Earth Planets Space, 50, 785–792, 1998 The Planet-B neutral gas mass spectrometer

The Planet-B neutral gas mass spectrometer is designed for in-situ measurements of the gas composition in the upper atmosphere of Mars. The sensor uses a dual frequency quadrupole mass analyzer with a mass range of 1– 60 amu (atomic mass units) and two electron multipliers to cover the dynamic range required. The ion source, which is collinear with the analyzer, operates in two different modes: 1) a closed source mode measuring non-surface reactive neutral species that have thermally accommodated to the gas inlet walls; and 2) an open source mode measuring chemically surface active species by direct beaming with no surface collisions. The in-line Retarding Potential Analysis (RPA) system selects the mode of operation. An onboard Field Programmable Gate Array (FPGA) is used to control the instrument operating parameters in accordance with pre-programmed sequences and to package the telemetry data. The sensor is sealed and maintained in a vacuum prior to launch and will be opened to the environment of Mars after orbit insertion. Measurements of He, N, O, CO, N2, NO, O2, Ar, and CO2 will be done at periapsis and the data will be used to determine the variation of the neutral atmosphere density and temperature with altitude, local solar time and season. Measurements are possible from 130–140 km to 500 km depending on the gas species, chemical background, and instrument measurement mode. The data will contribute to the studies of thermosphere energetics, lower atmosphere meteorology (e.g. dust storms) and serve as a resource for studies of the interaction of the upper atmosphere with the solar wind. 1