3:30 PM - 3:45 PM
[PPS06-01] Impact of water supply from interplanetary dust on the isotopic composition of the Martian atmosphere
Keywords:Mars, Atmosphere, Interplanetary dust, isotope, Photochemistry, Atmospheric evolution
The atmospheric D/H ratio on Mars is enhanced by ~5 times the value on Earth, suggesting that large amounts of water have escaped into space. Additionally, water supply processes into the atmosphere, like volcanic outgassing and ablation of interplanetary dust particles (IDPs), are considered important to satisfy the current isotopic composition. Nevertheless, the effect of IDPs ablation on the isotopic composition of planetary water is poorly understood. IDPs containing water as hydrous minerals with a relatively low D/H ratio ablate at high altitudes and supply water into the upper atmosphere. This study investigates the impact of the water supply from IDPs on the isotopic composition of the Martian atmosphere.
To verify whether the water supplied from IDPs affects the D/H ratio profile (the hydrogen isotopic composition), we use a 1-D atmospheric photochemical model (Nakamura et al., 2023) coupled with a simplified interplanetary dust ablation model. Assuming a CI chondrite-like composition, IDPs containing ~1 wt% of hydrogen as hydrous minerals with the same isotopic ratio as the VSMOW value are injected into the Martian atmosphere. The water injection flux is given as for the nominal model based on the observation made by the Long Duration Exposure Facility on the Earth. The vertical injection profile of the interplanetary dust water into the atmosphere is scaled with the ablation profile of the metallic species, assuming the water contained within the hydrous minerals is released along a similar altitude profile to Si and Mg.
Our results show that the HDO/H2O ratio is significantly changed by the water supply from IDPs in the upper atmosphere of Mars, while other species show little isotopic changes. The value decreases by ~400‰ above 100 km altitude of Mars by the water supply from IDPs for the nominal model, which corresponds to a ~7% decrease in the HDO/H2O density ratio. The change in the HDO/H2O ratio in the Martian upper atmosphere is caused by the high injection flux of water from IDPs compared to photochemical reactions and upward transport by diffusion. The isotopic ratios of OH and H show little changes even though they are the primary product of H2O photodissociation. This is because the lifetimes of OH and OD are so short that the isotopic change does not spread into the upper atmosphere. In addition, the densities of H and D are several orders of magnitude higher than that of H2O and HDO, high enough to ignore the isotopic change caused by the H and D originated from IDPs.
We further investigate the sensitivities of the atmospheric isotope profiles to parameters such as the dust ablation profile, temporal variations in the dust influx, and the D/H ratio of the IDPs. The sensitivity to the peak ablation altitude is investigated considering its uncertainty. We find that the atmospheric HDO/H2O ratio decreases with an increasing peak altitude of water injection. When the peak ablation altitude is increased by 10 km, 20 km, and 30 km from the nominal peak altitude of 90 km, the value decreases by ~800‰, 1200‰, and 1700‰, respectively, above 120 km altitude. The sensitivity test for temporal variations in the dust influx reveals that the isotopic change overcomes the diurnal variation in the dust influx and persists for several days. Therefore, we conclude that the water supply from IDPs changes the HDO/H2O ratio in the Martian upper atmosphere regardless of its local time and longitude. The sensitivity test for the D/H ratio of the IDPs is investigated considering the experimental results that the D/H ratio of the dust particles is enriched by hydrogen implantation by the solar wind (Jiang et al., 2024).
Furthermore, we verify whether the isotopic ratio in the escaping atmosphere is also changed because the escaping ratio is mainly controlled by the upper atmospheric composition. Our preliminary conclusion is that the change in the upper atmospheric HDO/H2O ratio does not affect the isotopic ratios of escaping hydrogens through Jeans escape and the dissociative recombination of HCO+, while it potentially affects those of escaping H2O+ and H3O+. Therefore, investigating the effect of the water supply from IDPs on the isotopic composition of ions will be important in future work.
To verify whether the water supplied from IDPs affects the D/H ratio profile (the hydrogen isotopic composition), we use a 1-D atmospheric photochemical model (Nakamura et al., 2023) coupled with a simplified interplanetary dust ablation model. Assuming a CI chondrite-like composition, IDPs containing ~1 wt% of hydrogen as hydrous minerals with the same isotopic ratio as the VSMOW value are injected into the Martian atmosphere. The water injection flux is given as for the nominal model based on the observation made by the Long Duration Exposure Facility on the Earth. The vertical injection profile of the interplanetary dust water into the atmosphere is scaled with the ablation profile of the metallic species, assuming the water contained within the hydrous minerals is released along a similar altitude profile to Si and Mg.
Our results show that the HDO/H2O ratio is significantly changed by the water supply from IDPs in the upper atmosphere of Mars, while other species show little isotopic changes. The value decreases by ~400‰ above 100 km altitude of Mars by the water supply from IDPs for the nominal model, which corresponds to a ~7% decrease in the HDO/H2O density ratio. The change in the HDO/H2O ratio in the Martian upper atmosphere is caused by the high injection flux of water from IDPs compared to photochemical reactions and upward transport by diffusion. The isotopic ratios of OH and H show little changes even though they are the primary product of H2O photodissociation. This is because the lifetimes of OH and OD are so short that the isotopic change does not spread into the upper atmosphere. In addition, the densities of H and D are several orders of magnitude higher than that of H2O and HDO, high enough to ignore the isotopic change caused by the H and D originated from IDPs.
We further investigate the sensitivities of the atmospheric isotope profiles to parameters such as the dust ablation profile, temporal variations in the dust influx, and the D/H ratio of the IDPs. The sensitivity to the peak ablation altitude is investigated considering its uncertainty. We find that the atmospheric HDO/H2O ratio decreases with an increasing peak altitude of water injection. When the peak ablation altitude is increased by 10 km, 20 km, and 30 km from the nominal peak altitude of 90 km, the value decreases by ~800‰, 1200‰, and 1700‰, respectively, above 120 km altitude. The sensitivity test for temporal variations in the dust influx reveals that the isotopic change overcomes the diurnal variation in the dust influx and persists for several days. Therefore, we conclude that the water supply from IDPs changes the HDO/H2O ratio in the Martian upper atmosphere regardless of its local time and longitude. The sensitivity test for the D/H ratio of the IDPs is investigated considering the experimental results that the D/H ratio of the dust particles is enriched by hydrogen implantation by the solar wind (Jiang et al., 2024).
Furthermore, we verify whether the isotopic ratio in the escaping atmosphere is also changed because the escaping ratio is mainly controlled by the upper atmospheric composition. Our preliminary conclusion is that the change in the upper atmospheric HDO/H2O ratio does not affect the isotopic ratios of escaping hydrogens through Jeans escape and the dissociative recombination of HCO+, while it potentially affects those of escaping H2O+ and H3O+. Therefore, investigating the effect of the water supply from IDPs on the isotopic composition of ions will be important in future work.