Measurements of the nonthermal helium escape from Mars

Abstract

The automatic space plasma experiment with a rotating analyzer (ASPERA) onboard the Phobos 2 spacecraft has recently revealed the presence of planetary He+ ions at Mars (Barabash and Norberg, 1994). In the present work the analysis is continued in order to estimate the total outflow of the He+ ions which are swept away by the solar wind. For the Phobos epoch the total He+ outflow rate was found to be (1.2±0.6)×1024 ions/s. The escape occurs mainly near the Martian magnetopause. Considering extreme errors in the measurements, the maximum helium outflow could range up to 2.4×1024s−1. From a scaling of the helium profile suggested by Moroz et al. (1990) to obtain the measured loss rate, one can deduce the helium abundance in the Martian upper atmosphere. It turns out that helium is a dominant gas in the Martian exosphere at altitudes between 500 and 1250 km. However, recently reported observations of the weak EUV emissions (108 photons) from the Martian He I suggest an abundance that is 18.5 times lower (Krasnopolsky et al., 1994). Possible reasons for this disagreement are discussed. The helium production rate near Mars can, in turn, be roughly estimated from the production rate for the Earth by using a scaling argument, since the only source of helium in the atmospheres of the terrestrial planets is radioactive decay of certain isotopes of uranium and thorium. Present estimates suggest a degassing rate of 8×1022 atoms/s only (Krasnopolsky et al., 1993). However, under steady state conditions one would expect the production and loss rates to be equal. The discrepancy leads us to the conclusion that either the helium degassing rate should be corrected (or the amount of uranium is higher on Mars than anticipated) or helium may also be delivered on Mars by other sources, for example, as solar wind α particles. The observed high total outflow of ions which are 4 times heavier than protons may result in an effective mass loading. Thus helium may play an important role in the solar wind‐Mars interaction.