Meteorite analogs of the Didymos binary asteroid system and implications for its origin

The Didymos binary asteroid system is a key target for planetary-defense science. Its satellite Dimorphos was impacted by NASA Double Asteroid Redirection Test (DART) mission, and the system will be further investigated by ESA Hera mission. Although Didymos is generally recognized as an S-type asteroid with ordinary-chondrite-like spectral characteristics (de León et al. 2010), its specific meteorite analog and the physical cause of the spectral changes observed after the DART impact remain debated.

Here we investigate the mineralogical composition, meteorite analog, and surface evolution of the Didymos system using visible and near-infrared reflectance spectra. We analyzed seven spectra of the Didymos system obtained around the DART impact, from the night before impact to several nights after impact (Polishook et al. 2023). To ensure internal consistency in the spectral interpretation, we compiled an expanded dataset of ordinary chondrite spectra with corresponding mineralogical measurements and established new calibration equations linking band parameters to olivine abundance, fayalite content, and ferrosilite content. These calibrations were then applied to the Didymos spectra after band-parameter analysis.

The derived mineralogical parameters indicate that Didymos is most consistent with L ordinary chondrites, with a preference for highly metamorphosed type 6 material. An LL-chondrite interpretation is less likely, although it cannot be completely excluded. This suggests that the Didymos system may have originated from the highly thermally metamorphosed inner region of an L-chondrite parent body.

We further used ratio spectra to distinguish between the effects of space weathering and grain size on ordinary-chondrite-like surfaces. Laboratory meteorite spectra show that, for H and L chondrites, grain-size variations commonly produce diagnostic deviations near the 1 and 2μm absorption bands when ratio spectra are compared with a space-weathering function. In contrast, the ratio spectra of the Didymos system are well described by a space-weathering trend and do not show the characteristic absorption-band-related features expected for dominant grain-size effects. Therefore, the spectral changes observed after the DART impact are most likely controlled by differences in space-weathering state, rather than by grain size or compositional heterogeneity.

Finally, we estimate a surface exposure age of approximately 0.39 Myr for Didymos from the degree of spectral weathering. This age is consistent with the reported surface crater retention age of Dimorphos, supporting the interpretation that Didymos and Dimorphos are compositionally homogeneous and that the binary system formed through rotational fission. These results provide new spectral evidence for the origin and evolution of the Didymos system and demonstrate that meteorite analog identification is essential for interpreting asteroid spectral changes caused by impact events.

References：

de León, J., Licandro, J., Serra-Ricart, M., Pinilla-Alonso, N., & Campins, H. (2010). Observations, compositional, and physical characterization of near-Earth and Mars-crosser asteroids from a spectroscopic survey. Astronomy & Astrophysics, 517, A23.

Polishook, D., DeMeo, F. E., Burt, B. J., Thomas, C. A., Rivkin, A. S., Sanchez, J. A., & Reddy, V. (2023). Near-IR spectral observations of the Didymos system: daily evolution before and after the DART impact indicates that Dimorphos originated from Didymos. The Planetary Science Journal, 4(12), 229.