Towards Regolith Subsurface Temperatures from Rover Wheel Tracks

Towards regolith subsurface temperatures from rover wheel tracks. S.G.Els1,4, A. Alblooshi 2, J. Hurrell 3, C. Perfetti1, Huygen W.1, V. Tommen1, C. Iorio1 ; 1Centre for Research and Engineering in Space Technologies (CREST), Université libre de Bruxelles, Bruxelles, Belgium ([email protected]), 2UAE University, Al Ain, UAE, 3Tohoku University Graduate School of Engineering School of Engineering, Sendai, Japan, 4 Mohammed Bin Rashid Space Centre, P.O. Box 211833, Dubai, UAE Introduction: The uppermost few centimeters of the lunar surface show a very large temperature gradient of up to 100 K over the first tens of millimters being the result of very low heat conductivity of the regolith. In principle, such gradients allow the presence of waterice near the surface even at non-polar latitudes. On the other hand, such large temperature gradients can be challenging for equipment being deployed on the surface but whose parts reach into the ground. Hence, measuring this gradient in-situ at various locations is of interest to enhance models of the thermal (sub)surface behavior and to identify locations which are of specific interest for further geological investigations. Wheel based vehicles on the Moon, especially those employing grousers to enhance traction, leave behind in their wheel tracks a mix of material from different depths. The resulting surface grain population inside the wheel tracks thus can be expected to show initial temperatures representing an average from grains from their original depths. In this poster paper we present some initial results from numerical simulations of such surface mixing from a traversing small rover and point out an experimental setup to test this approach in a lab facility. Track temperatures: To estimate the difference of temperatures within a wheel track and the undisturbed surrounding surface a two stage modelling approach was taken. Depth mixing from DEM models: First, the grain population within a wheel track was computed from the simulations of the wheel-surface interactions were used as described in [1]. These Discrete Element Model (DEM) simulations used a 10cm diameter and 8cm wide wheel, equipped with 14, 20mm long grousers, rolling over a surface consisting of grains with 2mm diameter. Of the resulting track, only an inner part of the 80mm wide wheel imprint was extracted. This was done in order to avoid eventual edge effects as the interest is to see the maximum mixing effect which can be expected to take place in the center of the tracks. As the depth of each grain is computed by the DEM before, during and after passing of the wheel, it is possible to generate a 3D map of the original depths of the grains at their final XYZ position after wheel passage. The above Figure shows the vertical displacement between before (ZB) and after (ZA) the wheel passed over the grains. Track temperatures: The vertical temperature profile of the undisturbed lunar surface was extracted from simulations of a 1.2x0.6x1.0m3 slab of regolith at a latitude of 60O and Sun elevation of 30O. This 20 element temperature profile samples in increments of 5mm to a depth of 100mm. Using the results from the DEM simulations of the inner track area, every grain was assigned the temperature corresponding to its original depth (i.e., before traversing of the wheel). Hence, no energy exchange is assumed between the grains during the reshuffeling of material. The resulting vertical temperature distribution is shown in the figure below. The colors show the temperatures in increments of 20OC, i.e., blue temperatures below -80OC, light-blue -80…-60OC, etc., until dark red indicating temperatures of >+40OC. As can be seen hot material is pushed down to depths of 20mm and cold material is moved up as well. In particular in the sinks of the tracks a temperature change will occur. Potential for in-situ observing: The science instrumentation on-board the original Rashid-1 lunar rover [2], and its successor Rashid-2 [3] which carries the same set of instruments, include a thermal imaging camera. This camera is mounted at the rear of the rover, hence, looking backwards and seeing the already traversed terrain, including the wheel tracks, from approximately 50cm on-wards behind the rover. This would allow to image the wheel tracks with low spatial resolution of approximately 0.5 mm/px and an expected temperature precision of 1 K, pending the final calibration of the camera. From such observations, and comparing to the modelling which will be based on the methodology presented here will allow to derive the vertical temperature distribution of the uppermost regolith layers. Also, repeated imaging will provide the heating rate of the tracks which thus allows to infer the regolith’s thermal properties. Experimental verification: A facility to test and experiment with small planetary rover systems is developed at CREST-ULB. This facility consists of a sandbox in which a small rover can traverse over sandy terrain which is heated by means of irradiation. The rover design which is presently employed resembles the dimensional and mobility properties of the Rashid-1/2 rover, in particular replicating the grouser and wheel design. At this point this facility is developing the methodology to introduce vertical temperature gradients similar to the ones expected on the Moon. Using a thermal imager with similar spatial resolution as the ones used on Rashid-1/2 is being used to measure the temperatures and its temporal evolution. References: [1] Hurrell J. et al. (2025) SSRv, in press, [2] Els S.G., et al (2021) European Geosciences Union General Assembly, id.EGU2112950, [4] AlMatroushi H. et al. (2025) this conference