GEM-Mars model explores fundamental Mars chemistry problem

2023-2024
The persistent underestimation by models of the highest ozone abundances on Mars may be indicative of a fundamental problem with our understanding of the Martian atmospheric chemistry. It was suggested before that heterogeneous reactions on water ice clouds could solve this, but there remain important issues at stake.

Using BIRA-IASB's Mars climate model GEM-Mars, a wide range of processes were explored that can contribute to solving the puzzle. This study helped to provide recommendations to guide the research community in finding solutions.

After decades of space-based observations and theoretical modelling, there remain persistent problems in our fundamental understanding of the Martian atmosphere. One of these is the underestimation of the highest ozone abundances by models. It was suggested before that the heterogeneous uptake of the hydroperoxyl radical (HO2) on water ice clouds could improve this, but there remain fundamental issues at stake.

The problem was recently addressed using BIRA-IASB's Mars climate model GEM-Mars. This work was done with support from dr. John Crowley of the Max Planck Institute for Chemistry (Germany), a world leading expert on heterogeneous chemistry.

The GEM-Mars model

GEM-Mars is a weather and climate model for Mars that was originally developed in Canada, and since 15 years developed and managed at BIRA-IASB. 

It allows to simulate in 3D how the main components of the Martian atmosphere interact, including dust, ices, and a range of atmospheric gases. GEM-Mars is one of only two such models in the world that contain a full description of the fundamental atmospheric chemistry on Mars.

Exploring new processes

Our study provided indications that the experimentally obtained reaction rate for the uptake of HO2 on ice is too large, and we demonstrated that with a reduced uptake coefficient, the impact on ozone is negligible. This triggered a search for alternative processes.

As a start, we investigated the low temperature behaviour of hydrogen peroxide (H2O2) and its impact on ozone and showed that H2O2 can be destroyed in surface ices on Mars. This was supported by a first, negative, search for H2O2 in observations of surface ices.

Next, in a collaboration with the Max Planck Institute for Chemistry, we tested all heterogeneous reactions on dust and water ice particles that are recommended for the terrestrial atmosphere. We found that the uptake of HO2 and H2O2 by mineral dust on Mars is too efficient when using the known terrestrial uptake coefficients.

Finally, we found that the shielding of incoming sunlight by water ice clouds leads to larger ozone abundances below the clouds, a process that was never well considered before.

Combining these processes improved the ozone simulation, but not sufficiently. Therefore, we concluded that more refinements are needed in atmospheric modelling, and that new observations and laboratory experiments are necessary. To this end, we provided detailed recommendations to guide the research community in solving this enigma.

 

Reference

Daerden, F., Crowley, J. N., Neary, L., et al. (2023). Heterogeneous processes in the atmosphere of Mars and impact on H2O2 and O3 abundances. Journal of Geophysical Research: Planets, 128, e2023JE008014. https://doi.org/10.1029/2023JE008014

Snapshot of a GEM-Mars simulation of ozone
Snapshot of a GEM-Mars simulation of ozone (O3, colour shading) near the surface during early northern summer, with a view of the north pole, and with a large volcano (Elysium Mons) just below the centre of the image. Contours: full white: polar ice cap; black: surface topography; white arrows: near-surface winds.
Total ozone column on Mars: differences observations and simulations
Panel (a) shows the total ozone column on Mars observed by the NASA/MRO MARCI instrument for a full year (x-axis) and for all latitudes (y-axis). Below, three GEM-Mars simulations are shown: (b) without heterogeneous chemistry, (c) with uptake of HO2 on ice using the yet unconfirmed uptake coefficient, (d) with the new processes explored in this study. The right side plots show the differences with the observations. Black contour lines indicate water ice clouds. Source: Daerden et al. (2023).
Amount of hydrogen peroxide (H2O2) simulated in the GEM-Mars model
Amount of hydrogen peroxide (H2O2) simulated in the GEM-Mars model in the gas-phase (top), the solid phase (second row), and the amount adsorbed on water ice (third row), and this for four cardinal seasons (left to right: northern spring, summer, fall and winter). The bottom row shows the simulated number of adsorbed H2O2 layers on water ice particles. Each figure shows height (y-axis) vs. latitude (x-axis). The white contours indicate temperature (2 top rows) and ice water content (2 bottom rows). Source: Daerden et al. (2023).