O/H ratio of Atmospheric Escape from Non-magnetized Ancient Earth

1. O/H ratio of Atmospheric Escape from Non-magnetized Ancient Earth M. Yamauchi1, H. Lammer2, J.-E. Wahlund3 1. Swedish Institute of Space Physics (IRF), Kiruna, Sweden 2. Space Research Institute (IWF), Graz, Austria 3. Swedish Institute of Space Physics (IRF), Uppsala, Sweden High EUV of early Sun means higher thermal loss of H but not O, predicting oxidation of the atmosphere. However, emergence of early life (prebiotic chemistry) requires reduced atmosphere. Can we solve this dilemma with non-thermal escape?

2. What type of interaction for ancient Earth? present Earth present Mars/Venus present Moon SW is stopped by the magnetic pressure (PB) of the planetary B Ionopause PB is enhanced until it balances both SW PD(swvsw2) and ionospheric plasma pressure PP (= ikTi) Conductivity of solid planet determines induction

3. Ancient solar forcing (Sun-in-Time) (a) Most likely high EUV/FUV flux (b) Most likelyhigh SW (solar wind) PD = vsw2 (c) Most likelystrong & active IMF due to faster rotation (d) Most likelyfrequent & intense SEP (Solar Energetic Particle) Ancient Magnetosphere/Ionosphere (e) Most likelyless geomagnetic field than present (a) + (e)  Most likely PP >> PB at all height In such a case, the pressure-balance boundary between SW and ancient Earth is determined by the ionopause, and is not by the magnetopause, with "mini-magnetosphere" like Mars.  Non-magnetized for SW interaction & magnetized for ionospheric heating

4. Various escape processes

5. We have to consider: • Observed non-thermal escape is as important as thermal escape. • Observed non-thermalescape increases with F10.7 flux. • Terrestrial non-thermal escape increases with geomagnetic activity. • Observed non-thermalescape increases during SEP event. • Observed ionopause height increases with F10.7 flux. Note: The ionopause = a magnetically shielding boundary whose magnetic pressure is balanced by the SW PD outside, and by the ionospheric plasma pressure inside, respectively, in a collision-free regime. Higher ionopause location means less neutrals (corona) beyond the ionopause.  Reduction of ion pick-up (of mainly H, He) Thick ionosphere also means more free electrons that impact on neutrals convert to ions. Such newly ionized neutrals inside the ionosphere are gyro-trapped by magnetized ionopause.  Reduction of Jeans escape (of mainly H, He)

6. Ionopause vs. EUV/FUV (major) • High ionopause during solar maximum for both Venus (Zhang et al., 2007) and Mars (Zhang et al., 1990). • Frequent SEP (during solar maximum) probably caused high balance altitude by heating of the ionosphere.  The ancient condition is this extreme. Ionopause vs. SW/IMF (minor) • High SW PD decreases the altitude of pressure balance (Luhmann et al., 1980; Phillips et al., 1984). (d) strong (stable) IMF  no change as long as SW PD > SW PB (e) variable IMF  lower balance altitude (by cancellation of B)

7. Qualitative Prognosis #1) depending on relative extent of ionosphere and exosphere #2) because non-thermal > thermal for Earth-sized planet

8. Conclusion To diagnose atmospheric evolution on early Earth and super-Earth, we qualitatively evaluate increases or decreases of non-thermal escape related to the ionosphere for nonmagnetized planets in response to changes in solar parameters. The ancient Earth can be considered as non-magnetized planet, whereas large part of the ionosphere is considered as magnetized (same as mini-magnetosphere of the Mars) where non-thermal heating is yet important. We expect much higher O escape & much higher O/H ratio of escape than present. The ancient atmosphere can be chemically quite reduced. Unclear parameters : atmospheric composition which is essential for both escape and O/H ratio of escape reference : Astrobiology J., 7(5), 783-800, 2007

9. CO2-rich vs. N2-rich atmospheres [Kulikov et al.SpaceSciRev,2007; Tian et al. JGR, 2008] [Lichtenegger et al. Icarus in press, 2010] in agreement with Tian et al. JGR (2008)