Probe the regolith characteristics of asteroids from 9-years infrared observations of WISE/NEOWISE: A case study of the Main-Belt Object (656) Beagle

TL;DR

This study analyzes 9-year infrared data of asteroid (656) Beagle from WISE/NEOWISE to determine its physical and surface properties, revealing insights into its composition, surface heterogeneity, and possible origin scenarios.

Contribution

It provides detailed thermophysical modeling of Beagle's regolith and challenges previous assumptions about its surface heterogeneity and relationship with nearby asteroids.

Findings

Beagle's diameter is approximately 57 km.

Surface albedo is unusually low and similar to nearby asteroids.

Infrared light curves suggest a more uniform surface than optical models indicate.

Abstract

This work presents data processing, fitting procedure, modelling and analyzing of 9-years infrared light curves provided by the WISE/NEOWISE telescope, by which the regolith characteristics of Main-Belt Object (656) Beagle is studied. We determine Beagle's effective diameter $D_{\rm eff}=57.3^{+4.5}_{-2.2}$ km, geometric albedo $p_{\rm v}=0.05^{+0.004}_{-0.007}$, mean roughness $\theta_{\rm RMS}=44\pm4^\circ$, mean grain size $b=100^{+350}_{-90}~\mu$m, mean specific heat capacity $c_{\rm p}=173\sim516\rm~JKg^{-1}K^{-1}$, mean thermal conductivity $\kappa=0.7\sim1.3\times10^{-3}\rm~Wm^{-1}K^{-1}$ and mean thermal inertia $\Gamma=14\sim32\rm~Jm^{-2}s^{-0.5}K^{-1}$. The albedo of Beagle is a little anomalous that the albedos of Beagle's neighbouring asteroids are more close to Themis, rather than Beagle itself. The W1-band near-infrared light curves don't reveal significant heterogeneous NIR features on the surface of Beagle, being inconsistent with the expectation of a family parent that has members with diverse NIR spectral types. These results add new clues of Beagle probably being an interloper or a sister, rather than the parent of its neighbouring asteroids including the first main-belt comet (MBC) 133P, hence may lead to new scenarios about the origin of famous MBC 133P. Besides, we found that asteroidal shape models from inversion of optical light curves are imperfect for modeling infrared lightcurves, thus could mislead evaluations of both the heterogeneity of regolith reflectivity at near infrared and thermophysical characteristics at thermal infrared.