In situ observations of the ionized environment of Mars: the antenna impedance measurements experiment, AIM, proposed as part of the Mars advanced radar for subsurface and ionospheric sounding, MARSIS

Although the priority is not very high, one of the scientific objectives of the Mars Express mission is to study the interaction of the Martian atmosphere with the interplanetary medium. Regarding Mars, the term “atmosphere” must be interpreted in its broadest sense, that is including the exospheric neutral and charged particles of planetary origin, which both extend deeply into the interplanetary medium and strongly interact with it. The antenna impedance measurements (AIM) experiment has been proposed within this framework. The main idea was to take advantage of the presence of a radar antenna onboard the Mars Express Orbiter and to measure its self-impedance in a frequency bandwidth that contains the plasma frequency. Both the real and imaginary parts of the antenna impedance are functions of the total plasma density and electron temperature. Consequently, with about 700 g of electronics, box, cables, and a rather simple interface to the radar antenna it becomes possible to monitor two local aeronomical parameters that play a fundamental role in the interaction between the Martian environment and the solar wind. Due to its lower-frequency bandwidth (8 kHz–2 MHz), which allows a full coverage from the shocked solar wind down to the ionosphere, its capability of measuring the electron temperature, and the reliability of its measurements the AIM experiment perfectly complements the Mars advanced radar for subsurface and ionospheric sounding (MARSIS) ionospheric investigations. In particular, careful computations show that the electron temperature can hardly be derived from the passive electric field measurements planned to be made by MARSIS. Unfortunately, due to a too late decision about the additional payload selection, the proposal of AIM as part of the MARSIS has been rejected.