Unraveling the opposite spectral evolutions on Ryugu and Bennu

Primitive asteroids may retain the record of volatile-rich planetesimals formed during the early solar system evolution. Our understanding of their diversity relies heavily on ground-based telescope observations of visible spectra. However, the interpretation of the featureless visible spectra is often challenging. Comparing two near-Earth primitive asteroids Ryugu and Bennu encountered by Hayabusa2 and OSIRIS-REx is crucial in this regard because their visible spectra differ : Ryugu has a reddish color (Cb-type) while Bennu is blue (B-type). Despite the spectral difference, recent analyses of samples returned from Ryugu and Bennu indicate that they are both consistent with low-petrologic-type carbonaceous chondrites, suggesting that their compositions may not differ as much as previously assumed. So, what makes the spectra of primitive asteroids different ?

To address this question, we compared the spectral and photometric properties of Ryugu and Bennu in the 0.48–0.85 μm wavelength range using data from both remote sensing and sample analyses. The precise comparison of the two asteroids was made possible by cross-calibrating the two remote-sensing instruments onboard Hayabusa2 and OSIRIS-REx. We show that the spectral distributions of craters on Ryugu and Bennu follow a common trend line in the reflectance–spectral slope diagram. In addition, the spectra of fresh craters on both asteroids are indistinguishable within the cross-calibration accuracy. The findings suggest that Ryugu and Bennu initially had similar visible spectra, but they evolved into spectrally distinct asteroids by processes such as (1) solar wind/micrometeorite bombardment, (2) solar heating, and (3) grain size/porosity evolution. We obtained multiple lines of evidence suggesting that the most plausible process may be (3). For instance, the spectral difference between coarse ( 1 mm) and fine (<300 µm) grained Ryugu samples qualitatively aligns with the observed spectral evolution trend. Our model calculation shows that such opposite evolution of grain size/porosity may be simply explained by their difference in asteroid size. This hypothesis implies that asteroids with different spectral types can actually have similar compositions, and thus Ryugu/Bennu-like materials may be more widespread in the solar system than previously assumed.