Global dust storms on Mars inject water high into atmosphere

Planetary aeronomy

One of the most striking phenomena on Mars is a planet‐encircling storm or “global dust storm.” Once it starts, the dust suspended in the atmosphere covers the whole globe for several weeks. Mid 2018, one of these storms occurred and measurements from the Nadir and Occultation for Mars Discovery (NOMAD) instrument on the ExoMars Trace Gas Orbiter (TGO) showed elevated water vapour in the upper atmosphere connected to the storm. We use a 3D numerical model of the Mars atmosphere to help explore the mechanisms during this event.

Connection between dust storms, water vapour and the evolution of Mars’ atmosphere

Surface features on Mars indicate that liquid water was present on Mars in the past. Recent studies suggest that global dust storms effectively transport water vapour from the near surface to the middle atmosphere of Mars. This behaviour is of interest in terms of the evolution of the planet, as water molecules at greater heights are more readily dissociated by sunlight and lost into space. The study of this process can tell us more about how the atmosphere of Mars may have changed over billions of years.

TGO/NOMAD observes high altitude water

In June to September 2018 and January 2019, a strong global dust storm and a regional storm occurred on Mars, respectively. We analysed the measurements taken at these periods by the NOMAD instrument onboard TGO currently in orbit around Mars. The objective was to investigate altitude profiles of water vapour in the martian atmosphere measured during the dust storms.

A significant increase of water vapour abundance in the middle atmosphere (40–100 km) was identified during the global dust storm - water vapour reached very high altitudes, up to 100 km. A noticeable increase was seen during the regional dust storm as well. The TGO-NOMAD observations have provided an unprecedented view of this important aspect of the Mars atmosphere.

Using numerical modelling to explain the observations

Using a three‐dimensional numerical model of the Mars atmosphere (GEM-Mars), we found that when dust from the storm is transported up to levels above ~40 km, it warms the atmosphere due to solar absorption. This in turn prevents ice clouds from forming at heights of 40–60 km and allows more water vapour to ascend to greater heights in the atmosphere.

We performed several simulations, varying the vertical distribution of the dust in the model and found that the formation of ice clouds is very sensitive to this. Numerical modelling experiments like these are crucial for the interpretation and understanding of the observations made by NOMAD.

References

Aoki, S., Vandaele, A.C., Daerden, F., Villanueva, G.L., Liuzzi, G., Thomas, I.R., Erwin, J.T., Trompet, L., Robert, S., Neary, L., Viscardy, S., Clancy, R.T., Smith, M.D., Lopez‐Valverde, M.A., Hill, B., Ristic, B., Patel, M.R., Bellucci, G., Lopez‐Moreno, J.-J., and the NOMAD team. (2019). Water Vapor Vertical Profiles on Mars in Dust Storms Observed by TGO/NOMAD. Journal of Geophysical Research: Planets, 124(12), 3482-3497. https://doi.org/10.1029/2019JE006109
Neary, L., Daerden, F., Aoki, S., Whiteway, J., Clancy, R.T., Smith, M., Viscardy, S., Erwin, J.T., Thomas, I.R., Villanueva, G., Liuzzi, G., Crismani, M., Wolff, M., Lewis, S.R., Holmes, J.A., Patel, M.R., Giuranna, M., Depiesse, C., Piccialli, A., Robert, S., Trompet, L., Willame, Y., Ristic, B., and Vandaele, A.C. (2020). Explanation for the Increase in High‐Altitude Water on Mars Observed by NOMAD During the 2018 Global Dust Storm. Geophysical Research Letters, 47(7), e2019GL084354. https://doi.org/10.1029/2019GL084354 .

Image of Mars before the onset of the planet-encircling global dust storm (left) and during the storm (right). (Credits: NASA/JPL-Caltech/MSSS)

Latitudinal variation of the water vapour vertical profiles retrieved from NOMAD data in the seasonal range between solar longitude Ls = 160–195° (Fig. (a), before the global dust storm), Ls = 195–202° (Fig. (b), during the growth phase of the storm), Ls = 210–220° (Fig. (c), during the mature phase of the storm), Ls = 220–240° (Fig. (d), during the decay phase of the storm), and Ls = 240–260° (Fig. (e), continued decay of the storm). Unit of water vapour amount is in parts-per-million (by volume).

Mars model result dust water vapour hydrogen

Model results show the impact of dust on water vapour and hydrogen. The figures above show the ratio of two simulations: global dust storm/no dust storm, for water vapour (left) and hydrogen (right). In the simulations, the dust storm causes an increase in water vapour by more than a factor of 150, followed by an increase of hydrogen by a factor of 40.

2021