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Abstract

Two different silicate/water ice mixtures representing laboratory analogs of interstellar and circumstellar icy grains were produced in the laboratory. For the first time, optical constants, the real and imaginary parts of the complex refractive index, of such silicate/water ice mixtures were experimentally determined in the mid-infrared (IR) spectral region at low temperatures. In addition, optical constants of pure water ice and pure silicates were derived in the laboratory. Two sets of constants were compared, namely, “measured” constants calculated from the transmission spectra of silicate/ice samples, and “effective” constants calculated from the optical constants of pure silicates and pure water ice samples using different mixing rules (effective medium approaches). Differences between measured and effective constants show that a mixing (averaging) of the optical constants of water ice and silicates for the determination of the optical properties of silicate/ice mixtures can lead to incorrect results. Also, it is shown that a part of the water ice molecules is trapped in/on silicate grains and does not desorb up to 200 K. Our unique data are well-timed with respect to the new and challenging space mission, James Webb Space Telescope (JWST), which will be able to provide novel and detailed information on interstellar and circumstellar grains. Suitable laboratory data are extremely important for the decoding of astronomical spectra.