Impact of water supply from interplanetary dust particles on the vertical D/H ratio profile of the Martian atmosphere

                 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 ablation of interplanetary dust particles (IDPs) and volcanic outgassing, are considered important to satisfy the current isotopic composition. IDPs, containing water as hydrous minerals with a relatively low D/H ratio, ablate at high altitudes and supply water into the upper atmosphere. Nevertheless, the effect of IDP ablation on the isotopic composition of planetary water is poorly understood.

               In this study, we use a 1-D atmospheric photochemical model of the Martian atmosphere (Nakamura et al., 2023) coupled with a numerical model of decomposition and dehydration of IDPs (Micca Longo et al., 2025) to clarify the effect of water supply from IDPs on the vertical D/H ratio profile. We assume a CI chondrite-like composition containing ~1 wt% of hydrogen as interlayer water or phyllosilicates -OH bonds with the same isotopic ratio as the VSMOW value. The water injection flux is given as  for the nominal model, scaled from the observed flux by the Long Duration Exposure Facility on the Earth (Love and Brownlee, 1993) to that on Mars. The vertical injection profile is given by our simple dust ablation model.

              Our results show that the water supply from IDPs significantly changes the HDO/H₂O ratio in the upper atmosphere, while other species show little isotopic change. The  value decreases by ~400‰ above 100 km for the nominal model, which corresponds to a ~7% decrease in the HDO/H₂O ratio. The HDO/H₂O ratio change in the Martian upper atmosphere is caused by the high injection flux of water from IDPs compared to photochemical reaction rates and upward transport rate of hydrogen. The isotopic ratios of OH and H show little change even though they are the primary products 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 background atmospheric densities of H and D are several orders of magnitude higher than those of H₂O and HDO, high enough to make the isotopic change caused by the water supply from IDPs negligible.

                 We further investigate the sensitivity of the atmospheric isotopic profiles to parameters such as the temporal variations in the dust influx and the D/H ratio of IDPs. The sensitivity test for temporal variations in the dust influx reveals that the isotopic change overcomes the local time variation in the dust influx and persists for several days. This suggests 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 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), which will also be presented.