The dark surface of (163693) Atira and implications for inner-Earth asteroid origins

Abstract

We compare the physical properties of inner-Earth asteroid (163693) Atira with the NEOMOD3 population model. Its radar-derived diameter of 4.9 km and absolute magnitude H$_V\simeq$16.4 imply a geometric albedo of p$_V\sim$0.02. Photometry with the Astrophysical Research Consortium 3.5-m telescope and ARCTIC yields g-r=0.49$\pm$0.03, r-i=0.20$\pm$0.02, i-z=0.02$\pm$0.03, a$^\ast$=$-0.05\pm0.03$, and a g-to-i spectral slope of 3.4$\pm$0.9$\%$/100 nm. The photometric reflectance favors a provisional X-type classification; combined with the low albedo, it supports a Tholen P-type interpretation. NEOMOD3 assigns an approximately 66$\%$ probability of delivery through inner Main Belt channels and 26$\%$ to central and outer Main Belt channels. Atira's NEOMOD3 albedo-category probabilities are $0.07\pm0.02$, $0.64\pm0.04$, and $0.29\pm0.04$ for p$_V<$0.1, 0.1$\leq$p$_V<$0.3, and p$_V\geq$0.3, respectively. Across 38 inner-Earth objects, including Atira and 'Ayló'chaxnim, inner Main Belt delivery generally dominates, and intermediate albedo is favored for 33 objects. Atira therefore represents an uncommon low-albedo outcome, compatible with delivery of dark material from the inner Main Belt or more distant primitive populations. Further characterization of inner-Earth asteroids will test whether current population models reproduce their compositional diversity.