TOPICSONC reveals the dark and carbon-rich
nature of Ryugu

We introduce a new paper on Ryugu with results revealed by the Optical Navigation Camera (ONC) onboard the Hayabusa2 spacecraft. Based on a half-year of intensive observations by ONC, we obtained precise reflectance properties of Ryugu. These results will be published in the journal Astronomy and Astrophysics.

Title: Global photometric properties of (162173) Ryugu
Authors: E. Tatsumi, D. Domingue, S. Schröder, Y. Yokota, D. Kuroda, M. Ishiguro, S. Hasegawa, T. Hiroi, R. Honda, R. Hemmi, L. Le Corre, N. Sakatani, T. Morota, M. Yamada, S. Kameda, T. Kouyama, H. Suzuki, Y. Cho, K. Yoshioka, M. Matsuoka, C. Honda, M. Hayakawa, N. Hirata, N. Hirata, Y. Yamamoto, F. Vilas, N. Takato, M. Yoshikawa, M. Abe, S. Sugita
DOI: 10.1051/0004-6361/201937096

Hayabusa2 is equipped with a telescopic multi-band imaging camera, ONC-T. In this study we used images obtained by the ONC-T from July 2018 to January 2019; approximately half a year. In order to measure the reflectance properties of Ryugu, we needed a variety of illumination and observation angles. Generally, Hayabusa2’s antenna and solar panels are pointing towards the Earth, and the attitude of Hayaubusa2 was decided by the geometry of the Earth, Ryugu, and the Sun (Fig. 1). As a result, the illumination and observation angles change as time passes. It therefore took a while to capture images with a variety of angles. In a previous study, Sugita et al. (2019) measured the reflectance of the asteroid surface. However, at that time the coverage of a variety of angles was not very great. This study constrained the reflectance better with wide range of angles. We had two objectives:
1. To compare ground-based and spacecraft-based observations in order to check the accuracy of the ONC calibration.
2. To reveal the reflectance properties of Ryugu and estimate the surface grain properties.
Our study confirmed the calibration accuracy of ONC. Also, we revealed that the standard reflectance of Ryugu is very low at just 1.87±0.14%, suggesting that the carbon content of Ryugu might be higher than 2wt.%. Moreover, we observed phase reddening effect (see below) on Ryugu, suggesting its surface is covered by very fine grains (~µm). Surprisingly, the reflectance and phase reddening degree of Ryugu are very similar to those of asteroid Bennu. The two asteroids may share similar surface reflectance properties.

Figure 1. Schematic diagram to show the relative positions of the Sun, Ryugu, and Hayabusa2. As time passes, this relative position changes and we could achieve a wide range of phase angles. © Eri Tatsumi

■Comparison between the ground-based and the ONC observations
In this study, we compared the spectrum of Ryugu observed by ONC with 24 ground-based telescopic observations, including three spectra obtained by the Hayabusa2 ground-based observation team at the GEMINI-South telescope in 2012. The ground-based spectra obtained in low Airmass (AM, the degree of atmospheric thickness) and large aperture have particularly low noise levels (high SNR). Comparing these high accuracy ground-based spectra with the spectrum obtained by ONC shows consistency within error bars (Figure 2). Both sets of results showed a flat spectra. Thus, we could ensure that the calibration of ONC is sufficiently good.

Figure 2. 24 ground-based observations (Red and Gray) and the ONC observation (Black) of Ryugu. They are ordered by smaller to larger noise levels (high to low SNR). The spectra we obtained at GEMIMI-South are S01, S02, and S03 (Tatsumi et al. 2020).

■Phase function
The phase function describes how the reflectance (brightness) of Ryugu changes as a function of phase angle. Ryugu’s phase function was obtained as shown in Fig. 3. You can see a sharp increase in reflectance value towards a phase angle of 0˚, where the Sun is just behind the observer. This sharp increase is called “opposition effect” and is often seen in the atmosphere-less bodies, for example the Moon and asteroid Itokawa (web article of Itokawa opposition effect (in Japanese): The phase function has different characteristics that reflects the physical properties, such as roughness or particle size, of the surface. Again, the ground-based and ONC observations show good agreement. By fitting and modeling this phase function curve, we could obtain Ryugu’s reflectance (albedo) map which simulates the reflectance with the same observation and illumination geometry condition (Fig. 4).

