Experimental study of frost detectability on planetary surfaces using multicolor photometry and polarimetry

When the temperature and pressure conditions allow it, water ice can deposit as frost on the regolith of planetary surfaces. Frost is an important indicator of the surface physical conditions, and may trigger geological processes by its deposition and sublimation. This works aims to explore, experimentally, the possibility of detecting early stages of frost formation and to characterize its spectrophotometric and spectropolarimetric signatures in visible reflected light. We deposit ice on top of different regolith simulants, measuring the dust temperature, the thickness, and the morphology of the frost through a microscope, while measuring the reflected light at phase angles of 50 degrees and 61 degrees, and the linear polarization at phase angles of 5 degrees and 16 degrees, at three different wavelengths (450, 550, and 750 nm). We show that both the spectral slope (in particular between 450-550 nm), and the difference of polarization between 450 and 750 nm are efficient methods to detect frost layers with thicknesses as low as 10 to 20 mu m. Furthermore, we find that the linear polarization at 16 degrees relates to the temperature of the regolith i.e. the type of the deposited ice crystalline structure.