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 atmosp here 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 deepl y into the interplanetary medium and strongly interact with it. The antenna impedance measurements (AIM) experiment has been proposed within this fram ework. The main idea was to take advantage of the presence of a radar anten na onboard the Mars Express Orbiter and to measure its self-impedance in a frequency bandwidth that contains the plasma frequency. Both the real and i maginary parts of the antenna impedance are functions of the total plasma d ensity and electron temperature. Consequently, with about 700 g of electron ics, box, cables, and a rather simple interface to the radar antenna it bec omes possible to monitor two local aeronomical parameters that play a funda mental role in the interaction between the Martian environment and the sola r 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 capab ility of measuring the electron temperature, and the reliability of its mea surements 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 ca n hardly be derived from the passive electric field measurements planned to be made by MARSIS. Unfortunately, due to a too late decision about the add itional payload selection, the proposal of AIM as part of the MARSIS has be en rejected. (C) 2001 Elsevier Science Ltd. All rights reserved.