Performance Modeling of Orbital Gamma-Ray Spectroscopy of Carbonaceous Asteroids: Monte-Carlo Modeling of the HPGe Mars Odyssey GRS

Abstract:

Orbital gamma-ray spectroscopy (GRS) experiments with high-resolution high-purity germanium (HPGe) detectors have successfully measured elemental abundances in the top ~50 cm of the surfaces of Mars and Mercury. GRS is sensitive to bulk concentrations of H, C, O, S, Fe, and Si among other elements. As these elements are also diagnostic of major carbonaceous and ordinary chondritic meteorite classes, we have simulated the science performance of a HPGe experiment in orbit around asteroids with model compositions corresponding to those of volatile-rich CI and CO carbonaceous chondritic meteorites. A circular orbit around a spherical asteroid was considered, with the altitude of the orbit equal to the radius of the asteroid (similar to the Dawn low-altitude mapping orbit "LAMO").

We simulated the gamma-ray and neutron emission from CI-like (~17 wt%
structural H₂O) and CO-like (<2 wt% structural H₂O) asteroids using
the MCNPX Monte-Carlo radiation transport code. The spacecraft
background (based on a Dawn-like spacecraft model) was also modeled
using MCNPX: this included background due to direct GCR/spacecraft
interactions and also background due to asteroidal neutron flux on the
spacecraft.

The detector model was based on the Mars Odyssey Gamma Ray Spectrometer ("MOGRS"; Boynton et al. 2004, 2007), the largest HPGe detector flown to date. The spectra from the MCNPX output were broadened to a resolution based on the in-flight performance of MOGRS, FWHM = 4.1 keV at 1332 keV (Evans et al. 2006). Doppler broadening was also modeled where applicable. Line fluxes were then extracted from the combined background + asteroid spectrum and statistical uncertainties evaluated.

Our simulations show that asteroids can be identified as having
CI-like vs. CO-like compositions in H/Si, O/Si, S/Si, and C/Si with
MOGRS within 4.5 months in a Dawn LAMO-like orbit. In addition, the
Fe/Si and S/Si sensitivity are sufficient to distinguish CO and other
low-hydrogen carbonaceous chondritic compositions from achondritic
carbon-rich (ureilitic) compositions.