Magnetic field and plasma measurements on board the Soviet spacecraft Phobos 2 have been analyzed during five elliptical orbits around Mars. The existence of at least one separate plasma boundary and an adjacent plasma layer, called the planetopause and the transition region, between the bow shock and the ionopause seems to be a characteristic feature of the solar wind interaction with an almost nonmagnetized planetary ionosphere. It is suggested that the planetopause is a multiple-ion discontinuity, where a large number of solar wind protons are deflected at an exospheric density ramp. Strong changes in magnetic field and plasma flow direction within the transition region are interpreted as signatures of current sheets or internal shocks. The detected eclipse boundary in the tail is perhaps an elongation of the ionopause found at Venus. New ideas concerning the formation of multiple-ion flow boundaries by electrostatic plasma-plasma interaction are discussed. Finally, a remarkable Deimos event has been detected during the fifth orbit. This is explained as an interaction of the subsonic solar wind with a thin cloud of charged dust particles. ¿ American Geophysical Union 1992