火星表面土壤热导率的地面模拟测量方法

Laboratory measurement of simulated Martian soil thermal conductivity

火星水冰及其他挥发分的埋藏和演化规律与火星表面土壤的热物理性质息息相关.热导率作为重要的热物理性质,一直是人们研究火星表面水/冰循环的焦点.本研究搭建瞬态热线法实验装置,建立了类火星环境下模拟火星土壤的热导率测量方法,实现了类似火星不同气压环境和不同含冰量模拟火星土壤热导率的测量功能.与其他测试方法以及前人经验模型的对比,本研究提出的测量方法易于实现,具有较高的可行性和可重复性,测量精度可达3%,测量可重复性大于95%.可以为未来进行火星表面热物理性质就位探测提供实验技术参考,也可为解译火星水的埋藏和循环过程提供地面验证方法,更好实现我国"天问一号"火星探测任务的科学目标.

The distribution and evolution of water ice and other volatiles on the surface of Mars are closely related to its surface’s thermal physical properties. The thermal conductivity of Martian soils is the most apparent and flexible thermal physical property and has been a consistent focus of Martian water/ice cycle and surface temperature research. The detection of soil properties and water ice is a major scientific objective of Tianwen-1, China’s first Mars mission. Because in-situ Mars exploration missions are rare, studies on the thermal physical properties of Martian soils remain largely limited to laboratory simulation experiments and the interpretation of remote sensing data. However, the latter approach is highly sensitive to the chosen model, which is based on theoretical predictions and experimental data. Although numerous experimental studies have sought to model the factors that affect the thermal conductivity of soils under Mars-like conditions, few have studied the influence of ice content. In this study, we measured the thermal conductivity of simulated Martian soils with variable water ice contents under Mars-like environmental conditions. The simulated Martian soil JMSS-1 was used as a base sample with physical properties and chemical components similar to those of Martian soils. The samples were pre-dried and carefully mixed with variable mass fractions of ultrapure water. Two groups of samples were used:(1) Dried samples and(2) samples mixed with 2% water. A vacuum freeze-dryer was adapted to apply low temperatures and pressures to resemble Martian surface conditions. Samples were then placed into the freeze-dryer chamber and cooled to as low as 40°C. The two sample groups were measured under ambient air pressure varying from 100 to 900 Pa. Prior to each measurement, the samples were maintained under the same pressure and temperature conditions to ensure thermal equilibrium, thus avoiding any temperature drift effects. The transient hot-wire method was used to measure the thermal conductivity. Constantan wire was chosen as the heating wire owing to its low temperature resistance coefficient. The temperature was monitored using an ultrathin T-type thermocouple placed adjacent to the heating wire in the middle of the sample using the minimum thermocouple size to reduce unwanted heat loss. A high-accuracy current source was used to supply the designated power, and a 24-bit analog-to-digital converter module was used to interpret the thermocouple data. All of the data were sent to a computer via serial communication. Measurements under each condition were repeated multiple times and the mean values were used for data analysis. The accuracy and stability of the experimental data output were confirmed by comparing with values acquired using the transient plane source method. The experiment accuracy was ~2%–3% compared with the results obtained from the same samples tested using a commercial transient plane source thermal analyzer(HotDisk TPS 2500 S). The obtained thermal conductivities of the dry soil samples under vacuum conditions agree with the fitted model of previous studies. The repeatability of this measurement approach is higher than 95%. The results show that for each sample group, the thermal conductivity increases with increasing ambient air pressure. The thermal conductivity data below 900 Pa are nearly four times higher than values obtained from the same samples below 100 Pa. The introduction of water ice into the samples also significantly altered the sample thermal properties. The thermal conductivities of samples with 2% water ice are 2–3 times higher than those of the dry soil samples measured under the same pressure conditions. This work provides an important foundation for interpreting China’s Tianwen-1 Mars probe to model and predict the distribution of ice on the Martian subsurface. The experimental setup is also useful for future Mars in-situ ice detection and resource utilization.