid with a spinning top shape. Moreover, the dehydration was suggested to have occurred from internal heating, likely due to the progenitor planetesimal of Ryugu forming early in the history of the Solar system, when the short-lived radionuclide 26Al was still very abundant. In this scenario, OM should have also been altered by the heating and bear similarities to that of heated carbonaceous chondrites.
Interestingly, the study also noted that interplanetary dust particles (IDPs) have a similarly low albedo, but preferred heated carbonaceous chondrites, due to their lack of the 0.7 um absorption band [6]. However, Cb-type asteroids were previously linked to IDPs and comet-like icy asteroids [7][8] and so the connection between these extraterrestrial objects and Ryugu permitted further evaluation. Using previous ground-based and Hayabusa2 spacecraft remote sensing data and reflectance spectra from irradiated OM and carbonaceous chondrites, a series of mass balance equations were solved to predict the OM content of Ryugu [9]. The results suggested an incredibly high OM content for Ryugu of between 14.6 and 59.3 volume % (vol.%). Such a finding was backed up by video footage from the Hayabusa2 space craft, which showed displaced material from the first touchdown being lighter on the surface facing away from the asteroid and darker on that pointing towards the interior of Ryugu. The only explanation for a carbonaceous chondrite-like material becoming brighter upon irradiation was that OM had been converted to graphite. If Ryugu had such a high OM content, then it would appear to be IDP-like in nature. As IDPs had been previously linked to comets or described as comet-like [8][10][11], a cometary origin for Ryugu was suggested as an alternative to the catastrophic collision and gravitational reaccumulation model [9].

2. The Formation and Evolution of Ryugu

2.1. Catastrophic Collsion Model

As it currently stands, the most accepted model for the formation and evolution of Ryugu is that of the catastrophic collision and gravitational reaccumulation model (Figure 1) [6][12][13]. In this model, the Ryugu progenitor planetesimal or asteroid was subject to a catastrophic collision, and this created many small debris including boulders and fine particles, which subsequently combined to form Ryugu through gravitational reaccumulation. Indeed, the spinning top-shaped current-day Ryugu has been explained through a combination of a high rotation rate imparted during the collision and the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect [12][13].
Figure 1. An illustration of the catastrophic collision and gravitation reaccumulation model. (1) The Ryugu progenitor planetesimal forms via the accretion of silicate dust and water-rich ice. (2) aqueous alteration leads to the alteration of the silicate dust and the formation of new minerals. Organic matter accreted within the ice is also altered to form new OM. (3) an impactor collides with the planetesimal reducing the body to debris. (4) the debris reaccumulates due to gravity and forms a rubble pile asteroid. (5) Rotation imparted by the impact and from the YORP effect leads to the generation of a spinning top-shaped asteroid and its movement into a near-Earth orbit. Credit for the image of Ryugu belongs with JAXA.
A moderately heated and thus dehydrated carbonaceous chondrite-like composition was one of the many attributes assumed by models that aimed to explain the formation of current-day Ryugu [6][12][13]. Such a characteristic was assumed as a result of the remote sensing data that found no band relating to hydrated silicates [6]. It is now known that Ryugu was not dehydrated or heated to temperatures beyond those of primitive carbonaceous chondrites and this is absolutely clear from the organic matter contained within the Ryugu return samples [14][15][16][17][18][19]. Nevertheless, some of the initial collisional models utilized to explain the formation of Ryugu assumed that the collision would be able to explain the heating and subsequent dehydration of Ryugu [6][13]. Whereas other models suggested that the catastrophic collision would not cause significant heating or shock throughout most of the progenitor body [20].
However, the initial input of the collisional simulation required a very large body (~100 km in diameter). It is unclear if the progenitor planetesimal of Ryugu was this large. In fact, such a large progenitor planetesimal may be very unlikely due to previous estimates of the size at which a body would facilitate fluid convection. Bodies greater than ~80 km in diameter would likely facilitate fluid convection and this would ultimately lead to elemental fractionation throughout carbonaceous chondrites [21][22]. Yet no fluid-related elemental fractionation was observed for the majority of Ryugu particles or for carbonaceous chondrites [14][23]. As such, it is unclear whether or not a catastrophic collision would lead to significant heating if the Ryugu progenitor planetesimal was more similar in size to the minimum estimates of ~20 km in diameter, based on the requirements for heat from 26Al decay to enable liquid water [14][24].

