Mapping the elemental composition of Ceres and Vesta: Dawn"s gamma ray and neutron detector

Abstract

Dawn is a NASA discovery mission that will explore the main belt asteroids (1) Ceres and (4) Vesta. Ceres and Vesta are among the oldest bodies in the solar system and represent very different evolutionary paths. By studying these ancient, complementary asteroids, we will answer fundamental questions about the early solar system and planetary formation processes.The Dawn payload consists of a Framing Camera (FC), a visual and infrared mapping spectrometer (VIR), and a Gamma Ray and Neutron Detector (GRaND). The instruments provide data needed to investigate the structure, geology, mineralogy, and geochemistry of the asteroids. GRaND provides the data for the geochemistry investigation, including maps of most major elements and selected radioactive and trace elements. An updated description of the GRaND instrument is given along with the expected performance of GRaND at Vesta and Ceres. Approaches to combine data from FC, VIR and GRaND are discussed.

K‐Th‐Ti systematics and new three‐component mixing model of HED meteorites: Prospective study for interpretation of gamma‐ray and neutron spectra for the Dawn mission

“…The Dawn spacecraft is equipped with a gamma‐ray and neutron detector (GRaND) that will enable measurement and mapping of elemental abundances (potentially O, Si, Fe, Ti, Al, Mg, Ca, K, U, Th, and H) on Vesta’s surface (Prettyman et al. 2003, 2004).…”

“…The Lunar Prospector Gamma Ray Spectrometer (LP‐GRS) can provide information on the analytical performance of GRaND, because LP‐GRS has a γ‐ray sensor (BGO; Bismuth germinate) with approximately the same volume, detection efficiency, and pulse height resolution (Prettyman et al. 2003, 2004).…”

“…The GRaND spatial resolutions at LAMO and HAMO are given by the full-width-at-half-maximum (FWHM) of the spatial response function of GRaND for the orbital altitudes of approximately 485 km and approximately 985 km (these values correspond to distances from subsatellite point of 200 km and 700 km, respectively, in Fig. 7 of Prettyman et al 2004). is especially well suited). We also briefly describe how GRaND neutron measurements might improve the mixing model.…”

The permanently shadowed regions of dwarf planet Ceres

“…Ceres has an ice-rich crust [Prettyman et al, 2016] that gradually retreats and feeds an exosphere. Based on model calculations of retreating ice, Fanale and Salvail [1989] estimated that the current global water supply rate is 0.03-0.30 kg/s, and Schorghofer [2016b] estimated that 0.2 kg/s of H 2 O is currently output as a direct result of the globally retreating ice; these estimates depend on assumptions about the physical properties of the top tens of meters.…”

Geochemistry of 4 Vesta based on HED meteorites: Prospective study for interpretation of gamma ray and neutron spectra for the Dawn mission

“…The performance of GRaND during the Dawn mission was investigated based on results from the Lunar Prospector Gamma Ray Spectrometer (LPGRS), because LPGRS has a gamma ray sensor with approximately the same volume, detection efficiency, and pulse height resolution . On this basis, Prettyman et al (2004) concluded that the spectrum obtained at LAMO is enough above background to map abundances of major rock forming elements, radioactive elements, and light elements such as H, C, and N at Vesta and Ceres. Therefore, GRaND data sets should allow construction of the mixing diagram employing [M/Si] plot between basaltic eucrite, cumulate eucrite, and diogenite, which is consistent with previous geochemical and petrographical studies (e.g., Mittlefehldt 2003;Mittlefehldt et al 1998).…”