Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p53a1428p&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P53A-1428
Other
0925 Magnetic And Electrical Methods (5109), 5400 Planetary Sciences: Solid Surface Planets, 6000 Planetary Sciences: Comets And Small Bodies, 6024 Interiors (8147), 6297 Instruments And Techniques
Scientific paper
The in-situ investigation of subsurface and atmospheric properties on planetary and cometary bodies or moons is a field of growing interest. We present a concept to measure the electrical properties using electric sensors at the feet of a planetary lander-system. Because of the expected high contact impedances, we suggest capacitive coupling for the injection of current into the regolith. This requires an alternating current, ideally in the frequency range from 100 Hz to 100 kHz, which at the same time provides a good resolution of both electrical resistivity and permittivity. We present a comprehensive theory covering all aspects such as the contact impedance of the electrodes, spurious currents in the lander, and the resolution of subsurface parameters depending on the geometry of the 4-point electrode configuration. Feeding sufficient current into the ground imposes special conditions on the design of the sensor- electrodes and the electronic components. Over resistive ground and at high frequencies the contact impedance will depend on the electrical properties of the subsurface and determines how much current can be injected into the ground. We calculate the contact impedance based on a spherical disc model and show that placing the electrode directly on the ground is always superior to the use of an insulating layer. Another design criteria is that the spurious current flow in the lander must be kept under a certain level. The interaction of the capacitive electrodes with the lander system and the ground is examined by an electric circuit which represents the properties of the subsoil and the lander system. The ratio between the spurious current in the lander and the current in the ground critically depends on the construction of the electrodes. We suggest a PEEK-vacuum solution which minimizes capacitive coupling to the lander while keeping the weight small. For an optimum resolution of the subsurface parameters, the geometry and especially the distance of the electrode to the ground are decisive. Near -surface information on the scale of the electrode size may be obtained directly from the contact impedances of the electrodes, while deeper information in the range of electrode distance may be obtained through a classical 4-point configuration. Within the limits of a space mission, not every position of the electrodes is possible for the geometry of the 4 electrodes. Two solutions for the placement of sensors and their influence on the expected results are discussed. The results show that ideally, the electrodes are as close to the ground as possible. However, a design where the electrodes are placed with a certain distance under the lander-body will also provide an acceptable resolution. Our suggested experiment might be realized within the Russian Phobos Grunt mission.
Auster Uli
Hördt Andreas
Pennewitz E.
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