Modeling the HDO cycle on Mars : focus on the dust effect

The deuterium/hydrogen isotopic ratio, D/H, is one of the main keys to understand the origin of water on several celestial 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 what is found on Earth. Although water is present only in very low quantities, this particularly high deuterium enrichment, supported by various geological indicators such as the presence of valleys, indicates a wet past for the red planet. To understand this result and how the water has escaped from Mars' atmosphere, the study of HDO – the main source of changes in the D/H ratio on the planet – and its annual cycle appears essential, particularly regarding its seasonal behavior in the upper atmosphere where water vapor can be photodissociated and then ejected.

The Mars PCM (Planetary Climate Model), formerly GCM (Global Climate Model), offers numerous possibilities and effectively models the planet's topography, seasonal phenomena, transport phenomena, and cloud-related phenomena such as condensation, each playing a significant role in the behavior of HDO in the Martian atmosphere throughout the year. This model, coupled with observations and data from the ACS (Atmospheric Chemistry Suite), has shed light on the HDO cycle in recent years. However, differences still exist between the model results and the observations. This is particularly the case for the vertical distribution of water vapor in the upper atmosphere. Some improvements, concerning, for example, dust, are examined, and their effects are studied and discussed. One of them is the implementation of a more realistic dust particles size distribution in the model, following a Gaussian function. These improvements provide a better understanding of the HDO cycle as well as a more reliable completion of observations. The goal is to further understand the nature and origin of the high deuterium enrichment on the red planet.