Laboratory study of the IR spectra of methane ices in the polar environment

We grew the pure methane ice and mixed binary ices of methane with polar molecules: water, carbon dioxide, methanol and ammonia under ultrahigh vacuum cryogenic conditions similar to those in cold prestellar clouds. The composition of ices was chosen to reflect current astrochemical notions. The following interstellar ice analogues were obtained: H$_2$O : CH$_4$ = 10 : 1, CO$_2$ : CH$_4$ = 5 : 1, CH$_3$OH : CH$_4$ = 2 : 1, NH$_3$ : CH$_4$ = 1 : 1, that are new to the literature and infrared spectra ice analogues databases. We report the significance of deposition temperature for the internal structure of ices, which gets revealed on infrared spectra during the warm up for ices deposited at 6.7 K and 10 K. We link the observed differences to solid methane being in amorphous phase ($T < 9.3$ K) and low-temperature crystalline (9.3 K $ < T < 20$ K) phases. We also note that the direct comparison between laboratory and observational spectra is possible since the deformation mode line shape doesn't depend on the grain size or shape near 7.7 $\mu$m, only on the temperature and molecular environment. Solid methane is a fairly abundant molecule in prestellar objects. With the James Webb Space Telescope (JWST) launch the quality of spectra has risen significantly and we hope that this work will be an asset to assigning the methane component in star-forming regions.