Terahertz imaging resolves ice porosity at the millimetre scale

Abstract The porosity and small-scale structure of ice and snow influence their physical properties and provide information about the processes that form and modify icy surfaces. However, existing in-situ methods for measuring ice porosity are generally either destructive or unable to resolve spatial variations at the millimetre scale. Here we show that terahertz time-domain spectroscopy (THz-TDS) enables non-destructive, spatially resolved measurements of ice porosity. We measure the complex refractive index of porous ice samples and retrieve porosity using effective-medium theory, producing two-dimensional maps of thickness-averaged apparent porosity with sub-millimetre spatial resolution and centimetre-scale penetration. Against gravimetric porosity measurements, a Bruggeman effective-medium inversion retrieves porosity with a root-mean-square error of 5.7 % and a systematic underestimation of 4.8 %. Measurements under both atmospheric and vacuum conditions demonstrate the applicability of the approach across different environmental conditions. These results establish THz-TDS as a non-destructive method for resolving small-scale variations in ice porosity, with potential applications in terrestrial cryospheric field measurements and the in-situ exploration of icy planetary surfaces.