On the outflow of O₂⁺ ions at Mars

Abstract

The ASPERA (automatic space plasma experiment with a rotating analyzer) instrument aboard Phobos 2 has detected a large outflow of molecular ions (possibly O₂⁺) from the Martian environment. We suggest that this molecular outflow results from horizontal transport of O₂⁺ ions from the dayside. We demonstrate that the in situ profiles of O₂⁺ 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. The profiles are considerably depleted compared with the corresponding diffusive equilibrium profiles. The difference between the diffusive equilibrium and the observed profile is the likely amount of O₂⁺ ions which flow over to the nightside, a part of which would escape the gravity of the planet. However, the estimated escape rate exceeds the observed total ionospheric escape rate by about an order of magnitude and that of the molecular ions by 1 to 2 orders of magnitude. This suggests that there is possibly a missing component in the observed molecular ion outflow from Mars.