Millimeter and submillimeter spectrometry is a sensitive and specific probe of gas phase (volatiles) composition and origin. First generation in-situ spectrometers have been built, but are too heavy and power hungry for space applications. This effort will apply new advances in millimeter wave CMOS technology to standard microwave techniques for the first time, with the goal of developing a highly compact, highly sensitive in-situ gas detection system. This simple system will generate the probing radiation inside a cavity resonator, where it interacts with the sample. The design increases sensitivity while simultaneously reducing mass and power. The demonstrated technique will be applicable across the THz spectrum and provide a template design for a multitude of planetary mission profiles.