Adapting a Cryogen-Free Measurement System for Geological Materials: Method Development for Thermal Conductivity Measurements Below 300 K with Applications to Planetary Science

Abstract

The thermal properties of airless planetary bodies like asteroids are essential to understanding their thermal evolution. While remote sensing missions collect thermal data, this is limited to surface properties. As such, little is known about internal heat flow. Thermal models are used to predict thermal evolution, but the limited availability of thermal property data constrains their accuracy. To improve these models, meteorites provide a way to investigate thermal properties under controlled laboratory conditions, as they are preserved fragments of planetary bodies. This study reports thermal conductivity measurements of meteorites between 5 and 300 K under vacuum (< 10^(−4) mbar), acquired using a Cryogen-Free Measurement System (CFMS) that was adapted for geological applications. Measurements were first collected for single-crystal minerals and obsidian to refine sample preparation and measurement procedures. This included developing techniques for handling friable samples, establishing thermal equilibration protocols, and implementing a data quality assessment method based on empirical observations of instrument performance. Above 100 K, radiative heat loss contributes to the measured thermal conductivity, resulting in values that exceed the true conductive behaviour. A correction procedure was applied using low-temperature model fits specific to each material. The single-crystal minerals exhibit anisotropic thermal conductivity and show trends consistent with phonon-dominated transport, where thermal conductivity is expected to increase as T^3 at low temperatures, peak, and then decrease as 1/T at higher temperatures; however, this full behaviour was not always observed within the measured range. In contrast, obsidian shows a plateau followed by a gradual increase in thermal conductivity with temperature, consistent with the behaviour of amorphous solids. Meteorites exhibit more complex behaviour. Despite being composed mainly of crystalline materials, phonon transport is suppressed by porosity and grain boundaries, resulting in thermal conductivity trends similar to disordered materials. These factors also drive anisotropy due to structural heterogeneity. The results of this study agree well with literature values, validating the use of the CFMS for geological analysis. The data collected in this study will support improved modelling of heat flow in small planetary bodies.