We propose a simple pyrometric algorithm to retrieve the temperature from a radiating body during heating or cooling with no prior knowledge of the spectral emissivity. We ratio the measured fluxes at two different temperatures in two narrow bands. The error budget of this method is calculated. We consider its application for the rover-based exploration of the Martian ground, which is subjected to daily and seasonal temperature variations. The method is optimized to increase accuracy in the retrieval of temperature variation, which is the most interesting parameter for ground heat-flux studies