Throughout the descent and at the surface, the downward-looking (DLIS) and upward-looking (ULIS) infrared spectrometers from the DISR/Huygens instrument recorded spectra of the atmospheric radiation and of the surface at the landing site. These spectrometers cover the range 850--1700 nm with a resolution of 15--20 nm. ULIS measurements of the downward flux in the stratosphere are consistent with the 1.6 % methane mole fraction inferred by Cassini/CIRS and Huygens/GCMS. ULIS spectra recorded in the lowest km of the atmosphere strongly vary in intensity depending whether the Sun is within the instrument field of view or not. The contrast in the methane windows, increasing from 4 at 940 nm up to 18 at 1600 nm, is strongly sensitive to the total aerosol optical depth. Using an aggregate haze particle model derived from visible spectra and solar aureole data from DISR, we derived optical depths of about 2 at 940 nm decreasing to 0.5 at 1600 nm. The set of ULIS spectra recorded during the descent provides constraints on the vertical profile of the aerosols in the range 150-40 km, using the residual intensity in the core of the methane bands. A constant-with-height particule concentration provides a good fit of the spectra whereas a cutoff in the lower stratosphere is at odds with the data. A spectrum at an altitude of 20 m with the DISR 20-W lamp turned on indicates a methane mole fraction of 5±1 %, corresponding to a relative humidity of 50 % near the surface. The reflectivity of the dark terrain at the landing site decreases from about 0.18 near 830 nm to 0.06 near 1500 nm and is relatively low and flat from 1500 to 1600 nm. This spectrum is consistent with water ice mixed with an unidentified dark material showing a featureless``blue slope'' in the infrared.