Radar Circular Polarization Ratio of Near-Earth Asteroids: Links to Spectral Taxonomy and Surface Processes

Abstract

The radar circular polarization ratio (CPR) is an indicator of near-surface, wavelength-scale complexity. Interestingly, previous surveys also noted an association between near-Earth asteroid (NEA) disk-integrated CPR and spectral taxonomic class, potentially reflecting differences in regolith mechanical properties and/or surface histories. Here, using Arecibo S -band radar measurements, we revisit this relationship, as well as associations with orbital and physical parameters, to test the impact of compositional class and surface processes on CPR. We found that the X complex, and potentially V-type NEAs, follow a bimodal CPR distribution, while the other studied taxonomies are unimodal. Considering S, C, Q, L, V, and X types, a CPR ≳ 1 is diagnostic of the high-CPR X complex, which includes all Xe types, while a CPR ≲ 0.4 is shared by S, C, Q, and L types. V types are not reliably identified by CPR alone. The high CPR tied to some X types may be reflective of significant roughness over multiple scales, but further analysis, including experimental insights, is needed to understand this signature. Furthermore, we found strong evidence that Aten NEAs have both a broader distribution and a higher median CPR than objects outside of Earth’s orbit. This may suggest that these Earth-crossing asteroids have their surficial physical properties altered by planetary encounters, indicating that such processes act over centimeter-to-decimeter scales. The 2029 close approach of NEA (99942) Apophis presents a unique opportunity to integrate ground-based radar observations with spacecraft measurements, such as by the Apophis Explorer (OSIRIS-APEX) mission, to reveal how resurfacing events alter NEA radar properties.

Keywords

• Near-Earth objects
• Radar observations
• Asteroid surfaces