DETECTION LIMITS FOR SUPERCAM’S TRANSMISSION SPECTROMETER ABOARD THE PERSEVERANCE ROVER: CAPABILITIES AND IMPLICATIONS FOR RAMAN SPECTROSCOPY ON MARS

Abstract:

Following the definitive identification of aqueously altered minerals in Gale Crater by CheMin, and the global detection of phyllosilicates by MRO and OMEGA, the Mars 2020 Science Definition team designated fine-scale and context mineralogical analysis as essential for the successor to the Curiosity rover. The purpose of this designation was to permit the differentiation of igneous parent, and authigenic, aqueously altered daughter rocks in cases where knowledge of elemental abundances is insufficient to infer stereochemistry of the target in question. Two Raman spectrometers were selected for flight as part of the Mars 2020 mission: (1) SHERLOC, a fine-scale Raman spectrometer, equipped with a NeCu 248.6 nm CW laser that relies upon λ^-4 and resonance Raman intensity effects to overcome limited sampling volumes in concert with wavelength-domain separation of competing radiation sources, and (2) SuperCam, a context-scale remote-sensing Raman spectrometer, outfitted with a Nd:YAG 532 nm pulsed laser and gated detector that utilizes high energy density laser pulses and signal intensification to overcome diminished solid angles in concert with time-domain separation of long-lived luminescence and solar reflectance. Regardless of the spectrometer design philosophy employed, the success of these instruments depends upon their ability to collect enough photons in a given spectral acquisition to overcome their background noise floor. During development and testing of SuperCam, the background signal (i.e. CCD bias, detector and intensifier dark currents) and noise dependence on instrument temperature, intensifier gain, intensifier gate-width and detector integration time were systematically characterized by placing SuperCam in a thermal-vacuum chamber, and cycling the instrument through the environmental and spectral acquisition conditions expected on Mars. Application of this comprehensive background signal and noise characterization in conjunction with measurements of the Raman laser irradiance within the field-of-view of the spectrometer as a function of distance, telescope étendue, wavelength-dependent instrument response, and a library of Raman scattering efficiencies enables a quantitative study of the signal-to-noise budget for this instrument.