Near infrared cavity ring-down spectroscopy for isotopic analyses of CH 4 on future Martian surface missions Y. Chen a,n , P. Mahaffy b , V. Holmes b , J. Burris b , P. Morey b , K.K. Lehmann c , B. Sherwood Lollar d , G. Lacrampe-Couloume d , T.C. Onstott a a Department of Geosciences, Princeton University, Princeton, NJ 08544, USA b Goddard Space Flight Center, Code 699, Greenbelt, MD 20771, USA c Department of Chemistry, University of Virginia, Charlottesville, VA 22904, USA d Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada M5S 3B1 article info Article history: Received 24 February 2014 Received in revised form 14 October 2014 Accepted 18 November 2014 Available online 26 November 2014 Keywords: Mars sciences Methane isotope Portable NIR CRDS Astrobiology abstract A compact Near Infrared Continuous Wave Cavity Ring-down Spectrometer (near-IR-cw-CRDS) was developed as a candidate for future planetary surface missions. The optical cavity was made of titanium with rugged quartz windows to protect the delicate super cavity from the harsh environmental changes that it would experience during space flight and a Martian surface mission. This design assured the long- term stability of the system. The system applied three distributed feedback laser diodes (DFB-LD), two of which were tuned to the absorption line peaks of 12 CH 4 and 13 CH 4 at 6046.954 cm 1 and 6049.121 cm 1 , respectively. The third laser was tuned to a spectral-lines-free region for measuring the baseline cavity loss. The multiple laser design compensated for typical baseline drift of a CRDS system and, thus, improved the overall precision. A semiconductor optical amplifier (SOA) was used instead of an Acousto-Optic Module (AOM) to initiate the cavity ring-down events. It maintained high acquisition rates such as AOM, but consumed less power. High data acquisition rates combined with improved long-term stability yielded precise isotopic measurements in this near-IR region even though the strongest CH 4 absorption line in this region is 140 times weaker than that of the strongest mid-IR absorption band., The current system has a detection limit of 1.4  10 –12 cm 1 for 13 CH 4 . This limit corresponds to 7 pptv of CH 4 at 100 Torr. With no further improvements the detection limit of our current near IR-cw-CRDS at an ambient Martian pressure of 6 Torr (8 mbar) would be 0.25 ppbv for one 3.3 minute long analysis. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Reports of up to 45 ppbv CH 4 in the Martian atmosphere (Formisano et al., 2004; Mumma et al., 2009) have generated multiple hypotheses about its possible origins, which include volcanic outgassing, lower temperature geological processes (water-rock reactions or CH 4 hydrate destablization), and micro- bial methanogenesis in the near subsurface of Mars (Krasnopolsky et al., 2004). The rate of photochemical destruction of CH 4 by UV photolysis is approximately 2.2  10 5 cm 2 s 1 , which translates into a CH 4 lifetime of  340 years for a  10 ppbv mixing ratio (Krasnopolsky et al., 2004). This short lifetime implies an active CH 4 source, for example, methanogenesis (4H 2 þ CO 2 ¼ 4CH 4 þ 2H 2 O), gaseous release through the cryosphere (Onstott et al., 2006) or volatilization of subsurface CH 4 hydrates (Madden et al., 2007; Max and Clifford, 2000). Although original satellite and ground-based observations suggest that the atmospheric CH 4 is not uniformly mixed and appears to be seasonal (Mumma et al., 2009), the latter conclusion has since then been ruled out (Villanueva et al., 2013). None- theless, to explain interannual differences an active sink for CH 4 must exist and must dominate over photolytic processes (Lefevre and Forget, 2009). The sink could be some undefined surface or atmospheric oxidation phenomena or biological CH 4 oxidation equivalent to aerobic methanotrophy (CH 4 þ 2O 2 ¼ 4CO 2 þ 2H 2 O) in terrestrial microorganisms (Onstott et al., 2006). The presence of an active sink implies that the CH 4 generation rate must be much greater than the 2.2  10 5 cm 2 s 1 required by the photo- lysis reaction, perhaps as much as 2  10 9 cm 2 s 1 (Lefevre and Forget, 2009). On Earth, microbially produced CH 4 is characterized by a somewhat more depleted 13 C isotopic composition relative to abiogenic or thermogenic CH 4 , although overlap in isotopic Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/pss Planetary and Space Science http://dx.doi.org/10.1016/j.pss.2014.11.016 0032-0633/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: yuhengc@princeton.edu (Y. Chen). Planetary and Space Science 105 (2015) 117–122