Cavity-enhanced laser absorption spectroscopy for high sensitivity in-situ trace gas detection

We propose to develop a laser absorption spectrometer for detection of methane (12CH4) , methane isotopes (13CH4, 12CH3D), and ethane (C2H6) in the Martian atmosphere. The presence of methane in the Martian atmosphere could be indicative of past or present microbial life on Mars. Methane signals have been detected from Earth-based and Mars-orbiting observatories, as well as in-situ at the surface, but the origin is currently unknown. The relatively short lifespan of methane in the Martian atmosphere (~400 years), and the multiple detections of short-term plumes suggest that there is a current or recently active source of methane within Mars' interior. Measuring the relative concentration of methane isotopes and higher hydrocarbons is key to identifying possible mechanism of methane production, such as thermogenic (biotic or abiotic) or microbial. However, such isotopologue measurements are below the sensitivity of any existing planetary instruments. The most precise data to date comes from in-situ detection by the Tunable Laser Spectrometer (TLS) instrument on NASA's Curiosity rover, which used a multi-pass Herriott cell to measure an average background concentration at the surface of Gale Crater of 400 parts per trillion by volume (pptv) with ~30% error margin as a result of being close to the instrument's limit of detection. Much high sensitivity can be achieved with a cavity-enhanced absorption spectrometer (CEAS), but current CEAS techniques have fallen short in attempt to address in-situ planetary needs due to various issues including size, complexity, measurement speed, and/or spectral resolution. Here, we propose an in-situ instrument based on a novel form of CEAS termed Scanning Optical-Feedback (SOF-) CEAS. The SOF-CEAS technique is much simpler than Pound-Drever-Hall (PDH) locking schemes, the setup can be made compact, and high-resolution line shapes can be measured at much higher speeds that traditional optical-feedback CEAS or cavity ring-down spectroscopy (CRDS) techniques. The proposed instrument will operate around 3.27 µm, where methane and ethane absorption is strongest, and a target detection limit of <10 pptv is expected with ~10 s of averaging time using a cavity that is 20 cm long but achieves an effective path length of >20 km. By scrubbing CO2 and enriching CH4 in ingested samples, such sensitivity will be sufficient to detect methane isotopes and ethane, as well as measuring total methane concentrations with higher precision and sampling rates than before. In addition to methane detection on Mars, the combination of high sensitivity, compact size, and fast measurement speeds can be enabling for various in-situ planetary missions, such as decent probes that require fast measurement times for high vertical resolution during a short decent.

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