|Title:||Collisional history of Ryugu’s parent body from bright surface boulders|
|Author：||E. Tatsumi, C. Sugimoto, L. Riu, et al.    Full List|
|Journal：||Nature Astronomy (2020)|
Hayabusa2’s Optical Navigation Camera (ONC) has found many anomalously bright boulders on the dark surface of C-type asteroid Ryugu. Observations with ONC and the near-infrared spectrometer (NIRS3) indicate that at least six of these bright boulders exhibit reflectance spectra consistent with an exogeneous origin; their spectra are similar to S-type asteroids. This means that these bright boulders resulted from a collisional mixing between Ryugu’s parent body and S-type asteroid(s). On Bennu, the asteroid explored by NASA’s OSIRIS-REx, bright boulders with spectra similar to V-type asteroids were found. This suggests that Ryugu and Bennu might have gone through different collisional histories. Moreover, the majority of bright boulders on Ryugu have spectra similar C-type asteroids. This type of bright boulder is likely endogenic, and may have originated from different parts within Ryugu’s parent body, which would have experienced variations in thermal history. Because the samples collected from Hayabusua2 may contain those bright materials, we may be able to obtain detailed knowledge on Ryugu’s collisional and thermal history from the analyses of the Hayabusa2 samples.
The rubble-pile asteroid Ryugu was formed by the catastrophic disruption of a parent body and subsequent re-accumulation of fragments. The initial global observations of Ryugu from the Home Position (20 km away from the Ryugu center) have shown that Ryugu is dark and uniform. However, high-resolution images (several mm/pix to several cm/pix) obtained with ONC at lower altitudes, reveal that many anomalously bright boulders were found everywhere on Ryugu’s surface (Figure 1).
Among those bright boulders, we analyzed the reflectance spectra of the 21 relatively large (>10s cm) boulders using both ONC and NIRS3. The ONC multi-band images show that six bright boulders show absorption around 1 µm, which is a typical property of Mg-Fe-rich anhydrous silicates (Fig. 2). As Ryugu is composed from moderately hydrated minerals, these anhydrous silicates might have been formed in a completely different environment. These six bright boulders were also observed by NIRS3 and were suggested to have similar spectra to S-type asteroids and ordinary chondrites, which are rich in Fe-Mg-rich anhydrous silicates. In contrast, the other 15 bright boulders have spectra similar to Ryugu, and are also relatively dark, suggesting that they may be a kind of carbonaceous chondrites.
Why were anhydrous silicates on Ryugu?
Because of the very different material properties between anhydrous silicates and bulk Ryugu materials, anhydrous silicates are highly unlikely to originate from the Ryugu parent body. Thus, the anhydrous silicates have had to be mixed with Ryugu in some way. There are two possible scenarios to mix them. (1) Anhydrous silicates recently impacted on Ryugu. (2) Mixing during a catastrophic disruption between Ryugu’s parent body and an anhydrous impactor. The first scenario, however, is less likely because of the presence of many large bright boulders, and that the impactors need to hit with a very small velocity to avoid the impactor fracturing. Thus, Ryugu’s parent body is likely to have experienced collision(s) with anhydrous silicate (i.e., S-type) asteroid(s), which resulted in a mixture between S-type bright boulders and the largely dark Ryugu (Figure 3). Ryugu is thought to have originated from the inner part of the asteroid main belt. There is a broadly distributed asteroid family (Nysa-Polana-Eulalia complex), which consists of carbonaceous asteroids, (i.e., C-complex), and anhydrous silicate asteroids, (i.e., S-complex). In this region, a collision between C-complex and S-complex asteroids could frequently happen (Fig. 4).
Figure 1. High-resolution images of the bright boulders (arrows) in proximity operations.
Especially, the images obtained during the first touchdown operation (a-c) show much
smaller bright fragments in regolith everywhere. (after Fig.1 of Tatsumi et al. 2020)
Figure 2. Reflectance spectra of 21 bright boulders obtained by ONC.
Some of them show absorption towards 1 µm, suggesting anhydrous silicates. (Tatsumi et al. 2020)
What are the carbonaceous bright boulders?
The difference in brightness of the carbonaceous bright boulders could be explained by differences in experienced temperatures. For example, previous studies during heating experiments of hydrous carbonaceous chondrites, such as Murchison and Ivuna, show that, depending on the experienced temperature, brightness can change by more than a factor of two. Difference in experienced temperatures could be due to temperature differences inside the parent body and/or the impact heating during the catastrophic disruption. The answer could be derived by the sample analyses.
Figure 3. Schematic image of the formation of bright boulders on Ryugu.
Figure 4. Nysa-Polana family is distributed in the inner asteroid main belt.
This family composes S-type and C-type asteroids, sharing similar orbital elements. © Eri Tatsumi
Bright boulders were also found on Bennu.
A paper published on the same day presents bright boulders on asteroid Bennu, identified by NASA’s OSIRIS-REx team.* Compared with the bright boulders on Ryugu, the bright boulders on Bennu show deeper absorption at 2 µm, suggesting the boulders are composed of basaltic howardite–eucrite–diogenite (HED)-meteorite-like materials, which can be seen in Vesta and the Vestoid family. Thus, the two different materials observed on Ryugu and Bennu indicate different collisional histories. This means that they are likely to have originated from different parent bodies.
Hayabusa2 will return to Earth with the samples from two touchdowns on Ryugu on December 6, 2020. Small amounts of the bright fragments could be included in the sampled materials. High-precision analysis of such a small amount of sample recently developed for Hayabusa2 mission will reveal more details of the history of Ryugu.
*DellaGiustina, D.N., et al. (2020) “Exogenic basalt on asteroid (101955) Bennu” Nature Astronomy, doi.org/10.1038/s41550-020-1195-z
Eri Tatsumi（Hayabusa2 Project)