Initial SAM calibration gas experiments on Mars: Quadrupole mass spectrometer results and implications

Highlights

• The SAM instrument measures the composition of the martian atmosphere.

• SAM's calibration gas cell was exercised in July and December 2015.

• Calibration constants for atmospheric composition have been adjusted.

• Revised SAM nitrogen abundance is similar to that measured by Viking.

• SAM accurately retrieved isotope ratios of CO₂ and Xe in the calibration cell.

Abstract

The Sample Analysis at Mars (SAM) instrument suite of the Mars Science Laboratory (MSL) Curiosity rover is equipped to analyze both martian atmospheric gases and volatiles released by pyrolysis of solid surface materials, with target measurements including chemical and isotopic composition (Mahaffy et al., 2012). To facilitate assessment of instrument performance and validation of results obtained on Mars, SAM houses a calibration cell containing CO₂, Ar, N₂, Xe, and several fluorinated hydrocarbon compounds (Franz et al., 2014; Mahaffy et al., 2012). This report describes the first two experiments utilizing this calibration cell on Mars and gives results from analysis of data acquired with the SAM Quadrupole Mass Spectrometer (QMS). These data support the accuracy of isotope ratios obtained with the QMS (Conrad et al., 2016; Mahaffy et al., 2013) and provide ground-truth for reassessment of analytical constants required for atmospheric measurements, which were reported in previous contributions (Franz et al., 2015, 2014). The most significant implication of the QMS data involves reinterpretation of pre-launch contamination previously believed to affect only CO abundance measurements (Franz et al., 2015) to affect N₂ abundances, as well. The corresponding adjustment to the N₂ calibration constant presented here brings the atmospheric volume mixing ratios for Ar and N₂ retrieved by SAM into closer agreement with those reported by the Viking mission (Owen et al., 1977; Oyama and Berdahl, 1977).

Introduction

The Sample Analysis at Mars (SAM) instrument suite, which includes a Quadrupole Mass Spectrometer (QMS), Gas Chromatograph (GC), and Tunable Laser Spectrometer (TLS), was designed to perform precise in situ measurements of the chemical and isotopic composition of the martian atmosphere and volatiles associated with surface materials (Mahaffy et al., 2012, Webster and Mahaffy, 2011). SAM measurements of volume mixing ratios (VMR) for the most abundant martian atmospheric species were initially computed with empirical calibration constants derived from pre-launch data (Franz et al., 2014). These constants were later revised on the basis of laboratory experiments and instrument performance during the first Mars year of SAM operations (Franz et al., 2015). The later report described enhanced modeling of instrument background, pressure-related behavior, and mass interferences that improved calculations of VMR for the minor atmospheric species O₂ and CO. At the time of its publication, explanation for the difference in ratio of Ar to N₂ obtained by SAM (Mahaffy et al., 2013) and Viking (Owen et al., 1977, Oyama and Berdahl, 1977) remained an outstanding question of interest, but prior to execution of the calibration cell experiment, insufficient data were available to assign this difference to a SAM instrument effect with confidence (Franz et al., 2015).

The first two experiments utilizing the SAM calibration cell on Mars were performed in July and December 2015 (on MSL mission Sols 1042 and 1204) and designated SAM experiments #25274 and #25321, respectively. For simplicity in this manuscript, we will refer to them as CG1 and CG2 (i.e., Calibration Gas 1 and 2). These experiments provided opportunities to assess the continued robustness of the calibration constants through their application to a gas mixture of known chemical and isotopic composition. Results of QMS analysis of the calibration gas suggested a need to update the constant required for calculation of atmospheric N₂ abundance, while indicating accurate retrieval of CO₂ and Xe isotope ratios.

SAM experiment design and analysis are supported by efforts using laboratory facilities at the NASA Goddard Space Flight Center (GSFC). The SAM breadboard and test bed models incorporate quadrupole mass spectrometers fabricated at GSFC to the same design as the flight model and controlled with identical electronics and flight software, rendering these instruments high-fidelity facsimiles of SAM. The breadboard employs the SAM prototype QMS in a laboratory setting that allows flexibility in plumbing configuration for a range of experiments supporting both atmospheric and solid sample analyses. The test bed comprises a replica of SAM, including all instrument and gas processing system components. The test bed is housed in an environmental chamber that simulates martian ambient conditions. All experiments executed on Mars, including those using the calibration cell, are first performed on the SAM test bed for script validation. For the studies described here, both instruments utilized operating parameters analogous to those of the flight model as implemented in ground calibration or on Mars.