Figure 3 Phase function of Ryugu (the relationship between phase angle and reflectance). Color markers indicate the ONC observations (Box-A, Box-B, etc.) and gray markers indicate the ground-based observations. The two observations are consistent, indicating the accuracy of the ONC calibration. Sharp increase of reflectance can be seen around phase angle 0˚. (Tatsumi et al. 2020)

Figure 4. Reflectance (albedo) map of Ryugu. This map was made from 12 images which were corrected to the viewing geometry of phase angle 30˚ based on the phase function. Some places including the equatorial ridge show high reflectance value. However, the variation of reflectance is relatively small ~10%.
(Tatsumi et al. 2020)

■Dark Ryugu
The blackness/whiteness of an object can be expressed by reflectance (albedo). This study showed Ryugu’s reflectance (albedo) to higher accuracy through using a larger quantity of image data. There are different definitions of reflectance depending objectives. Here we explain most commonly used reflectance;
Geometric albedo: The ratio brightness of the object observed at phase angle 0˚ and a disk with the perfectly diffusive surface. This value is often used to describe the brightness of ground-based observations.
Standard reflectance (Reflectance factor at standard geometry): The ratio brightness of the object observed at phase angle 30˚ and a disk with the perfect diffusive surface. This value is often used to describe the brightness of meteorite samples in laboratory.
We found that the geometric albedo is 4.0±0.5% and standard reflectance is 1.87±0.14% for Ryugu. This reflectance updated the value in Sugita et al. (2019) to a slightly lower value. This suggests that the carbon contents of Ryugu is very high >2wt.%. Figure 5 shows the reflectance and carbon contents of thermally metamorphosed carbonaceous chondrites (Cloutis et al. 2012) which are considered to be similar to Ryugu composition. This relationship indicates the lower the reflectance, the higher the carbon contents. We still don’t know what kind of compounds the carbon is forming. However, so far organic compounds have been widely found in the carbonaceous chondrites. We therefore expect to see large amount of organic compounds from Ryugu’s returned samples.

Figure 5. Reflectance and carbon contents thermally metamorphosed carbonaceous chondritic samples (after Cloutis et al. 2012). Filled squares are powder samples and open squares are chip samples. As we found the reflectance of Ryugu is ~1.9% (red line), we can estimate that the carbon contents of Ryugu is very high (Orange region). This might be found as organic compounds.

■Phase reddening effect
The phase reddening effect is a known effect on many atmosphere-less objects, such as asteroids, comet Churyumov-Gerasimenko and the Moon. For all of these bodies, the larger phase angle, the redder the object looks. This effect had been thought to be not evident for dark objects, but we confirmed this effect on Ryugu. The precise cause of the phase reddening effect is still under investigation, but multiple scatterings caused by wavelength-scale particles or physical structures on the surface is considered to be a key to this effect. Thus, our observational results suggest that the presence of fine grains or structure on the Ryugu surface.
Surprisingly, the reflectance and degree of phase reddening effect of Ryugu are very similar to those of Bennu, which is a target of the OSIRIS-REx mission (Fig. 6). This suggests that the surface properties of the two objects may be very similar. So far, we knew of a few similarities between Ryugu and Bennu, for example density and shape, and now we found a further similarity in reflection properties. On the other hand, there is a difference in hydration degree; Bennu is more hydrated than Ryugu. The new results make the mystery between two objects go deeper and deeper…

Figure 6. Geometric albedo and degree of phase reddening for various bodies. Y-axis shows the degree of phase reddening and larger value means a stronger effect. Ryugu and Bennu place very close each other, suggesting similar surface properties.

Samples from Ryugu will be on Earth this December. Results from remote-sensing data will be verified by the sample. We are looking forward it, but at the same time we feel impatient to know the answer… What treasure will Hayabusa2 bring back to us?

Eri Tatsumi(Hayabusa2 Project)