CONTRIBUTION OF SURFACE MAGNETIC RECORDINGS TO PLANETARY EXPLORATION

The transient variations of the magnetic field at the surface of a pla net have a primary external source, the interaction between the enviro nment of the planet and solar radiation, and a secondary source, the e lectric currents induced in the conductive planet. The continuous reco rding of the time variations of the magnetic fields at the surface of Mars by means of three-component magnetometers installed on board land ers would therefore allow study of both the internal structure of Mars and the dynamic of its ionized environment. The depth of penetration of an electromagnetic wave in a conductive medium depends on both the period of the wave and the electrical resistivity of the medium. The l arger the period and the resistivity, the greater the depth of penetra tion (skin effect). The high frequency spectrum will therefore enable one to estimate the resistivity in the uppermost kilometres of the pla net, and to give information about the presence (or absence) of liquid water under the permafrost. The low frequency spectrum of the transie nt variations will give information on the presence (or absence) of sh arp variations in the resistivity in the uppermost hundreds of kilomet res of Mars, and thus on the thermodynamic conditions within the upper mantle of this planet. Averages of the measurements made during ''qui et time measurements'' would provide a very good estimate of the field of internal origin at the locations of the surface stations. If in ad dition a total duration of one year or more for the mission can be exp ected, and a drift on the order of 1 nT per year for the ground-based magnetometer, it might even be possible to detect some dynamo-related secular variation. In addition to the map of the Martian magnetic fiel d which will be produced by the Mars Surveyor 1 orbiter, these ground- based local main field measurements will provide original information on the present and past magnetic field of Mars, and then on its presen t and past core dynamics. As is the case for the Earth, different poss ible controlling plasma processes will lead to different convection pa tterns inside the magnetosphere and therefore different magnetic signa tures at the planetary 1 surface. Continuous recordings of the transie nt variations of the magnetic field on board landers will then provide constraints on the convection within the Martian magnetosphere, that is a small magnetosphere where the ionosphere lies at great heights re lative to the dimensions of the magnetospheric cavity. Copyright (C) 1 996 Elsevier Science Ltd