Modeling of HDO in the Martian
atmosphere

**L. Rossi**¹, **Margaux Vals**¹, **Franck Montmessin**¹, **François Forget**²,
**Ehouarn Millour**², **Kevin S. Olsen**¹, **Anna Fedorova**³,
**Alexander Trokhimovskiy**³, **Oleg Korablev**³ and the ACS team.

¹ : LATMOS/IPSL, UVSQ, Université Paris-Saclay, Sorbonne Université, CNRS, Guyancourt, France (loic.rossi@latmos.ipsl.fr)

² : LMD, Paris, France

³ : Space Research Institute (IKI), Moscow, Russia

Twitter : @AstroLR

EGU 2020
# Why HDO?

* The D/H ratio is used to study the inventory of water on Mars

- Preferential escape of H atoms over D atoms

- D/H ratio can thus be used to estimate the past water inventory

* The current value is about 5 times the Earth's value (SMOW).

* The martian climate can also intervene and change the D/H ratio, so we need to understand it!

<figure><img src="image_1.png" /></figure>

D/H maps from Encrenaz et al. (2018) (CC-BY)
# HDO Fractionation

* HDO has saturation pressure different from that of H₂O

- Preferential condensation of HDO

* Fractionation Factor:

$$ \alpha_c = \frac{(HDO/H_2O)_{ice}}{(HDO/H_2O)_{vap}} = \exp\left(\frac{16288}{T^2} - 9.34 \times 10^{-2}\right) $$

* The ice will therefore be enriched in deuterium with respect to the vapour phase.

* One can expect strong fractionation in the cold areas (in particular, the polar areas)
Processes considered

* Condensation/sublimation in the clouds

- Everything is condensed or sublimated at once with respect to the saturation pressure

- So no realistic microphysics

* Condensation/sublimation at the surface

* Sedimentation of ice crystals

For the most part, HDO is treated like H₂O, except for the fractionation.
Model parameters

* We use the IPSL Mars GCM (Forget et al. 1999)

* HDO is represented with 2 tracers (vapour, ice)

* D/H initialized with 5 (SMOW) everywhere on the planet

* The ice in the north polar cap is assumed to have D/H=5

* Clouds are not radiatively active

* We use a 64x48x32 (lon x lat x alt) grid

* Unlike in Montmessin et al. (2005), the dust is not prescribed but transported with a semi-interactive scheme (Madeleine et al. 2011)
HDO Seasonal Cycle

<figure><img src="image_2.png" /></figure>

Scenario Climatology

HDO follows well H₂O, except for the polar areas, where the fractionation traps HDO in the ice and reduces the D/H ratio of the column.
Latitudinal variations

<figure><img src="image_3.png" /><figcaption>Zonally averaged profiles of water vapour (filled line) and D/H ratio (dashed line)</figcaption></figure>
# The dust storm of MY 34

* We apply the dust scenario from MY34 to the HDO cycle to explore the effect of the dust storm.

<figure><img src="image_4.png" /></figure>

Cloud formation constrains the vertical distribution of HDO

Before the storm
# The dust storm of MY 34

* We apply the dust scenario from MY34 to the HDO cycle to explore the effect of the dust storm.

<figure><img src="image_5.png" /></figure>

Higher
temperatures
affect cloud
formation

Less cloud
formation means
HDO can reach
higher altitudes
# The dust storm of MY 34

* We apply the dust scenario from MY34 to the HDO cycle to explore the effect of the dust storm.

<figure><img src="image_6.png" /></figure>

Higher
temperatures
affect cloud
formation

Less cloud
formation means
HDO can reach
higher altitudes
# The dust storm of MY 34

* We apply the dust scenario from MY34 to the HDO cycle to explore the effect of the dust storm.

<figure><img src="image_7.png" /></figure>

Higher
temperatures
affect cloud
formation

Less cloud
formation means
HDO can reach
higher altitudes

During the storm
# The dust storm of MY 34

* We apply the dust scenario from MY34 to the HDO cycle to explore the effect of the dust storm.

<figure><img src="image_8.png" /></figure>

Higher
temperatures
affect cloud
formation

Less cloud
formation means
HDO can reach
higher altitudes
# The dust storm of MY 34

* We apply the dust scenario from MY34 to the HDO cycle to explore the effect of the dust storm.

<figure><img src="image_9.png" /></figure>

Higher
temperatures
affect cloud
formation

Less cloud
formation means
HDO can reach
higher altitudes
# The dust storm of MY 34

* We apply the dust scenario from MY34 to the HDO cycle to explore the effect of the dust storm.

<figure><img src="image_10.png" /></figure>

After the storm
# Genealogy of tracers

Need to conserve the isotopic ratio during the dynamic transport: it is necessary to transport the isotopic ratio and not HDO alone (Risi et al. 2009). Numerically, HDO is transported “within the mass of water”: “HDO is a son of H₂O”

<figure><img src="image_11.png" /></figure>

*Without genealogy*

<figure><img src="image_12.png" /></figure>

*With genealogy*

This provides a huge improvement in places where there is little water vapour, and solves many of the spurious values of D/H we had.
D/H ice ratio with sedimentation of HDO

<figure><img src="image_13.png" /></figure>
# D/H ice ratio with sedimentation of Ratio=HDO/H₂O

Transporting the ratio seems to also improve the sedimentation

<figure><img src="image_14.png" /></figure>
# Conclusions / summary

* General agreement with

- previous models from Montmessin et al. (2005)

- observations of latitudinal variations (e.g. Khayat et al. 2019)

* The dust storm has a visible effect on the D/H ratio:

- the temperatures are changed

- cloud formation and circulation is affected

- fractionation of HDO happens at higher altitudes, if at all.
Perspectives/ameliorations

* Implementing the microphysics of the cloud formation

* Comparison with ACS and NOMAD observations of HDO and D/H profiles

* Experimenting with new fractionation formulas based on lab experiments conducted at temperatures closer to the Martian situation (Lamb et al. 2017)

* Possibility of simulating paleoclimate scenarios, in collaboration with Eran Vos (Weizmann Inst., Israël) and Joseph Naar (LMD).