Optical properties of zinc sulfate hydrate in terahertz frequency band

As a typical sulfate mineral on Martian surface, zinc sulfate hydrate experiences a hydration process due to variations in environment temperature. In this paper, the characteristics of zinc sulfate under different heating temperatures and durations have been investigated using terahertz time-domain spectroscopy. Based on the absorption peak at 0.89 THz, optical parameters- peak intensity, peak area, refractive index and dielectric constant-were employed to characterize the crystal water content change. Among those optical parameters, the peak intensity exhibits the highest correlation coefficient (R²) of 0.969 at 318 K. Meanwhile, the refractive index shows the highest correlation coefficient (R²) of 0.974 at 348 K. Furthermore, a correlation between the dehydration rate and the absorption peak area was established, revealing dehydration rates of 0.0139 at 318 K and 0.0094 at 348 K, respectively. Finally, the optical properties of zinc sulfate irradiated by protons at a fluence of 2 × 10¹⁰ protons/cm² were measured and a reduced refractive index is observed. Those results offer new insights for establishing a relationship of proton with water content, and to further understand the mechanism of water formation in the minerals of Martian surface.