Laboratory Evaluation of a Methane Spectrometer for the Martian Atmosphere

Introduction:  Methane observed in the Martian atmosphere is not stable on geological timescales, suggesting a source on Mars that is active today [1]. Several theories have been proposed that link this atmospheric methane to emission from ancient subsurface reservoirs (such as methane hydrate clathrates) [2,3], hot water/rock reactions (e.g. Fisher Troph-Type Processes) [4], exogenous input of organic material (e.g. Interplanetary Dust Particles), [5] or  biological activity [4]. However, methane is currently measured in the near-surface atmosphere only a few times per year, making testing these theories challenging [6, 7].  

Methane on Earth is primarily biological, making the methane cycle on Mars of great astrobiological interest. This instrument could also provide important thermochemical data about the Martian subsurface[1].

Figure 1: The ABB breadboard prototype spectrometer.

Figure 1: The schematic for the experimental breadboard spectrometer.

The Martian Atmospheric Gas Evolution (MAGE) experiment was proposed to provide the data needed to distinguish between different theories of the origin and evolution of methane in the Martian atmosphere. A small gas spectrometer, based around the ABB Integrated Cavity-enhanced Optical Spectroscopy (ICOS), is a Canadian instrument that could be deployed to sample the concentration of atmospheric methane on an hourly basis [3].

Methodology: For this work, we use an experimental ICOS spectrometer which represents a prototype of a future spectrometer capable of flight to Mars. Images of the breadboard instrument (Figure 1) and its schematic (Figure 2are shown. The prototype spectrometer contains 2 lasers, a 1659 nm laser analyzing concentrations of ¹²CH₄ and ¹³CH₄, and a 1650 nm laser analyzing concentrations of ¹²CH₄ and CO₂.

The prototype spectrometer’s 1659 nm laser was tested using with a balance gas of carbon dioxide. Mass flow controllers were used to make methane concentrations of 1000 ppmv and 10 ppmv. Tedlar bags were used to make theoretical concentrations of methane of 1 ppmv and 10 ppbv by serial dilution. As there was difficulty obtaining a concentration of methane below 10 ppm due to methane contamination of the CO₂ background gas, the runs of 10 ppm, 1 ppm, and 10 ppb are considered equivalent due to background gas contamination. The graphs below show the raw data, processed data with a uniform window filter of size 10 applied, and the data with the drift removed by subtracting the filtered data from the raw data. A mean line goes through the center of the graph. The integration times discussed further in the results section are calculated using the results with the drift removed.

Figure 3: Concentrations of ¹²CH₄ from the prototype spectrometer

Figure 4: Concentrations of ¹³CH₄ from the prototype spectrometer.

Results:  As the seasonal background of methane on Mars ranges in concentration from 0.2-0.7 ppbv, with periodic high spikes of up to 45 ppbv [7], sensitivity in this range would make a flight instrument based on ours suitable for the detection of methane. The instrument takes an average of 16 hours of operation to reach 95% confidence that the concentrations of ¹³CH₄ detected are within 1 ppb of the true value, while it takes an average of 56,647 hours to reach 95% confidence that the values of ¹²CH₄ are within 1 ppb of true values. The significant drop in integration time when observing ¹³CH₄ vs ¹²CH₄ is due to a more stable signal response at lower concentrations of gas. The results from the 10 ppm, 1 ppm and 10 ppb runs are averaged to obtain the operational times discussed above, as they are considered replicas. The long measurement time is largely due to instability caused by noise within the instrument, clearly seen in the results for both ¹²CH₄ and ¹³CH₄ (Figure 3 and Figure 4).

Future Work: Future modifications include testing under Mars-like conditions, including testing the instrument with Mars analogue mixtures of gases. This will include pressure changes in the atmosphere to observe how the limits of detection change. If deployed on a Mars rover, or on a rotorcraft, the instrument could help to constrain the potential sinks and sources of Martian methane.       

Acknowledgments: This work was funded by the Canadian Space Agency FAST program grant No, 19FAYORA13 and is being pursued in collaboration with ABB Inc.

References: [1] Atreya, S. K., Mahaffy, P. R., & Wong, A.‐S. (2007). Planetary Space Science, 55(3), 358–369. [2]Stevens, A. H., Patel, M. R. & Lewis, S. R. Icarus 281, 240–247 (2017). [3] Lasue, J., Quesnel, Y., Langlais, B. & Chassefière, E. Icarus 260, 205–214 (2015). [4] Oehler, D. Z., & Etiope, G. (2017)., 17(12), 1233–1264.1657 [5] Schuerger, A.C., Moores, J., Clausen, C., Barlow, N., Britt, D., 2012. J. Geophys. Res. Planets 117, E08007.. [6] Webster, C. R., Mahaffy, P. R., Atreya, S. K., Moores, J. E., Flesch, G. J., Malespin, C., et al. (2018). Science, 360(6393), 1093–1096. [7] Moores, J.E., Sapers, H.M., Oehler, D., Newman, C. and Whyte, L. (2021) 8 pp. published in Bulletin of the American Astronomical Society, Vol 53, issue 4. https://baas.aas.org/pub/2021n4i125/release/1