Thermophysical modelization of the ExoMars 2022 landing site

Abstract

We performed thermophysical simulations of Oxia Planum, the landing site of the ExoMars 2022 mission [5]. The numerical simulations concern: I) the influence of the thermal inertia on the subsurface temperature at the latitude of Oxia Planum; II) the heat released in the subsurface by the drill installed on the ExoMars rover. The numerical simulations are performed using a 3-D model using the discretization technique of the finite element method (FEM).

1. Introduction

Numerical simulations are required to characterize, from a thermophysical point of view, Oxia Planum, the landing site of the mission ExoMars 2022. A drilling system is installed on the ExoMars rover and it will be able to analyze up 2 meters in the subsurface of Mars. The spectrometer Ma_Miss (Mars Multispectral Imager for Subsurface) [1] will investigate the lateral wall of the borehole generated by the drill, providing hyperspectral images. Among the scientific objectives of Ma_Miss there are the   characterization and the mapping of possible volatiles. In this regard, numerical simulations are useful to understand if the temperatures in the subsurface are such as to preserve volatiles, especially after the heating provided during the drilling operations.

2. Numerical Method

We performed our simulations by using a 3-D finite element model [2, 3], which solves the classical heat equation in a parallelepipedal domain representing a portion of Oxia Planum, the landing site of the ExoMars mission. The top of this domain is modeled with a Gaussian random surface in order to simulate the roughness of the surface. The dimensions of the domain are 1cm x 1cm x 5cm. The depth (5 cm) has been chosen since it is compatible with the likely skin depth. At the top we imposed a radiation
boundary condition, while on the other sides zero heat flux is imposed. The initial temperature is set at 200 K, which is compatible with the surface equilibrium temperature. Self-heating between the facets of our domain is taken into account. We investigate: I) the dependence of the subsurface thermal response to different thermal inertia; II) the
heat released by the drill in the subsurface of Mars. In particular, for the point II, we assume: a) the drilling is instantaneous in a well-defined “drilling temporal window”; b) thrust and rotational velocity are constant. The contribution of the drill is taken into account by applying an heat flux (depending in particular on the thrust, angular velocity and frictional heating) on the wall of the rock matrix in contact with the drill.

3. Summary and conclusions

In Fig.1 we report an example of the results we obtained by applying our numerical model. Fig.1 shows the temperature profile vs time at different depths for two cases: case

Fig.1: Temperature profile vs time at different depths. Case (A): K = 0.045 W m⁻¹K⁻¹; Case (B) K = 0.0045 W m⁻¹K⁻¹.

(A) with a thermal conductivity K=0.045 W m^-1 K^-1, compatible with Insight estimation [4] and case (B) with a thermal conductivity an order of magnitude smaller than the case (A). Case (A) is characterized by a thermal inertia of 270 TIU (Thermal Inertia Units) and a skin depth of 3 cm, while case (B) is  characterized by a thermal inertia of 85 TIU and a skin depth less than 1 cm. Our numerical results suggest that: a) the surface temperature ranges from 180 K to 270 K if thermal inertia is high (300 TIU); b)  surface temperature ranges from 140 K to 280 K if thermal inertia is low (<100 TIU); c) the self heating can increase surface temperature of about 10 K. In Fig.2 we shows the increase of the temperature due to the drilling process. In the x-axis is plotted the distance from the hole in the direction perpendicular to the drilling direction. Four cases are explored: rpm (rotation per minute) = 30 and 60 and frictional coefficient (η) = 0.5 and 0.9. The frictional coefficient computes the efficiency of the heat transfer by the drill. We observe an increase of temperature up to 300 K from the initial temperature, in case of rpm = 60 and frictional coefficient = 0.9 Future simulations with different parameters will be carried out. Moreover, a model validation with laboratory experiments is required.

Fig.2: Left panel: rpm = 30 and η= 0.5; right panel: pm = 60 and η= 0.9.

Acknowledgements

The Italian Space Agency (ASI) has founded the experiment.

References

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