Development of Thermal Sensors for Planetary Surface Layers

Surface layers composed of so-called regolith are common on many airless bodies in the solar system. They have remarkable thermal properties, as they belong to the best thermally insulating materials known, and thus they act like a thermo-blanket covering the surface. Very little is known today about extension and depth variation of these grainy surface layers. Only for the moon there exists a limited amount of data on thermo-physical properties, stemming from the early lunar lander missions and from the Apollo missions. The same can be said concerning data on the global lunar heat flux, which has been measured at two Apollo missions landing sites by drilling boreholes into the lunar surface and measuring the variation of the temperature profile with depth. No in situ data exist to date from the surface of other airless bodies, like asteroids, planetary satellites, or Mercury. In view of the renewed interest in the exploration of the moon, reflected in various lunar missions planned for the near future, it is worthwhile to consider the thermo-physical properties of regolith layers in more detail. The applied project aims to contribute in three areas: 1. Contributing in the development of thermal conductivity sensors for materials with very low conductivity, aiming for application in space missions and industrial processes, as well as the creation of calibration standards for powders in a vacuum environment. 2. Performing experiments with lunar/asteroid soil simulants and determine their thermo-physical properties in dependence of grain size and texture in a thermal vacuum environment. 3. Performing theoretical/numerical modelling in order to understand and interpret the experimental results properly. The project will basically be performed with equipment available at IWF Graz from previous FWF projects. It will benefit from the experience in thermal measurements gathered during the development phase of the "MUPUS" experiment flown aboard the ESA comet mission Rosetta. The company HUKSEFLUX as partner for the production of the necessary hardware has been selected because they have the best experience in producing similar sensors for terrestrial applications. In view of anticipated future applications we have also established a link to the Harbin Institute of Technology (HIT), which is working on the development of lunar rovers for the Chinese lunar program.

The central topic of this TRP project was to develop suitable sensors and evaluation methods for determining the thermal properties of the materials constituting the near surface layers of extraterrestrial planetary bodies, like Moon, Mars, asteroids and come nuclei. These parameters are of high importance if one wants to understand the thermal evolution of these bodies. Hereby the key material property to be measured is the so-called thermal conductivity, which depends not only on the chemical composition of the material in question, but also to a high extent on structural parameters like bulk porosity, grain size distribution and the shape of individual grains. Another important influential factor is the environmental gas pressure, which is low on many of these bodies as compared to terrestrial conditions. A standard method to determine the thermal conductivity of such materials is the so-called heated needle method, where a slender metal rod is heated by a constant power. However, commercially available measurement probes suffer from various weaknesses, which make them unsuitable for use on planetary lander missions. Therefore, in the frame of this 3 years project, we have designed several new sensor types, which were built by the Dutch company Hukseflux as our contractor and tested on various materials of interest for extraterrestrial applications in our cryo-vacuum laboratory at the Space Research Institute, Graz. The new sensors built and tested within this project are robust in the sense that they are mechanically strong and thus can be more easily used in cohesive materials, which are to be expected for example on the surface of the Moon. They would survive mechanical actions like hammering or drilling into the surface soil and thus can be suggested as a potential payload for planetary missions. The sensors and the evaluation methods for the thermal conductivity developed in the frame of this project are of interest for various space missions already under way or in the proposal phase. In particular we mention ESA`s comet mission Rosetta and the physical properties instrument MUPUS on its Lander Philae. Moreover, there is some interest to use such sensors as a payload on the forthcoming Chinese lunar lander missions in the frame of the Chang`e program as well as on the lunar lander proposals currently under study at NASA and ESA. Finally it should be noted that the sensors developed and tested within this project have also the potential to be used for field experiments in harsh environments on the Earth, where the thermal properties of the underground are if interest for engineering projects. Some useful results in this interdisciplinary field was achieved from our cooperation with the State Key Laboratory of Frozen Soil Engineering in Lanzhou, China, which conducts infrasructure engineering projects in high altitude areas.