Mars Atmospheric Wind Map Along the 2018 Global Dust Storm

This work presents ground-based wind velocity measurements of Mars during the 2018 global dust storm using Doppler velocimetry techniques based on observations made with the Ultraviolet and Visual Echelle Spectrograph (UVES) at the European Southern Observatory’s Very Large Telescope (VLT) facility in Chile.

This instrument’s high resolution ( R ∼ 100 000) allows for the dust cloud velocity to be measured, by computing the Doppler shift induced in the Fraunhofer lines (in the λ of 420-1100 nm) in the solar radiation that is back-scattered in the dust suspended in the Martian atmosphere, by the motion of that same dust, with an average error of approximately 5 ms^-1. This allows us to sound Mars' atmosphere during a global dust storm and obtain latitudinal wind profiles and a first approach of a planetary wind map at the altitude level sounded, i.e. the altitude level where the optical depth reaches the unity.

The purpose of this research project is to successfully apply and validate a new approach to investigate Mars’ middle atmosphere wind velocities from ground-based observations. This is the first time that a Doppler velocimetry method based on observations made in the visible and ultraviolet wavelength range is employed to study the Martian atmosphere.

Global dust storms are complex stochastic events that can drastically alter the atmospheric dynamics. During such events dust can be lifted to heights above 50 km across all latitudes and longitudes, increasing the optical depth form on the sand consequently the heating rates. The increased heating rates strengthen the Martian circulation. These storms usually develop in the southern hemisphere during southern Summer and Spring (Ls≈180°-360°), however, the 2018 storm started developing in the northern hemisphere on Ls ≈ 185°.

Global dust storms are unique to Mars and are, perhaps, the most spectacular atmospheric event taking place anywhere in the Solar System. These storms induce thermodynamic responses throughout the whole atmosphere however, their occurrence is unpredictable and shows inter-annual variability and despite the massive development our understanding of Mars has suffered over the last 50 years, these storms are still viewed as stochastic events and the issue of their year-to-year variability remains largely unresolved.

One could see the evidence that Global dust storms do strengthen the atmospheric circulation, however, due to the low mass of the Martian atmosphere, the winds carry little momentum. Their characterization is nonetheless crucial, as they are critical to the Martian climate, and is important to study the atmospheric role of the energy input due to the infrared radiation absorption by the dust grains while in atmospheric suspension.

These storms can lift enough dust into suspension to dramatically increase the opacity of the atmosphere for several months, as it was the case of the major global dust storm observed on 2018. Furthermore, recent findings suggest that such storms may have played an important role in Mars’ loss of water [3].

Global dust storms are the more complex and least understood events in what appertains to the dust cycle and perhaps even the whole climate. These events cover large fractions of the planet with optically thick dust (τ_visible>3) can even envelop the entire planet.

Observations and Method: Unlike Venus, Mars’ atmosphere is very transparent in the visible and ultraviolet ranges and the radiation in those wavelength ranges that is back-scattered in the atmosphere is negligible which precludes the application of our method. However, during global dust storms, the opacity of the atmosphere increases and allows for the scattering of enough light in the suspended dust in the middle atmosphere for the application of our method to be feasible.

The wind velocities were retrieved using the Doppler velocimetry technique developed and fine-tuned for the case of Venus [14, 15, 16, 17].

The adaptation of our Doppler velocimetry method for the case of Mars atmospheric studies, took in account the geometry of our observations. Spherical geometry was used to locate the observations within the planet, as seen from Earth at the time of each observation, and compute the de-projection factors for each point of the slit and for each exposure, in order to de-project the radial Doppler velocities from the observer’s (Earth) line-of-sight.

The rotation velocity’s contribution to the overall Doppler shift was removed by computing and subtracting the rotation velocity at each point on Mars sounded by the spectroscopic slit, this was done for all the positions surveyed on Mars. The wind velocities retrieved from the motion of the dust particles in atmospheric suspension, during the 2018 Global Dust Storm on Mars, were computed and we will present the output of this work.

Results and Discussion: The scope of this work is to study the behaviour of Mars’ middle atmosphere during a global dust storm using ground-based observations made with VLT-UVES and Doppler velocimetry techniques for the first time, to complement observations of orbiter instruments. The success and validation of the application of this method to the atmosphere of Mars may provide a new way to investigate the Martian atmosphere during dust storms.

The main goal of this research line is therefore, to provide direct wind measurements using high resolution spectroscopy and Doppler velocimetry, based on Fraunhofer lines scattered at Mars’ dust hazes, which allows spatial wind variability studies and will make possible to obtain a latitudinal profile of the wind along the cited global dust storm and a wind map of the dust storm as a function of the latitude and local time over the planet as seen from Earth.

Acknowledgements: We acknowledge support from the Portuguese Fundação Para a Ciência e a Tecnologia (ref. PD/BD/ 128019/2016 and ref. PTDC/FIS-AST/29942/2017) through national funds and by FEDER through COMPETE 2020 (ref. POCI-01-0145 FEDER-007672).

References:

[1] Vandaele, et al., Nature, 568(7753), 521-525.doi:10.1038/s41586-019- 1097-3, 2019

[2] Machado, P., et al., Icarus, 221, 248- 261.doi:10.1016/j.icarus.2012.07.012, 2012

[3] Machado, P., et al., Icarus, 243, 249-263. doi:10.1016/ j.icarus.2014.08.030, 2014

[4] Machado, et al. Icarus, 285, 8- 26.doi:10.1016/ j.icarus.2016.12.017, 2017

[5] Gonçalves R., Machado P., et al., Icarus, vol 335, article 113418, 2020