Gamma-Ray Spectroscopy of Asteroid 16 Psyche: Expected Performance of the Psyche Gamma-Ray Spectrometer

47th Lunar and Planetary Science Conference (2016) 1394.pdf

GAMMA-RAY SPECTROSCOPY OF ASTEROID 16 PSYCHE: EXPECTED PERFORMANCE OF THE PSYCHE GAMMA-RAY SPECTROMETER. Patrick N. Peplowski1, David J. Lawrence1, John O. Goldsten1, Morgan Burks2, Andrew W. Beck1, Linda T. Elkins-Tanton3, Insoo Jun4, Timothy J. McCoy5, Thomas H. Pretty- man6, 1Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA ([email protected]); 2Lawrence Livermore National Laboratory, Livermore, CA 94550; 3Arizona State University, Tempe, AZ 85287; 4NASA Jet Propulsion Laboratory, Pasadena, CA, 91109, USA; 5Smithsonian Insti- tution, Washington, DC, 20560, USA; 6Planetary Science Institute, Tucson, AZ 85719, USA.

Introduction: The high radar albedo (0.42, [1]) elemental composition of near-surface (depths of 10s and density (6.98±0.58 g/cm3, [2]; 6.49±2.94 g/cm3, cm) materials. [3] 7.6±3.0 g/cm3, [4]) of the M-class asteroid 16 Psy- The Psyche spacecraft payload includes a Gamma- che is taken as evidence that it is a metal-rich world, Ray and Neutron Spectrometer (GRNS) to provide similar to (and perhaps a parent body to some) Fe me- global, and in some instances spatially resolved, meas- teorites. Iron meteorites contain appreciable bulk Ni urements of Fe, Ni, Si, K, S, Al, Ca, Th, and U concen- content (>5 wt%, ~8.4 wt% on average) [5], suggestive trations on 16 Psyche. Measurements of H and C are of the range of Ni concentrations that might be present also possible, depending on their total concentrations. on 16 Psyche. Reflectance spectroscopy of 16 Psyche GRNS has two subsystems - a MESSENGER- reveals an absorption feature at 0.9 µm which is inter- heritage Gamma-Ray Spectrometer (GRS) and a Lunar preted to originate from silicates, specifically high-Mg Prospector-heritage Neutron Spectrometer (NS). The orthopyroxene, covering ~10% of the surface [6]. NS subsystem, and all neutron science returned by Taken together, these observations suggest that 16 GRNS, are presented in a companion abstract [8]. Here Psyche is the exposed core of a melted and differenti- we present details on the GRS subsystem and its ex- ated protoplanet, perhaps the end result of silicate- pected performance. Our analysis is restricted to the stripping hit-and-run collisions (e.g. [7]). Under this 1.3-body-radius (low altitude) orbit planned for the scenario, exploration of 16 Psyche offers the first and primary mission. The higher-altitude orbits are not possibly only opportunity to directly investigate a optimized for GRNS science and are therefore not dis- planetary core. Core formation is a critical but poorly cussed here. understood stage of planetary formation, so an investi- Gamma-Ray Spectrometer: The GRS consists of gation of 16 Psyche will have profound implications a high-purity germanium (HPGe) sensor surrounded by for our understanding of the formation and evolution a borated plastic anti-coincidence shield (ACS). The of Mercury, Venus, Earth, Mars, and asteroid 4 Vesta. HPGe sensor will provide superior energy resolution Key questions regarding 16 Psyche’s formation and (4.0 keV @ 1332 keV) measurements of gamma-ray evolution include determining if 16 Psyche is indeed emissions from 16 Psyche. The ACS will characterize an exposed planetary core, constraining its thermal GRS-incident charged particles, primarily galactic evolution, and characterizing its elemental composi- cosmic ray (GCR) protons, for the purpose of remov- tion. The Psyche mission – one of five Discovery mis- ing GCR-induced backgrounds in the gamma-ray spec- sion concepts selected by NASA for Phase A study – tra via a veto rejection. At 1.3 body-radius orbital alti- seeks to answer these questions via a focused orbital tude, the ACS veto reduces the background continuum investigation of 16 Psyche. of the gamma-ray measurements by a factor of ~6 at 7 Nuclear Spectroscopy of 16 Psyche: The key MeV and ~2.5 @ 3 MeV. Note that the ACS is also a questions for the Psyche mission lead to a clear need to sensitive neutron detector, and provides a measure of characterize the surface composition of the asteroid. both low-energy and fast neutrons [8]. Particularly important are: Expected Performance: The in-flight performance 1. Characterizing the bulk Ni content, which is di- of the MESSENGER GRS serves as the basis for un- agnostic of the thermal evolution of the core, 2. Characterizing the composition of the silicates, derstanding the sensitivity of the Psyche GRS subsys- which may be remnants of 16 Psyche’s original tem. MESSENGER GRS measured gamma rays from mantle, and Mercury at orbital altitudes ranging from 0.1 to 6.8 3. Measuring the light element (H, C, O) content body radii, which includes 1.3 body radii – the mean of the metals to understand light-element parti- orbital altitude for the prime Psyche GRNS science tioning into cores during differentiation. orbits. The MESSENGER GRS 1.3 body radii data With an orbital mission, these objectives can only be serve as the basis for characterizing the Psyche GRS achieved using nuclear spectroscopy. Nuclear spec- signal and backgrounds. Unlike MESSENGER GRS, troscopy uses measurements of neutron and gamma- which was mounted directly on the spacecraft bus, the ray emissions from planetary surfaces to derive the Psyche GRS will be located at the end of a 2-meter- 47th Lunar and Planetary Science Conference (2016) 1394.pdf

