15:45 〜 16:00
[PCG19-08] Enhancement of hydrogen escape on early Mars induced by solar energetic particles
キーワード：火星、水素散逸、太陽高エネルギー粒子
Hydrogen escape from the Martian atmosphere is crucial for understanding Mars' habitability throughout its history. Geomorphological and geochemical features of the Martian surface suggest that Mars possessed liquid water on the surface and lost it to space over the geological timescale. Water photolysis below the cold trap produces hydrogen accumulating in the atmosphere as a long-lived hydrogen molecule, which is transported to the upper atmosphere and escapes to space by thermal or non-thermal escape processes. Previous studies highlighted a gap in water loss between the geological evidence and atmospheric modeling and observations based on isotopic fractionation, with the latter values significantly smaller than the former estimates (e.g., Cangi et al., 2023). This fact brings a necessity of seeking unknown processes driving hydrogen escape from Mars.
Nakamura et al. (2023a) suggested that precipitation of solar energetic particles (SEPs) into the present-day Martian atmosphere enhances the dissociation of water vapor due to cluster ion chemistry driven by SEP-induced atmospheric ionization. The flare observations of solar-type G stars by the Kepler mission suggested that our Sun should have been much more active, and intense SEP events could have hit the planetary atmospheres repeatedly 4 billion years ago (e.g., Shibayama et al. 2013; Lingam et al., 2018). Such continuous precipitation of SEPs into the early Martian atmosphere could have enhanced the dissociation of water vapor at altitudes higher than the photolysis altitudes and increased hydrogen escape.
In this study, we explored the impacts of SEPs on the hydrogen escape on early Mars using an atmospheric ionization model based on continuous-slowing-down approximation and a one-dimensional photochemical model PROTEUS (Nakamura et al., 2023a, 2023b). We found that the dissociation of water vapor by SEP-induced ion chemistry exceeds the photolysis, enhancing the hydrogen thermal escape rate by more than an order of magnitude. The enhancement of hydrogen escape decreases with increasing the H2 degassing flux but is non-negligible up to 1010 cm-2 s-1. The SEP-induced water dissociation and the forced hydrogen escape lead the atmosphere redox to unbalance from ~10 days to 106 years, after which the atmosphere establishes a new steady state of chemical composition with enhanced hydrogen escape and suppressed CO concentration due to self-regulation.
References:
Cangi, E., et al. (2023). Journal of Geophysical Research: Planets, 128, 7.
Lingam, M., et al. (2018). The Astrophysical Journal, 853(1), 10.
Shibayama, T., et al. (2013). The Astrophysical Journal, 209:5.
Nakamura, Y., et al. (2023a). Journal of Geophysical Research: Space Physics, 128, 12.
Nakamura, Y., et al. (2023b). Earth Planets and Space, 75(1), 140.
Nakamura et al. (2023a) suggested that precipitation of solar energetic particles (SEPs) into the present-day Martian atmosphere enhances the dissociation of water vapor due to cluster ion chemistry driven by SEP-induced atmospheric ionization. The flare observations of solar-type G stars by the Kepler mission suggested that our Sun should have been much more active, and intense SEP events could have hit the planetary atmospheres repeatedly 4 billion years ago (e.g., Shibayama et al. 2013; Lingam et al., 2018). Such continuous precipitation of SEPs into the early Martian atmosphere could have enhanced the dissociation of water vapor at altitudes higher than the photolysis altitudes and increased hydrogen escape.
In this study, we explored the impacts of SEPs on the hydrogen escape on early Mars using an atmospheric ionization model based on continuous-slowing-down approximation and a one-dimensional photochemical model PROTEUS (Nakamura et al., 2023a, 2023b). We found that the dissociation of water vapor by SEP-induced ion chemistry exceeds the photolysis, enhancing the hydrogen thermal escape rate by more than an order of magnitude. The enhancement of hydrogen escape decreases with increasing the H2 degassing flux but is non-negligible up to 1010 cm-2 s-1. The SEP-induced water dissociation and the forced hydrogen escape lead the atmosphere redox to unbalance from ~10 days to 106 years, after which the atmosphere establishes a new steady state of chemical composition with enhanced hydrogen escape and suppressed CO concentration due to self-regulation.
References:
Cangi, E., et al. (2023). Journal of Geophysical Research: Planets, 128, 7.
Lingam, M., et al. (2018). The Astrophysical Journal, 853(1), 10.
Shibayama, T., et al. (2013). The Astrophysical Journal, 209:5.
Nakamura, Y., et al. (2023a). Journal of Geophysical Research: Space Physics, 128, 12.
Nakamura, Y., et al. (2023b). Earth Planets and Space, 75(1), 140.