Convective Heat Transfer Measurements at the Martian Surface

We present a new sensing method to characterize the convective wind activity on the surface of Mars. This method is based on the use of three rods that monitor the air temperature, and can provide information about the wind speed, direction and temporal oscillations. The sensor monitors the heat transfer coefficient h of the sensing element, on a Mars surface spacecraft. This sensing method will be implemented in the HABIT (HabitAbility: Brines, Irradiance and Temperature) instrument which has been included as scientific payload for the future ExoMars 2018 (ESA-Roscosmos) Surface Platform. This Surface Platform is the responsibility of Roscosmos and the Space Research Institute of Russian Academy of Sciences (IKI), and includes two European contributions. One of them is HABIT, whose goal is to investigate the habitability of the present day Mars. The convective heat method that is here presented is based on the use of three Air Temperature Sensors (ATS) placed at three different positions with different orientations. Each ATS consist of a rod with three Resistance Temperature Thermodetectors (RTD) which measure the temperature of this fin at 3 different locations along the axial direction. The thermal profile of this fin depends on the convective heat transfer with the Martian atmosphere and on the exposure to the local wind. It also depends on the temperature and density of the fluid. Under certain approximations the thermal profile of each fin can be analytically solved. By measuring simultaneously the three temperatures of the rod, the method retrieves the fluid temperatures Tf and an average adimensional parameter. This method has already been used on the Rover Environmental Monitoring Station (REMS) that has been operating now for three years on board the Mars Science Laboratory, on the surface of Mars. In this work we show in which way the m parameter is related to the heat transfer coefficient h. Given that h is dependent on the local forced convection, it can be used to characterize the wind on the surface of Mars. In this preliminary study a computational fluid dynamics code is used to illustrate the method. We obtain numerical results of the convective heat transfer h and m parameter for different wind speeds. The results are compared with the predictions based on theoretical calculations. The different values of each ATS when exposed to the same wind flow can be used to retrieve information about the orientation of the wind.