long boom. The boom will provide a significant reduc- Ni gamma-ray peaks are also observed in the spec- tion in the spacecraft-originating backgrounds seen by trum, and our simulations indicate that Ni concentra- GRS and facilitates making measurements from alti- tions >4 wt.% can be characterized by GRS. A survey tudes >1 body radius. Our models include the of the Ni content of hundreds of iron meteorites reveals MESSENGER-derived spacecraft background, re- a mean of 8.4 wt% and, more importantly, no samples duced as appropriate to estimate the improvements with <5 wt% Ni [5], indicating that our sensitivity is resulting from placement on a boom. appropriate for the mission. For concentrations of 8 Gamma-ray emissions from 16 Psyche are modeled wt.% or greater, mapping of Ni variability will be using the radiation transport code MCNPX. MCNPX achieved in the 1.3-body radius orbit. Gamma-ray emissions from non-metallic phases has a long history of application for planetary nuclear are also apparent in the simulated spectra. This in- spectroscopy applications (e.g. [9-12]). MCNPX was cludes Si, K, S, Al, and Ca. For pyroxene concentra- used to model gamma-ray emission from 16 Psyche for tions of 20 vol.% or higher, O gamma rays are also a range of metal compositions (2-30 wt.% Ni) and py- seen at robust detection levels. The light elements H, roxene abundances with the metal (0-90 vol.% pyrox- C, and S can be characterized when their bulk concen- ene). The MCNPX outputs were convolved with the trations exceed ~1 wt.% [10, 11, 13], and modeling to energy response and detection efficiency of the GRS determine our exact sensitivity to these elements is system, following which the expected background sig- ongoing. nals, derived from MESSENGER data, were added to References: [1] Shepard et al. (2010), Icarus 208, produce a simulated spectrum. Note that our models 221-237. [2] Kuzmanoski and Koracevic (2002), were first benchmarked to flight data, as we produced Atron. Astrophys. 395 L17-L19. [3] Baer et al. (2011), an MCNPX model of Mercury gamma-ray emissions Celestial Mech. Dynamical Astron. 100, 27-42. [4] for comparison MESSENGER GRS measurements. Shepard et al. (2008) Icarus 195, 184-205. [5] Results: Figure 1 shows the modeled gamma-ray McSween (2000), Meteorites and their parent bodies, spectrum for our nominal 16 Psyche composition (90 p. 187. [6] Hardersen et al. (2005), Icarus 175, 141- vol.% metal + 10 vol.% pyroxene; IVA iron meteorite 158. [7] Asphaug and Reufer (2014), Nat. Geosci. [8] metal (8.4 wt.% Ni) and pyroxene (bronzite) were Lawrence et al. (2016) LPSC, this session. [9] Pretty- used). The spectrum reports the total measurement as man et al. (2006) JGR 111 E12. [10] Boynton et al. expected from the 1.3 body-radius orbital phase (70 (2007) JGR 112 E12. [11] Evans et al. (2012) JGR 117 days). Comparison of the 1.3-body radius signal to the background highlights which spectral features can be E12. [12] Peplowski et al. (2015) Meteorit. Planet Sci. attributed to 16 Psyche. The spectrum is dominated by 50 353-367. [13] Peplowski et al. (2015) Planet. Space gamma-ray peaks originating from Fe, as expected for sci. 108, 98-107. a body composed of >87 wt.% Fe.

Figure 1. A portion of our simulated gamma-ray spectrum, highlighting the total (red) and background (blue) com- ponents separately. The simulated measurement was calculated from a modeled mixture of 10 vol.% metal/90 vol.% pyroxene, both with IVA-like compositions. Those gamma-ray peaks with the highest signal-to-background are labeled. Gamma-ray emissions from higher energy, >1850 keV, are not shown. That portion of the spectrum includes additional Ni and Fe peaks, along with S, C, and O signatures.