Quantitative grain size estimation on airless bodies from the negative polarization branch

II. Dawn mission targets (4) Vesta and (1) Ceres

¹ Korea Astronomy and Space Science Institute (KASI), 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Republic of Korea
e-mail: ysbach93@gmail.com
² Department of Physics and Astronomy, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 08826, Republic of Korea
e-mail: ishiguro@snu.ac.kr
³ SNU Astronomy Research Center, Department of Physics and Astronomy, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 08826, Republic of Korea
⁴ Center for Astronomy, University of Hyogo, 407-2 Nishigaichi, Sayo, Hyogo 679-5313, Japan
⁵ Japan Spaceguard Association, Bisei Spaceguard Center, 1716-3 Okura, Bisei, Ibara, Okayama 714-1411, Japan
⁶ Nayoro Observatory, 157-1 Nisshin, Nayoro, Hokkaido 096-0066, Japan
⁷ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Received: 12 December 2023
Accepted: 10 January 2024

Abstract

Context. Sunlight scattered from the surface of an airless body is generally partially polarized, and the corresponding polarization state includes information about the scattering surface, such as albedo, surface grain sizes, composition, and taxonomic types. Most polarimetric studies on airless bodies thus far have focused on optical wavelengths (λ ≲ 1 µm).

Aims. We conducted polarimetry of two large airless bodies, the Dawn mission targets (1) Ceres and (4) Vesta, in the near-infrared region. We further investigated the change in the polarimetric phase curves over the wavelengths expected from previous works.

Methods. We used the Nishiharima Infrared Camera (NIC) installed at the Nishi-Harima Astronomical Observatory (NHAO) to observe these objects at multiple geometric configurations in the J, H, and K_s bands (λ ~ 1.2–2.3 µm).

Results. Polarimetric parameters were determined and compared with previously reported experimental results. In particular, Vesta exhibits a characteristic change in the negative polarization branch as the wavelength increases to the K_s band, which we interpret as an indication of the dominant existence of D ~ 10–20 µm particles. Our approach is supported by empirical reasoning and coincides well with an independent, theory-driven approach based on thermal modeling.

Conclusions. This work demonstrates how near-infrared polarimetry can be utilized to quantitatively determine the particle size of airless objects. This finding will have important implications for asteroid taxonomy and regolith evolution.