The plasma environment of Mars: from the shocked solar wind down to the ionosphere
Résumé
Despite the gnat number of missions devoted to the exploration of the planet Mars (including: Mariner 4, in 1965; Mars 2, 3, & 5, from 1971 to 1974; Viking landers, in 1976; and Phobos 2, in 1989), actually little is known about the near-Mars plasma environment and its interaction with the interplanetary medium. Mars Global Surveyor has recently confirmed that no intrinsic magnetic field could be invoked to stand off significantly the solar wind. Nevertheless, complex localized magnetic anomalies have been reported. They could be the signature of a past intrinsic field fossilized in rocks. It has long been recognized that the Mars-solar wind interaction strongly depends on solar activity, solar zenith angle, and altitude, as well. Unfortunately, the limited space and time coverages of the observations do not allow us to characterize the full nature of interaction. The solar wind pressure is thought to be balanced by the ionosphere and/or upper atmosphere, in analogy with Venus and comets. While the atmosphere of Venus is 10 000 times denser than that of Mars near the surface, the densities of the neutral atmospheres of the two planets are indeed of the same order of magnitude at altitudes of 250-450 km. In addition, mass loading of the solar wind by the ionization of suprathermal atmospheric neutrals is likely to play a role comparable to a comet one. Similar to Venus there is evidence that there are important inhomogeneities inside and outside the Martian ionosphere (plasma clouds and holes) and it is likely that there is also a strong axial asymmetry in the plasma distribution around the wake. Erosion processes produced by the solar wind are claimed to be important in the quantification of the Mars atmospheric mass budget. Frictions exerted on the wake planetosphere plasma have been observed. They should not be restricted to the outermost region and might also occur deep inside so that the amount of volatiles lost in the past through this phenomenon could be important.