The plasma environment of Mars: from the shocked solar wind down to the ionosphere

Despite the gnat number of missions devoted to the exploration of the plane t Mars (including: Mariner 4, in 1965; Mars 2, 3, & 5, from 1971 to 1974; V iking landers, in 1976; and Phobos 2, in 1989), actually little is known ab out the near-Mars plasma environment and its interaction with the interplan etary medium. Mars Global Surveyor has recently confirmed that no intrinsic magnetic held 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 h as long been recognized that the Mars-solar wind interaction strongly depen ds on solar activity, solar zenith angle, and altitude, as well. Unfortunat ely, 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 ana logy with Venus and comets. While the atmosphere of Venus is 10 000 times d enser than that of Mars near the surface, the densities of the neutral atmo spheres of the two planets are indeed of the same order of magnitude at alt itudes of 250-450 km. In addition, mass loading of the solar wind by the io nization of suprathermal atmospheric neutrals is likely to play a role comp arable to a comet one. Similar to Venus there is evidence that there are im portant inhomogeneities inside and outside the Martian ionosphere (plasma c louds and holes) and it is likely that there is also a strong axial asymmet ry in the plasma distribution around the wake. Erosion processes produced b y the solar wind are claimed to be important in the quantification of the M ars atmospheric mass budget. Frictions exerted on the wake planetosphere pl asma have been observed. They should not be restricted to the outermost reg ion and might also occur deep inside so that the amount of volatiles lost i n the past through this phenomenon could be important. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.