Primordial argon isotope fractionation in the atmosphere of Mars measured by the SAM instrument on Curiosity and implications for atmospheric loss

Abstract

[1] The quadrupole mass spectrometer of the Sample Analysis at Mars (SAM) instrument on Curiosity rover has made the first high-precision measurement of the nonradiogenic argon isotope ratio in the atmosphere of Mars. The resulting value of ³⁶Ar/³⁸Ar = 4.2 ± 0.1 is highly significant for it provides excellent evidence that "Mars" meteorites are indeed of Martian origin, and it points to a significant loss of argon of at least 50% and perhaps as high as 85-95% from the atmosphere of Mars in the past 4 billion years. Taken together with the isotopic fractionations in N, C, H, and O measured by SAM, these results imply a substantial loss of atmosphere from Mars in the posthydrodynamic escape phase.

Keywords: Mars; argon isotopes; atmospheric loss; habitability; planetary atmospheres; solar system.

Fig 1

Results from the dynamic enrichment of argon 36 and 38 on Mars by the SAM instrument on MSL on sol 231. (a) The successive ingestion of samples and enrichment cycles of the Mars atmosphere increases the counts per second (cps) and S/N of the argon isotopologues (symbols, top left axis), while the major ion m/z 44 of atmospheric CO₂ is scrubbed down to background levels (black trace, top right axis) via adsorption onto the SAM scrubbers. During a normal atmospheric ingestion, the m/z 44 peak would saturate the detector. The data shown for m/z 36 and 38 have been corrected for background signal as described in the text. Enrichment cycles 1a and 1b reference measurements of the same samples of atmosphere as transferred into the QMS through two different valves. Cycle 1a used a low-conductance valve; cycles 1b through 10 used the same higher-conductance valve. Cycle 1b is thus is the first true sample in this series. (b) The average ratio for each enrichment cycle is given as a function of argon 36 counts, with error bars representing the uncertainties introduced by scatter in the data and the multiple background subtraction methods used. All data from the final six enrichment cycles, where the ³⁸Ar S/N > 3, are averaged to determine a ³⁶Ar/³⁸Ar ratio of 4.26 ± 0.08 for the dynamic enrichment run. Sol 0 is referenced to Curiosity's landing at Gale Crater (4.5895°S, 137.4417°E) on Mars at 15:03 local mean solar time or 05:17 UTC on 6 August 2012, in Mars Year 31.

Fig 2

Same as Figure 1 but for the semistatic enrichment experiment on sol 341. All data from the final six enrichment cycles, where the ³⁸Ar S/BG > 5, are averaged to determine a ³⁶Ar/³⁸Ar ratio of 4.16 ± 0.14 for the semistatic enrichment run. The uncertainty estimate includes statistical noise and the background correction.

Fig 3

Comparison of the ³⁶Ar/³⁸Ar ratio measured in the atmosphere of Mars by Curiosity's SAM-QMS in 2013 with the Viking GCMS result in 1976, Mars meteorites, Earth, Jupiter and the Sun. See Table 1 for the values and references.