2.2. Cometary Nucleus Model

An alternative to the catastrophic collision and gravitational reaccumulation model is provided by the cometary nucleus model (Figure 2) [9][14][25][26]. In such a scenario, the progenitor planetesimal of Ryugu was broken up by an impact to form ice-rich fragments that would have lost their ice through sublimation to yield rubble pile asteroids. While the planetesimal accretion could have occurred within or near the main belt, it may be more plausible for such an event to have occurred within the outer solar system, due to evidence from Ca and Cr isotopes indicating Ryugu contains the least thermally processed solar system material measured to date [14]. Furthermore, a cometary origin for the Orgueil (CI1) carbonaceous chondrite was proposed previously [27][28] and at current cometary bodies are believed to have formed in the trans-Neptunian disk of the outer solar system [29].
Figure 2. An illustration of the cometary nucleus model. (1) The Ryugu progenitor planetesimal forms via the accretion of silicate dust and water-rich ice within the trans-Neptunian disk (TND). (2) aqueous alteration leads to the alteration of the silicate dust and the formation of new minerals. Organic matter accreted within the ice is also altered to form new OM. (3) during the massive disk phase of the TND, an impactor collides with the planetesimal causing the body to break up into many large chunks (catastrophic impact) or multiple impact events lead to the erosion of the planetesimal to leave behind a much smaller body (sub-catastrophic impacts). (4) After the dispersal of the TND, via a Nice model-like scenario, the Ryugu progenitor planetesimal fragment is transferred into the inner Solar System. In the inner Solar system, the fragment displays cometary behavior, due to the loss of volatiles, which in turn spin-up the body. (5) as a result of the spin-up, the rotation speed of the body increases, and this forms a spinning top shape for the material left behind. Eventually, the body sublimes off all of its surface ice and a rubble pile asteroid is formed. The current day Ryugu may thus resemble an extinct or dormant cometary nucleus. Credit for the image of Ryugu belongs with JAXA.
Accordingly, the planetesimal progenitor of Ryugu could have formed within the trans-Neptunian disk (TND) and suffered a disruptive event during the massive disk phase, where collisions between trans-Neptunian objects would have been large [30]. The collision could have been either catastrophic [31] or sub-catastrophic [32], and thus the resulting object could have represented a true collisional fragment or a substantially eroded portion of the original body. The subsequent dispersal of the TND that formed the Kuiper belt and Oort cloud [29][33], such as that proposed by the Nice model [34][35][36], could have implanted TND objects into the main belt [37]. Indeed, the main belt has been proposed to have begun empty [38] and active asteroids/main belt comets could represent the descendants of implanted TND objects [29][39][40]. If such a scenario is true, then Ryugu may represent an extinct or dormant comet [9][14][40].
Evidence for past cometary activity on the cometary progenitor of Ryugu was provided by the occurrence of unaltered olivine and pyroxene clasts and the presence of a clast-like domain in the matrix of a Ryugu particle with a distinct lithology [14]. It was suggested that ice sublimation would lead to the generation of gas jets, as seen on the comet 67P and 103P/Hartley 2 [41][42][43]. The jets may have then led to the formation of fractures and caused portions of the cometary surface to collapse. Such events could have trapped any exogenous material picked up by the comet as it traveled through the dusty remnants of collisions within the main belt. The sublimation of ice has also been shown to result in the spin-up of the body and can explain the formation of the spinning top shape of Ryugu [25].
Furthermore, cometary jets are thought to deposit a portion of their material back onto the comet’s surface and this can become sintered in place, trapping any organic or dusty material entrained in the jet [41]. Such a process was employed to explain some of the geochemical differences observed between TD1 and TD2 particles and the larger geochemical heterogeneity found among the TD1 particles [14]. As material from many different depths can be entrained within the jets, if jets were operating in the past at or near the TD1 site, material from distinct regions of the cometary progenitor of Ryugu could be accumulated within a narrow region on its surface. Meanwhile, if the TD2 site did not experience jet activity the samples may just record a single region of Ryugu and thus appear more similar.
In terms of organic matter, Ryugu appears to contain CI-chondrite-like organic matter including its amino acids. Accordingly, it has been proposed that CI chondrites, such as Orgueil, may have a cometary origin due to the distinct differences between their amino acids and those of CM2 chondrites [44]. Furthermore, the discovery that Ryugu contains more aromatic or highly aromatic material related to organic particulates or nanoglobules [19] than carbonaceous chondrites may imply that it accreted a more primitive composition. In agreement, the finding that Ryugu MOM/IOM has a high CH2/CH3, which is similar to that observed in IDPs and for ISM material, may indicate that Ryugu accreted more comet-like or ISM-like organic matter than most carbonaceous chondrites. Together with the overlap of H and N isotopic values and N/C ratios for Ryugu organic matter and values determined for some comets, the notion of a cometary origin for Ryugu is certainly plausible. Moreover, a cometary origin for Ryugu proposes the advantage of not requiring a catastrophic impact of a rocky asteroid, which may cause significant heating and shock-related features

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