Orbital bistatic radar observations of asteroid Vesta by the Dawn mission

Abstract

We present orbital bistatic radar observations of a small-body, acquired during occultation by the Dawn spacecraft at asteroid Vesta. The radar forward-scattering properties of different reflection sites are used to assess the textural properties of Vesta’s surface at centimeter-to-decimeter scales and are compared to subsurface hydrogen concentrations observed by Dawnʼs Gamma Ray and Neutron Detector to assess potential volatile occurrence in the surface and shallow subsurface. We observe significant differences in surface radar reflectivity, implying substantial spatial variations in centimeter-to-decimeter-scale surface roughness. Our results suggest that unlike the Moon, Vesta’s surface roughness variations cannot be explained by cratering processes only. In particular, the occurrence of heightened hydrogen concentrations within large smoother terrains (over hundreds of square kilometers) suggests that potential ground-ice presence may have contributed to the formation of Vesta’s current surface texture. Our observations are consistent with geomorphological evidence of transient water flow from Dawn Framing Camera images.

Fates of hydrous materials during planetesimal collisions

“…Some studies report that C-type asteroids contain hydrous materials (e.g., Takir et al, 2015;Rivkin et al, 2015). Additionally there are reports of evidence of hydrogen on the dwarf planet Ceres (de Sanctis et al, 2016) and asteroid Vesta (Palmer et al, 2017) via the Dawn mission. Recent work suggests that hydrous minerals (and water ice) on Ceres would be the result of Ceres's current activity (Carrozzo et al, 2018;Raponi et al, 2018).…”

Coherent backscatter enhancement in bistatic Ku-/X-band radar observations of dry snow

“…Bistatic radar measurements of surfaces of solar system bodies are possible by using an orbiting spacecraft in combination with the deep space network receivers on Earth (Simpson, 1993;Palmer et al, 2017). However, such experiments require a very specific geometric alignment of the spacecraft's orbit with respect to the Earth and are thus not common.…”

Processing and Analysis for Radio Science Experiments (PARSE): Graphical Interface for Bistatic Radar

“…In a first step toward addressing this deficiency, we develop herein a public, intuitive software tool with a graphical user interface that automates the processing and analysis of BSR surface occultation data acquired by NASA's Deep Space Network (DSN) ground stations. The development of this tool, PARSE ("Processing and Analysis for Radio Science Experiments"), was spurred on by the challenges encountered during the opportunistic BSR experiment performed by NASA's Dawn mission at Asteroid Vesta (Palmer et al 2017). In this grazing-angle surface-scatter experiment, the telecommunications high-gain antenna (HGA) aboard the spacecraft was used to transmit a continuous, direct signal with right-hand circular polarization (RCP) to be received by DSN stations on Earth (Figure 1).…”

“…B 2008)-took several years, however, owing to complex acquisition geometries around such a small body with an unknown and weak gravitational field prior to mission encounter. Opportunistic forward-scatter BSR experiments performed at small bodies like Vesta occur at grazing incidence angles and a slow spacecraft orbit that result in surface echoes with unexpectedly small Doppler shifts from the direct signal (δf ), making these surface-scattered echoes difficult to identify in the raw data without using proper processing parameters tailored to the mission (Palmer et al 2017;. Furthermore, unlike backscatter BSR experiments that are performed at smaller bistatic angles (i.e., θ bs < 180°), forward-scattered echoes are not distinguishable from the direct signal by any differences in polarization, as they maintain the same sense of circular polarization as the transmitted (direct) signal (in this case, right-hand circularly polarized or RCP; Palmer et al 2017).…”

“…Opportunistic forward-scatter BSR experiments performed at small bodies like Vesta occur at grazing incidence angles and a slow spacecraft orbit that result in surface echoes with unexpectedly small Doppler shifts from the direct signal (δf ), making these surface-scattered echoes difficult to identify in the raw data without using proper processing parameters tailored to the mission (Palmer et al 2017;. Furthermore, unlike backscatter BSR experiments that are performed at smaller bistatic angles (i.e., θ bs < 180°), forward-scattered echoes are not distinguishable from the direct signal by any differences in polarization, as they maintain the same sense of circular polarization as the transmitted (direct) signal (in this case, right-hand circularly polarized or RCP; Palmer et al 2017). Since the method of measuring the ratio of RCP to LCP signal (the circular polarization ratio; Campbell 2002) to retrieve absolute surface roughness is not applicable to forwardscatter observations, to derive the relative surface roughness, Palmer et al (2017) instead measure the relative strength of reflected power from each echo site.…”