Temperature dependence of complex permittivity of calcium sulfate dihydrate investigation by terahertz time-domain spectroscopy.

As a common sulfate mineral on Martian surface, calcium sulfate hydrate experiences wide temperature variations. However, the permittivity properties of calcium sulfate hydrate as a function of temperature remains underexplored. In this study, this gap has been addressed by systematically investigating the complex permittivity of calcium sulfate dihydrate (CaSO·2HO) in THz frequency band using terahertz time-domain spectroscopy over a temperature range from 100 K to 320 K. Base on the effective medium theory of Landau-Lifshitz-Looyenga (LLL), the permittivity has been extracted from the matrix and compared with that of the calcium sulfate (CaSO). It is found that as the temperature increases from 100 K to 320 K, the real part and the imaginary part of the permittivity for CaSO·2HO increases from 5.3 to 5.8, and 0.25 to 0.32 at 1.0 THz, respectively. For CaSO, the corresponding values change from 5.2 to 5.3 and 0.28 to 0.4 respectively. The difference in the permittivity properties is mainly attributed to the temperature-dependent changes in crystal-water molecular polarizability, as well as its frequency-dependent response. Finally, to investigate the effect of solar-wind on these properties, calcium sulfate dihydrate irradiated by proton with a fluence of 2 × 10 protons/cm has also been measured and discussed. Following proton irradiation, at 220 K the real part of the permittivity increases approximately 0.3, while the imaginary part of the permittivity decreases about 0.2. These findings provide valuable insights into the temperature-sensitive permittivity behavior of hydrated minerals as well as to quantitatively identify minerals on the Mars.