The Small Business Innovative Research Phase I proposal seeks to develop a compact, robust in situ spectrometer capable of detecting multiple gas-phase species in planetary atmospheres with ultra-high sensitivity and selectivity. This instrument will employ a novel room-temperature, widely tunable mid-infrared laser source in conjunction with cavity ringdown spectroscopy. During Phase I, both the novel laser and the cavity ringdown technology were demonstrated in the 3.3 micron spectral region. During Phase II, a flight-ready instrument will be constructed, enabling access to a variety of species, including methane, ethane, ammonia, and formaldehyde. The ultra-high sensitivity of the proposed system will enable these species to be detected at concentrations below 7*10^7/cm^3 per minute, which corresponds to a detection limit of <30 pptv in Earth's atmosphere. The unique laser source to be constructed in Phase II will also be capable of being integrated into other spectroscopic platforms, in many cases providing a direct replacement for cumbersome cryogenic diode lasers while at the same time significantly improving the spatial mode quality and increasing spectral coverage.