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Two different silicate/water ice mixtures representing laboratory analoguesof interstellar and circumstellar icy grains were produced in the laboratory.For the first time, optical constants, the real and imaginary parts of thecomplex refractive index, of such silicate/water ice mixtures wereexperimentally determined in the mid-infrared spectral region at lowtemperatures. In addition, optical constants of pure water ice and puresilicates were derived in the laboratory. Two sets of constants were compared,namely, measured constants calculated from the transmission spectra ofsilicate/water ice samples and effective constants calculated from the opticalconstants of pure silicates and pure water ice samples using different mixingrules (effective medium approaches). Differences between measured and effectiveconstants show that a mixing (averaging) of optical constants of water ice andsilicates for the determination of the optical properties of silicate/icemixtures can lead to incorrect results. Also, it is shown that a part of waterice molecules is trapped in/on silicate grains and does not desorb up to 200 K.Our unique data are just in time with respect to the new challenging spacemission, James Webb Space Telescope, which will be able to bring novel detailedinformation on interstellar and circumstellar grains, and suitable laboratorydata are extremely important for the decoding of astronomical spectra.

Low temperature optical properties of interstellar and circumstellar icy silicate grain analogues in the mid-infrared spectral region