A twelve-year survey of the HDO and H2O cycles using the Mars Planetary Climate Model from MY26 to MY37

Context : The deuterium/hydrogen isotopic ratio, D/H, is one of the keys to understand the origin of water on terrestrial bodies within the Solar System and its evolution over time in their atmospheres.

In the atmosphere of Mars, this D/H ratio is on average 5 to 6 times higher than the Vienna Standard Mean Ocean Water (VSMOW), the Earth’s oceans reference. Although water vapor is present only in very low quantities on Mars (100 ppmv on average), it is rich in deuterium, which points to a wetter past for the Red Planet, a fact corroborated by various geological indicators (valleys, ancient lakes, shorelines). This enrichment is understood as a cumulative effect of differential escape between H and D atoms, the latter being more prone to gravity than its lighter isotopologue. This differential escape is deeply rooted into the seasonal behavior of HDO and H₂O, sole precursors of D and H on Mars, which release H and D at high altitude, when climatic conditions allow photochemistry. [1, 2]

Model : The Mars PCM (Planetary Climate Model) simulates the physical, chemical and dynamical processes in the Martian atmosphere, including water ice cloud-related phenomena [3], such as condensation, that fully control the relative behavior of HDO [4, 5]. This model, coupled with observations and data from ACS (Atmospheric Chemistry Suite), has shed light on the HDO cycle recently. However, differences still exist between the model and the observations. This is particularly the case for the vertical distribution of water vapor in the upper atmosphere [5, 6]. Some improvements to the Mars PCM, namely new dust injection scheme and non-orographic gravity waves [7] have been implemented.

Results : This study presents a 12-Martian-year simulation, covering Martian Years 26 to 37, with a focus on interannual variations in the HDO and H₂O cycles. The results from the model are compared with multiple observations from instruments such as ACS and SPICAM, capturing a wide range of dust events and revealing key processes to which the HDO and H₂O cycles are particularly sensitive and predicted by the Mars PCM. A third moment is being implemented in the dust particles size distribution to improve its vertical distribution with respect to observations. The first outcomes of this modification will also be addressed.

This study is part of a broader effort to better understand the origin and the long-term evolution of the water on Mars. By investigating the processes behind the Mars’ deuterium enrichment, it contributes to unraveling the history of atmospheric escape and climate change on the Red Planet.

References

[1] Villanueva, G. et al. (2015), Strong water isotopic anomalies in the martian atmosphere: probing current and ancient reservoirs, Science

[2] Owen, T. et al. (1988), Deuterium on Mars: The Abundance of HDO and the Value of D/H, Science

[3] Navarro, T. et al. (2014), Global climate modeling of the Martian water cycle with improved microphysics and radiatively active water ice clouds, JGR Planets

[4] Bertaux, J-L. & Montmessin, F. (2001), Isotopic fractionation through water vapor condensation: The Deuteropause, a cold trap for deuterium in the atmosphere of Mars, JGR Planets

[5] Vals, M. et al. (2022), Improved Modeling of Mars' HDO Cycle Using a Mars' Global Climate Model, JGR Planets

[6] Rossi, L. et al. (2022), The HDO cycle on Mars : Comparison of ACS observations with GCM simulations, JGR Planets

[7] Gilli, G. et al. (2020), Impact of Gravity Waves on the Middle Atmosphere of Mars: A Non-Orographic Gravity Wave Parameterization Based on Global Climate Modeling an