Escape of high-altitude water and D/H fractionation driven by non-thermal processes on Mars
Abstract
Mars has lost most of its initial water inventory to space through a number of escape processes acting on its constituents, H and O atoms. A classical model assumes that H and O escape in 2:1 ratio over time scales greater than 0.1 Ma, with H escape responding to O loss. In this picture, variations due to seasonal effects and climate cycles are considered to average out. Recently, seasonal variations in H escape have been observed and attributed to stronger coupling of the lower and middle atmospheres during martian summers that can be further enhanced by dust storms. Atmospheric water vapor on Mars at altitudes as high as 80 km has been observed during global and regional dust storms1,2. The ExoMars Trace Gas Orbiter observed that HDO followed the same trend during the 2018 global dust storm. These data were used to construct D/H ratios at high altitudes and confirm the vertical transport of water during a global dust storm. During a regional 2019 C-storm, Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter measurements of the H corona derived from Ly α line brightness suggested that water transport from mid-altitudes (60-80 km) to the corona took place. Photochemical modeling predicted that high-altitude water could lead to enhanced H escape3. Here, we present the results of our Mars atmospheric escape model applied to transport of water constituents (H, D, H2, OH, H2O) from mid-altitudes for conditions corresponding to regional and global dust storms, as well as non-stormy atmosphere, for different seasons. We use MAVEN multi-instrument data as model inputs and consider dissociative recombination of O2+and CO2+molecular ions with electrons as a major source of suprathermal (vs thermal background) O atoms, as well as newly constructed state-to-state cross sections for O+CO2 collisions4. We introduce and quantify two new hypotheses: 1) high-altitude water and D escape is aided by solar wind energetic neutral particles capable of penetrating deep into the martian atmosphere; 2) direct escape of molecules driven by collisions with superthermal neutral atoms affects water transport and D/H ratio. 1A. C. Vandaele et al., Nature, 568, 521 (2019) 2L. Maltagliati et al., Science, 333, 1868 (2011) 3M. S. Chaffin et al., Geophys. Res. Lett., 41, 8013 (2014) 4M. Gacesa et al., arXiv:1906.11368 (2019)
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P52C..05G
- Keywords:
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- 0343 Planetary atmospheres;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 6225 Mars;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5405 Atmospheres;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 5445 Meteorology;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS