Polarimetric Decomposition of Arecibo Radar Observations of Near-Earth Asteroids

Abstract:

Traditional planetary radar polarimetry is limited to analyzing the circularly polarized component of the received signal, which when combined with knowledge of the known circularly polarized transmitted signal, gives information about the scattering properties of the target surface. This type of analysis neglects the randomly polarized (depolarized) and linearly polarized components of the received signal, which provide information on the surface morphological complexity and subsurface scattering. Measuring the full polarization state, or equivalently the Stokes vector, of radar echoes from the Moon (Stacy, 1993 PhD, Cornell) and Venus (Carter et al., 2004 JGR 109; 2006 JGR 111) has revealed geomorphology such as buried lava flows, pyroclastic deposits, and fine-grained crater ejecta. Preliminary full-polarimetric analyses of archival radar observations have shown potential in identifying fine-grained regoliths on near-Earth asteroids (NEAs) (Carter, 2005 PhD, Cornell; Carter et al., 2007 39^th DPS). The discrepancy between the physical interpretation of radar polarimetry of (1019555) Bennu (hereafter Bennu) and the surface as revealed by OSIRIS-REx further necessitates a re-evaluation of our understanding of radar scattering on NEAs.

We will present recent work in decomposing the full polarization state of archival radar observations of NEAs from Arecibo Observatory (AO). For this work, we will discuss our analyses of AO radar observations of NEAs (53319) 1999 JM8, Bennu, and (33342) 1998 WT24. We create delay-Doppler images of each Stokes parameter and use these to derive delay-Doppler images of the circular polarization ratio, degree of linear polarization, degree of depolarization, as well as the m-chi decomposition. We augment the interpretation of the m-chi decomposition, originally applied to the lunar surface, for NEA surfaces through numerical simulations of m-chi decompositions of morphologically complex particles, similar to those observed on the surfaces of Bennu and (162713) Ryugu. The methodology for improving polarimetric calibration of future planetary radar observations at AO will be discussed.