Rotationally-Resolved Radar Scattering Properties of Near-Earth Asteroid (433) Eros

Studies of the regolith of near-Earth asteroids (NEAs) can give us insight into the evolutionary processes that affect these bodies. The regolith’s physical, mechanical, thermal, and scattering properties can tell us about, e.g., the asteroid’s collisional environment, its overall structure, how its surface has responded to space weathering, and the geophysics of the topographic features.

Asteroid (433) Eros, with diameter 34-by-11-by-11 km, is in particular a useful target for such studies as it was extensively observed by the NEAR Shoemaker rendezvous mission, which had a visit with Eros in 2000-2001. NEAR firmly established (among other things) Eros’s shape, spin-state, reflectance, and albedo [e.g. 1,2,3,4]. All of these make it possible to put Earth-based observations of Eros, which are point-source hemispherical-averages, into better context.

The work we report here is part of a broader study of Eros at wavelengths that were not sampled by NEAR. In earlier work [5], we analyzed an extensive dataset of 25 epochs of near-to-mid-IR spectroscopy. Each spectrum simultaneously sampled the reflected and thermal parts of the asteroid’s spectral energy distribution. Those data cover numerous sub-Earth latitudes and rotational longitudes, which, along with the known physical parameters, allowed us to use thermal modeling [6] to assess Eros’s global thermophysical properties as well as to look for hints of heterogeneity in the regolith.

Here we report our analysis of multi-epoch radar observations of Eros, extending our investigation of regolith into the decimeter- and meter-scale [7,8]. Dual-polarization, S-band (at wavelength 12.6 cm) radar observations of Eros were made at the Arecibo Observatory 305-m telescope in early 2019. We have CW data from January 25 to 31 and February 8, 13, and 16, and delay-Doppler imaging from those January dates. We obtained data at each observing session for up to 2.6 h. Eros’s sub-Earth latitude was –30º to –20º during that time span. With so many observation epochs we saw virtually all Erosographic longitudes at least once. (Eros’s rotation period is 5.2703 h.) Eros was 0.21-0.26 au from Earth during this span. Our analysis so far has focused on two products. First, we have used the CW spectra to derive the rotationally-resolved SC/OC polarization ratio. We note that the ratio varies and is correlated with the broad-end vs short-end of the asteroid. Our second product is an assessment of the scattering law that is most applicable to Eros. Normally when interpreting radar data, the dependence of the differential radar cross section σ₀ (a.k.a. dσ/dA) on the incidence angle must be assumed since there is no a-priori, independent shape model to use that is detailed enough to take its effect out. A typical assumption is that σ₀ is proportional to the cosine of the incidence angle all raised to an assumed exponent [9,10]. Since Eros’s shape is already well-known, we can instead independently constrain this. We will present our analysis of Eros’s scattering behavior using both the CW and delay-Doppler data.

Acknowledgments: We acknowledge support from NASA’s YORPD program via award 80NSSC21K0658, NASA’s SSERVI program via award 80NSSC19M0214, and from NSF’s AAG program via award 1856411. We also thank Arecibo Observatory staff and observers for their efforts in acquiring the data presented here. References: [1] Veverka, J. et al. (2000) Science, 289, 2088-2097, [2] Miller J. K. et al. (2001) Icarus, 155, 3-17, [3] Riner M. A. et al. (2008) Icarus, 198, 67-76, [4] Li J. et al. (2004) Icarus, 172, 415-431, [5] Hinkle M. L. et al. (2022) Icarus, 382, 114939, [6] Magri C. et al. (2018) Icarus, 303, 203-219, [7] Magri C. et al. (2001) Meteoritics & Planet. Sci., 36, 1697-1790, [8] Virkki A. et al. (2023) Remote Sensing, 15, 5605, [9] Mitchell D. L. et al. (1996) Icarus, 124, 113-133, [10] Nolan M. C. et al. (2013) Icarus, 226, 629-640.