This Small Business Innovation Research Phase I project concerns the novel application of cavity-enhanced absorption spectroscopy to quantify biogenic gases (CH4, CO2, and NH3) and their isotopes (13C/12C and 18O/16O) on Mars and other future planetary missions. These species and their isotope ratios are of critical importance to NASA, because they have been implicating in biological activity and can provide decisive evidence for a biosphere on other planets. In Phase I, we will demonstrate technical feasibility by building a miniature, lightweight Off-Axis ICOS spectrometer and employing it to accurately quantify biogenic gases and their associated isotopes at concentrations approximating the expected Martian atmosphere. The prototype will be tested under both mechanical vibrations and thermal fluctuations to validate its robustness. Final Phase I work will entail developing a gas sampling strategy suitable for Mars and designing a complete analyzer system. In Phase II, Los Gatos Research will build and deliver this analyzer with a fully-integrated electronics package that satisfies NASA's size, weight, and power requirements for Mars deployment. The proposed instrument will be the first sensor capable of measuring the concentrations and isotopes ratios of critical biogenic gases in a compact, lightweight, and low-power package.