A New Iteration of the Asteroid Taxonomy

The classification of the minor planets of the Solar System has been revisited regularly since its first outline in the 1970s as dichotomy of carbonaceous and silicaceous asteroids. The reformulations followed insights granted by new data (e.g. surveys such as ECAS, SMASS) or extensions of the observable feature space (e.g. CCD spectroscopy).

Since the last major update of the taxonomy by DeMeo et al. in 2009, we have seen a wealth of new data provided by targeted campaigns, which show that the spectroscopically-defined class boundaries do not align sufficiently with mineralogical and meteoritic population trends. Examples are the continuous trend between B- and C-type objects (Clark et al. 2010, De Leon et al. 2012) and the large subclassing of the continuous S-type asteroids (Vernazza et al. 2014). Furthermore, any compositional interpretation of members of the X-complex is tentative as shown by the diversity of radar albedos, densities, and spectral properties observed among them (Shepard et al. 2010, 2015, Carry 2012, Neeley et al. 2014).

Given the increase of data and insights into the compositional asteroid Main Belt, we derive a new iteration of the asteroid taxonomy, focusing primarily on a methodological improvement. Acknowledging that future survey efforts will contribute asteroid spectra in different wavelength ranges (e.g. Gaia (visible), SPHEREx (NIR), MITHNEOS (visible-NIR)), we evolve the current method into a probabilistic model approach which enables us to classify complete and partially observed spectra in the same scheme. We further re-introduce the visual albedo into the classification space to disentangle the degenerate X-complex.

By means of a clustering analysis of almost 3,000 minor body spectra and albedos, we devise a taxonomic scheme consisting of 17 classes with close resemblance to the Bus-DeMeo and Tholen systems (refer to Fig. 1): A, B, C, Ch, D, E, K, L, M, O, P, Q, R, S, X, V, Z. The two main complexes remain C and S. We resolve the X-complex and replace it by the new M-complex. Subclasses in the Bus-DeMeo system are replaced by a mineralogical interpretation of the continuous distribution of minor bodies in the classification space. The new class Z entails extremely-red objects in the inner Main Belt including the supposed TNO-implants (203) Pompeja and (269) Justitia (Hasegawa et al. 2021).

A python-tool to classify minor body observations in the derived taxonomic scheme is freely accessible to the community (https://classy.readthedocs.io).

Figure 1: Evolution of the asteroid taxonomy from the Tholen- (Tholen 1984) over the Bus-DeMeo- (Bus and Binzel 2002, DeMeo et al. 2009) to the system established in this work.

 

References

Clark, B. E., Ziffer, J., et al. (2010), Spectroscopy of B-type asteroids: Subgroups and meteorite analogs.
Carry, B. (2012), Density of asteroids.
Bus, S. J. and Binzel, R. P. (2002), Phase II of the Small Main-Belt Asteroid Spectroscopic Survey. A Feature-Based Taxonomy.
DeMeo, F. E., Binzel, R. P., et al. (2009), An extension of the Bus asteroid taxonomy into the near-infrared.
de León, J., Pinilla-Alonso, N., et al. (2012), Near-infrared spectroscopic survey of B-type asteroids: Compositional analysis.
Hasegawa, S., Marsset, M., et al. (2021), Discovery of Two TNO-like Bodies in the Asteroid Belt.
Neeley, JR., Clark, BE., et al. (2014), The composition of M-type asteroids II: Synthesis of spectroscopic and radar observations.
Shepard, M. K., Clark, B. E., et al. (2010), A radar survey of M- and X-class asteroids II. Summary and synthesis.
Shepard, M. K., Taylor, P. A., et al. (2015), A radar survey of M- and X-class asteroids. III. Insights into their composition, hydration state, & structure.
Vernazza, P., Zanda, B., et al. (2014), Multiple and Fast: The Accretion of Ordinary Chondrite Parent Bodies.