ON THE OUTFLOW OF O-2(+) IONS AT MARS

The ASPERA (automatic space plasma experiment with a rotating analyzer ) instrument aboard Phobos 2 has detected a large outflow of molecular ions (possibly O-2(+)) from the Martian environment. We suggest that this molecular outflow results from horizontal transport of O-2(+) ion s from the dayside. We demonstrate that the in situ profiles of O-2(+) ions as observed by the Viking 1 and 2 landers at Mars are eroded by the solar wind interaction down to an altitude of about 150-160 km. Th e profiles are considerably depleted compared with the corresponding d iffusive equilibrium profiles. The difference between the diffusive eq uilibrium and the observed profile is the likely amount of O-2(+) ions which flow over to the nightside, a part of which would escape the gr avity of the planet. However, the estimated escape rate exceeds the ob served total ionospheric escape rate by about an order of magnitude an d that of the molecular ions by 1 to 2 orders of magnitude. This sugge sts that there is possibly a missing component in the observed molecul ar ion outflow from Mars.