We present the expected performance for a ground-based terahertz (THz) radiometer, a plan to be launched on the TERahertz EXplore-1 (TEREX-1) Mars exploration microspacecraft. The small THz passive radiometer has been developed for the TEREX series of future microspacecrafts. This spacecraft is an opportunity for organizations with limited resources and technology to conduct frequent missions to Mars well suited for resource exploration in contrast to all of the current and past Mars missions of large/giant class missions with fully government lead. The observation frequencies of the TEREX-1 radiometer are 474.64–475.64 and 486.64–487.64 GHz with a 100-kHz resolution, and the double-sideband noise temperature less than 3000 K. A theoretical error analysis is performed with the instrument characteristics to assess for the first time up-looking observations of atmospheric oxygen molecules (O 2 ) and water vapor (H 2 O). Measurement errors for O 2 and H 2 O are 7%–22% and 14%–25% with 8–17- and 5–10-km vertical resolution in the vertical ranges 0–55 and 0–25 km, respectively. TEREX-1 is also capable to measure minor species, O 3 and H 2 O 2 , with a precision better than 30% within two independent layers. We used the integration time of 1 h for all simulations. Our theoretical simulation showed the instrument characteristics of the TEREX-1 sensor are able to observe vertical profiles of O 2 and H 2 O abundances with the same level of the large class missions.