Modeling of the effect of the MY34 Global Dust Storm on the martian HDO cycle.
Authors: Rossi Loïc , Vals Margaux , Montmessin Franck , Forget François , Millour Ehouarn , Olsen Kevin , Fedorova Anna , Trokhimovskiy Alexander , Korablev Oleg 
Published: October 2020

Abstract&#160;HDO and the D/H ratio are important in order to understand Mars past and present climate, in particular with regards to the evolution through ages of the Martian water cycle. We present here new modeling efforts aimed at rejuvenating the representation of HDO in the LMD Mars GCM, with the perspective of comparison with the new observations provided by the Atmospheric Chemistry Suite (ACS) on board the ESA/Roscosmos Trace Gas Orbiter.&#160;&#160;&#160;Introduction&#160;Mars is known to have had a significant liquid water reservoir on the surface and the D/H ratio derived from the HDO/H2O abundance ratio is a sensitive tool to constrain the primordial abundance of the water reservoir on Mars and its evolution with time. The current ratio is at least five times that of the Vienna Standard Mean Ocean Water (SMOW) (Owen et al. 1988 , Encrenaz et al. 2018, Krasnopolsky 2015, Villanueva et al. 2015).&#160;&#160;H and D atoms in the upper atmosphere are coming from H2O and HDO, their sole precursor in the lower atmosphere. The lower mass of H over D atoms and the fact that H2O is preferentially photolysed over HDO (Cheng et al. 1999) explain the differential escape of these two elements. Also, the heavier isotope, HDO, has a lower vapor pressure than H2O, which results in an enrichment of the deuterated isotope in the solid phase of water. This effect is known as the Vapor Pressure Isotope Effect (VPIE) and can&#160; reduce the D/H ratio above the condensation level to values as low as 10% of the D/H ratio near the surface (Bertaux et al. 2001, Fouchet et al. 2000).&#160; Fractionation should affect the amount of HDO depending on latitude, longitude, altitude and season.&#160;&#160;Modeling HDO&#160;In particular, a previous model (Montmessin et al., 2005) has predicted that an isotopic gradient should exist between the cold regions where condensation depletes the atmosphere in HDO and the warmer, condensation-free, regions where&#160; D/H is found to be maximum. This leads to a latitudinal gradient of D/H (with variations greater than a factor of 5) between the warm and moist summer hemisphere and the cold and dry winter hemisphere. This gradient was later confirmed by several Earth observing assets (Krasnopolsky 2015, Villanueva et al. 2015, Encrenaz et al. 2018, Aoki et al. 2019). Yet some observations (Villanueva et al., 2015, Khayat et al., 2019) have revealed longitudinal variations of D/H ratios which have remained explained and are not reproduced by any model.&#160;It is therefore essential to model the HDO cycle and the associated processes of fractionation in a 3D general climate model (GCM). With the Trace Gas Orbiter (TGO) mission now in its mission phase, very strong and precise constraints will be soon available to evaluate the GCM prediction capability.&#160;We present here the results from our (re)integration of the HDO cycle into the LMD Mars GCM, taking into account HDO in its vapour and ice phases in the atmosphere, and as surface ice. The model presented here uses a simplified cloud formation scheme, without condensation nuclei. The updated model is in good agreement with the results of Montmessin et al. (2005), showing the latitudinal gradient between cold and warm regions and its seasonal variation (see fig 1)&#160;Figure 1 : Latitudinal and seasonal variation of the zonally averaged D/H ratio in the vapor phase, as predicted by the model.&#160;The present study gives a 3D climate model projection of the behavior of HDO in the context of a dust annual scenario mimicking the dust seasonal and spatial evolution observed during Martian Year 34 (MY34) and including the occurrence of the Global Dust Storm (GDS). The simulations reported here emphasize the contrast in HDO behavior between a perturbed year such as MY34 and a more traditional MY where the seasonal evolution of dust obeys a recurrent and more quiet, so-called "climatological" scenario.&#160;The simulations (see fig. 2) show that in the climatological scenario the cloud formation, and the subsequent fractionation processes, trap HDO within the ice particles, depleting the vapour phase above the condensation level (the so-called "deuteropause").&#160;Figure 2 : Zonally averaged meridional profiles of temperature, HDO ice mixing ratio and D/H ratio of vapour for (left) the climatology scenario and (right) the MY34 scenario, at Ls&#8239;=&#8239;210, during the GDS.&#160;By comparison, during the GDS, the dust has a strong effect on the temperature field, and therefore on the circulation and the cloud formation. Since clouds are forming at higher altitude (if at all), the deuterium is less constrained vertically and can extend to higher altitudes. A similar effect being observed for water vapour (Fedorova et al., 2020).&#160;Early observations with the ACS and NOMAD instruments, on board the ExoMars Trace Gas Orbiter, indicate that HDO vertical distribution is indeed affected by the GDS, in qualitative agreement with our model (Vandaele et al., 2019).We will here present the results of these comparative simulations and discuss the differences and similarities with the observations from ACS and NOMAD.&#160;Acknowledgements&#160;ExoMars is the space mission of ESA and Roscosmos. The ACS experiment is led by IKI Space Research Institute in Moscow. The project acknowledges funding by Roscosmos and CNES. Science operations of ACS are funded by Roscosmos and ESA. Science support in IKI is funded by Federal agency of science organizations (FANO). L.R. acknowledges support from CNES and from the Excellence Laboratory "Exploration Spatiale des Environnements Plan&#233;taires (ESEP)".