Ion Loss due to Momentum Transport Effects

The outflow of cold ionospheric ions from the Martian terminator regions was also observed by the ASPERA instrument on board of the Phobos 2 spacecraft, which indicates a strong interaction between the solar wind plasma and the cold ionospheric plasma in the Martian topside ionosphere in a way that the solar wind plasma transfers momentum directly to the ionosphere in a dayside transition region to the deep plasma tail [47]. PVO measured on Venus the median velocity field of O+ ions on the outbound portion of orbits from periapsis to the ionopause and found that the bulk velocity of the ions near the terminator is about 5 km s-1 [58], which is the escape velocity of O atoms on Mars.

The results of several studies [58, 59, 60] suggest that the solar wind momentum transport seems to be capable of accelerating ionospheric O+ ions to velocities > 5 km s-1 (> 2 eV) resulting in energies larger than the Martian escape energy. In a previous study [58], it was shown that cool ion escape due to momentum transport effects may have removed water from Mars equivalent to a global ocean with a thickness of about 10 to 30 m, depending on the uncertainties of solar wind parameters and the Martian plasma environment in the past.

We studied this loss process by using the observed average stellar wind data of young solar-like stars [22, 52] and the ionospheric density profiles corresponding to the Martian history [50] and found an additional escape rate of oxygen of about 1 x 1025 s-1 at present, about 2 x 1026 s-1 2 Ga ago and about 2 x 1027 s-1 3.5 Ga ago. One can see that this loss process may have played an important role in the Martian history because it is strongly related to the solar wind and ionospheric density, which was much larger due to the more active young Sun.

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