The Regolith X-ray Imaging Spectrometer (REXIS) aboard NASA’s OSIRIS-REx asteroid sample return mission is an instrument devoted to the characterization of the elemental surface composition of the asteroid (101955) Bennu. REXIS conducted science observations of the asteroid surface from a terminator orbit in two separate 1-month intervals during July and November 2019. The REXIS instrument has two parts: (1) a primary wide field of view coded aperture X-ray telescope with a 2 x 2 CCD focal plane to enable measurements over the 0.3 to 10 keV, and (2) a solar X-ray monitor (SXM) consisting of a single colimated silicon-drift diode for characterization of the solar X-ray spectrum incident on the asteroid surface in the terminator orbit geometry. The low energy threshold of the X-ray instrument was selected to enable characterization of O, Fe, Mg, Al, Si, and S during periods with quiet or slightly elevated solar X-ray fluxes.
For evaluation of the instrument performance potential and interpretation of the data collected, this presentation describes an X-ray fluorescence (XRF) simulation routine created to generate synthetic observations simulating a variety of asteroid compositions and geometries, as well as to enable characterization of the instrument response. Here we discuss the methodology, expected discrimination power between various meteoritic analogs, and how these simulations connect with the observations conducted at Bennu.