Atmosphere and water loss from early mars under extreme solar wind and extreme ultraviolet conditions

The upper limits of the ion pickup and cold ion outflow loss rates from the early martian atmosphere shortly after the Sun arrived at the Zero-Age-Main-Sequence (ZAMS) were investigated. We applied a comprehensive 3-D multi-species magnetohydrodynamic (MHD) model to an early martian CO ₂-rich atmosphere, which was assumed to have been exposed to a solar XUV [X-ray and extreme ultraviolet (EUV)] flux that was 100 times higher than today and a solar wind that was about 300 times denser. We also assumed the late onset of a planetary magnetic dynamo, so that Mars had no strong intrinsic magnetic field at that early period. We found that, due to such extreme solar wind-atmosphere interaction, a strong magnetic field of about ∼4000 nT was induced in the entire dayside ionosphere, which could efficiently protect the upper atmosphere from sputtering loss. A planetary obstacle (∼ ionopause) was formed at an altitude of about 1000 km above the surface due to the drag force and the mass loading by newly created ions in the highly extended upper atmosphere. We obtained an O⁺ loss rate by the ion pickup process, which takes place above the ionopause, of about 1.5 × 10²⁸ ions/s during the first ≤150 million years, which is about 10⁴ times greater than today and corresponds to a water loss equivalent to a global martian ocean with a depth of ∼8 m. Consequently, even if the magnetic protection due to the expected early martian magnetic dynamo is neglected, ion pickup and sputtering were most likely not the dominant loss processes for the planet's initial atmosphere and water inventory. However, it appears that the cold ion outflow into the martian tail, due to the transfer of momentum from the solar wind to the ionospheric plasma, could have removed a global ocean with a depth of 10-70 m during the first ≤150 million years after the Sun arrived at the ZAMS.