The winds of Mars


Although Mars is by far the best-known planet of the Solar System, there is a basic feature of its atmosphere dynamics about which, in contrast, there is a surprising lack of observational data: the wind. This means a constraint for the appropriate advance in the Martian environment characterization in several aspects, so its study is a preferential goal to be reached for the missions to come.

Credits: NASA/JPL-Caltech/University of Arizona

Up to 19 spacecrafts have successfully reached the planet out of the 43 ones which have been sent to Mars since 1960, and several orbiters and two rovers (Opportunity and Curiosity) are still operating and providing a constant flux of information. As a result, Martian geography, geology, history, climate and atmosphere are known in some detail nowadays, making Mars a quite familiar world, not only for scientists but even for the general public, it could be said.
Just to mention an example, throughout the five years it has been exploring the basin of Gale crater and the foothills of its central peak, the Mount Sharp, Curiosity rover has contributed remarkable discoveries. Evidence of past bulks of liquid water on the surface, native organic compounds and fixed nitrogen in the ground, or the confirmation of transient increases of the methane concentration in the atmosphere are among the most noteworthy findings of the mission, which in addition include the definition of a water exchange cycle between the soil and the atmosphere.
Regarding this latter, the atmosphere, it is quite well-characterized in terms of temperature, pressure, and aerosol and water content. The last spacecraft arrived to Mars so far, the Trace Gas Orbiter (TGO, from the ExoMars 2016 mission), will greatly improve the knowledge of the atmosphere composition by detecting the presence of rare gases in it when it starts its operations once its definitive orbit is reached, which will take place at the end of 2017.
However, all this amassed knowledge can hardly be put together in a general depiction of the atmospheric dynamics without counting on accurate data of the wind within the boundary layer, a core factor which has been poorly measured for different kinds of reasons during all these exploration efforts. Air movement is the fundamental determining mechanism for the exchange of energy, momentum, gases, and aerosols with the ground, as well as a main factor in the course of large scale phenomena such as dust storms or water cycle. Besides, wind is currently the dominant geological force shaping the Martian surface, so its geological implications are of the greatest importance to understand the planet’s evolution.
Furthermore, and with regard to the next foreseen missions reaching the surface (ESA/Roscosmos’s ExoMars 2020 and NASA’s 2020 rover), the possibility of forecasting reliably the wind activity at a given point and date would be an invaluable help when designing Entry, Descent and Landing (EDL) systems, and establishing their optimal configuration to minimize the huge risks that this delicate series of manoeuvres entails. Perhaps (only perhaps; this is a mere speculation) the failure in the landing of ExoMars’ Schiaparelli Entry, Descent and Landing Demonstrator Module (EDM), whose root cause was the unexpected angular rate acquire by the module after the parachute deployment, could have been prevented if a reliable model to simulate the wind activity had been available.
Currently, the principal sources of wind estimations are the General Circulation Models used to simulate the Martian atmospheric dynamics. These models rely on coarse observation by orbiters, which in turns should be refined with missing observational data of wind from the boundary layer.
Therefore, it is imperative to achieve extensive and accurate winds measurements for the better comprehension of the Martian environment and for the proper development of further exploration. However, “the winds on Mars are almost completely unknown” (N.J. Livesey and W. G. Read, 2016). The wind data available are scarce, and have been gathered from distant points with very low temporal resolution. It must be pointed out that the Martian atmosphere is very thin, and the typical winds are so weak that it is very difficult to get accurate measurements, or even to retrieve them from the electronic noise caused by the harsh environmental conditions on the planet. Different methods and instruments have been developed so far, but the most innovative were mounted in failed missions (Beagle 2, of ESA's Mars Express Mission) or were broken down at some point of the mission (Curiosity rover’s wind sensor).

Credits: NASA/JPL

The Beagle 2 lander was the first mission equipped with a wind sensor intended to obtain a long-term dataset of wind speed and direction at high frequency to characterize the air flux in the boundary layer, but it crashed on the surface. Curiosity rover mounts a wind sensor as part of its Rover Environmental Monitoring Station (REMS) instrument, which was designed to acquire continuous measurements of wind speed and direction along every Martian sol (day and night). It consists of two sets of bi-dimensional sensors, one of which was damaged during the touchdown of the rover, so the data provided is being limited and not as accurate as expected, despite the new calibration for the working set of sensors which was implemented to try and palliate the problem..
Apart from the extension in the lack of high quality measurements of wind these failures have supposed, they have thwarted the ultimate testing of the respective instruments in real conditions so, at this moment, there is no device with a contrasted readiness level to rely on for the recording of accurate wind values.
The aforementioned missions to be launched in 2020 (ExoMars and 2020 rover), will include new instruments with dedicated sensor to measure the wind. But in the case of the ExoMars 2020 Surface Platform (the Russian module of the mission together with the ESA’s rover), something more will be done. Members of the Atmospheric Science Group, responsible for the development of the instrument HABIT (as well a part of the Surface Platform scientific payload), is designing a method conceived to support the validity of the wind records eventually taken on Mars by the proper wind sensor on board the platform. It consists in the retrieval of air temperature measurements (HABIT mounts three air temperature sensors -ATS- on its Environmental Package -ENVPACK- unit), to calculate air movement around the platform, and then to infer the wind activity by means of the analysis of the thermal profile and heat transfer along the rods composing the sensor.

Credits: Álvaro Soria-Salinas

This novel method has been “learnt” throughout the exploitation of environmental data provided by the ATS of REMS instrument, which is basically the same to be used on HABIT, so it has a contrasted operational heritage and robustness.
At the very least, this analytical tool will reinforce the reliability of the wind measurements to be gathered by the Surface Platform’s METEO unit, and the heat transfer within the boundary layer studies it will allow to carry out, will contribute relevant information to reach one of the main scientific goals of the ExoMars 2020 mission, namely, the assessment of the habitability potential of present day Mars.
Moreover, by applying the method to the retrieval of REMS’ data, it will permit to refine their wind measurements, which undoubtedly will mean an advance in the knowledge of the wind regime, and will constitute a reference for the adjustment of data taken by any other instrument provided with air temperature sensors on board future missions from here on out.