Nordic-based Observations Of Binary Asteroid (163693) Atira Reveal A Hydrated CM Chondrite Composition

[astro-ph.EP] We present photometric, spectroscopic, and polarimetric observations of (163693) Atira collected on the night of 05 March 2024. A dual-band lightcurve acquired in the V and R_C filters using the Metsähovi Observatory (L08) in Kirkkonummi, Finland spans nearly 8~hours.

From these measurements we estimate a V-R_C = 0.43 +/- 0.07 mag. A visible spectrum and linear polarisation measurement were obtained with the Nordic Optical Telescope in La Palma, Spain. We employ a semi-empirical model to recover the spectral flux that was lost due to a mis-aligned spectroscopic slit.

The corrected model reflectance spectrum indicates a primitive Cgh taxonomic type with a distinct absorption feature from 0.60 to 0.85 um with a depth of ~7.4 %. Direct comparison with laboratory spectra of meteorites indicates close matches with CM carbonaceous chondrites.

We detect a large degree of linear polarisation at a phase angle of 83 degrees which is consistent with (3200) Phaethon and (155140) 2005 UD, but lower than (162173) Ryugu and (152679) 1998 KU2. We use a CM carbonaceous chondrite density to estimate its bulk porosity of 51 +/- 31 % from previously estimated bulk density.

The uncertainty in bulk density and bulk porosity is dominated by the volume of the primary. Future near-infrared spectroscopic and thermal infrared observations can be used to refine the composition and bulk density.

Finally, we discuss implications for ground-based physical characterisation studies of the Atira population.

Eric M. MacLennan, Grigori Fedorets, Zuri Gray, Teemu Willamo, Tuomas Salo, Anne Virkki, Mikael Granvik, Karri Muinonen

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2609.27722 [astro-ph.EP] (or arXiv:2609.27722v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.27722
Focus to learn more
Submission history
From: Eric MacLennan
[v1] Wed, 23 Sep 2026 11:39:32 UTC (4,263 KB)
https://arxiv.org/abs/2609.27722

Astrobiology, Astrogeology, Astrochemistry,