Visible color of the artificial crater on Ryugu created by Small Carry-on Impactor

Introduction:  The Japanese spacecraft Hayabusa2 made the rendezvous with a C/F-type asteroid Ryugu from June 2018 to November 2019 [1]. After the global mapping and the first touch-down operation, Small Carry-on Impactor (SCI) operation was successfully conducted on 5 April 2019 to make an artificial crater (SCI crater) [2]. This operation aimed to reveal cratering mechanism on asteroids with very small gravity. Mass of the impactor is ~2 kg and the impact speed was estimated as ~2 km/s. Moreover, this operation gave us the precious opportunity to uncover the internal material of Ryugu and observe it by the remote-sensing instruments. The observation of SCI crater might give us the insights of space weathering, heating, and layering on Ryugu. Hayabusa2 is equipped with a telescopic multiband camera, ONC-T with seven color filters in UV to NIR wavelength; ul: 0.40 µm, b: 0.48 µm, v: 0.55 µm, Na: 0.59 µm, w: 0.70 µm, x: 0.86 µm, p: 0.95 µm [3].

We investigate spectral slope, UV down/up-turn, and 0.7-µm band absorption around the SCI crater. In the global observation, the spectral slope was the most prominent variation in visible spectra [3]. The bluer regions, such as the equatorial ridge and the pole regions, of Ryugu correspond to the higher gravitational potential regions, suggesting the mass wasting from higher to lower potential regions leads to expose the fresh bluer subsurface material on Ryugu [3]. Recently more detailed mapping on the pole regions were conducted and possible 0.7-µm band absorption and UV less upturn (flat UV) were suggested [4]. These two features could be indicative of hydrated minerals, such as Fe-bearing phyllosilicates. This observational result leaded a scenario of space weathering by solar wind in the past [4]. The observational facts from the SCI crater could be also strong constraints on space weathering effect on Ryugu. In this study, we report the visible color observation results of the SCI crater and discuss the processes occurred on the surface of Ryugu in the context of both the global observation and the SCI crater observation.

Data processing:  Before (8 March 2019) and after (16 May 2019) the SCI operation, ONC-T multi-band observations at the SCI target site were conducted. The spatial resolution of before and after the SCI observations are 13 and 5.5 cm/pixel, and the phase angles are 17˚ and 30˚, respectively. After the conversion to radiance factor (I/F) [5,6] and co-registration to v-band images, we measured the spectral indexes, such as spectral slope (0.48 - 0.86 µm), 0.7-µm band depth, and UV-index. UV-index measures the excess from the spectral slope.

Color of the SCI crater:  Figure 1 shows the spectral slope map for the SCI crater. The floor of the SCI crater is found to be bluer than surrounding region. The clear color difference is observed between inside and outside of the SCI crater. The floor material might be excavated sublayer of the impact site. It should be also noted that the possible impact site between two boulders, Mobile boulder and Stable boulder, are redder in comparison with the global average. We also see the relatively red part on the eastern wall of the crater. Moreover, after the SCI operation, Mobile boulder was excavated and we see the bluer color for the part which had been lied underground. The UV-index value of inside the SCI crater is slightly smaller than surrounding area. However, the 0.7-µm band depth seems to be similar for inside and outside of the SCI crater.

 

Discussions: Based on the global observation, we had expected to see blue material inside of the SCI crater. Even we see the relatively bluer material inside of the SCI crater than its vicinity, the degree of blueness of the SCI crater is similar to the equatorial ridge and is far redder than the material on both north and south poles which has negative value of spectral slope. Relatively blue material inside of the SCI crater supports the hypothesis of the fresh bluer material under the red surface material as suggested by [3,7]. However, since we see the bluest material on the pole regions, the sublayer of the SCI target region might have experienced space weathering process in the past. Moreover, absence of 0.7-µm band feature also suggests that the space weathering had affected not only the outermost surface but also meter-scaled substrates. Thus, we hypothesize mass wasting by the YORP spin-up on the formation of the equatorial ridge [8], gardening by impacts, or granular convection [9] could form the space weathered subsurface layer.

Another interesting color feature is the redder material in the middle of the SCI crater, at the impact point. We can see the similar feature for some of the natural craters of several tens meter scale. This could be explained by the metamorphism by high pressure and temperature process [10], breaking up of material just at the impact site which can be sometimes observed by the impact experiments, or excavating another layer underneath.

Acknowledgments: We are grateful to the entire Hayabusa2 team for making the encounter with Ryugu possible. This study is supported by the JSPS core-to-core program “International Network of Planetary Sciences”. ET acknowledges financial support from the project ProID2017010112 under the Operational Programs of the European Regional Development Fund and the European Social Fund of the Canary Islands (OP-ERDF-ESF), and the Canarian Agency for Research, Innovation and Information Society (ACIISI). 

References: [1] Watanabe, S. et al. (2019) Science 364, 268. [2] Arakawa, M. et al.(2020) Science 368, 67. [3] Sugita, S. et al. (2019) Science 364, eaaw0422. [4] Tatsumi, E. et al. (2019) Asteroid Science in the Age of Hayabusa2 and OSIRIS-Rex, #2091. [5] Tatsumi, E. et al. (2019) Icarus 325, 153. [6] Tatsumi, E. et al. (2019) LPSC 50, Abstract #1745. [7] Morota, T. et al. (2020), Science 368, 654. [8] Sugiura, K. et al. (2019) Asteroid Science in the Age of Hayabusa2 and OSIRIS-Rex, #2012. [9] Tsuchiyama, A. et al. (2011) Science 333, 1125. [10] Hiroi, T. et al. (2020) LPSC 51.