The SPECTRAL ice chamber: A NASA-GSFC experimental setup to study the formation processes, the chemistry and optical properties of planetary nitrile-containing organic ices

We present a new experimental equipment, the SPECtroscopy of Titan-Related ice AnaLogs (SPECTRAL) high-vacuum chamber and the experimental protocol, used to investigate on planetary organic ices of the Solar System, particularly ices containing cyanides or mixed with them. Located in the Spectroscopy for Planetary ICes Environments (SPICE) laboratory at NASA Goddard Space Flight Center (GSFC), the SPECTRAL chamber is aimed to characterize the physico-chemical and optical properties of pure and mixed organic ices at low temperatures (from 14 K to 200 K) and under high-vacuum (8×10^-8 mbar). We particularly follow how ices evolved optically and chemically with the temperature by using a Fourier Transform Infrared (FTIR) spectrometer and a quadrupole mass spectrometer (QMS) that are coupled to the chamber. UV irradiation of ices can be carried out as well. The FTIR measures quantitative thin-film infrared transmission spectra from the far- to near-IR spectral range (50 cm^-1 to 11700 cm^-1 / 200 - 0.85 μm). The QMS detects ion masses of molecules released during the sublimation process of the icy samples through thermal heating and is used as an analytical tool to confirm the ice composition. Double laser interferometry technique is used to determine the thickness of the ice films in order compute the complex indices of refraction, n and k.

The SPECTRAL chamber was built initially to determine the chemical compositions, structural and optical properties, and formation processes of the unidentified Titan's stratospheric ice clouds observed by Cassini's Composite InfraRed Spectrometer (CIRS) during the fourteen years of the Cassini mission to Saturn. In this context, we have studied several pure and mixed nitrile ices, as well as -CN ices combined with benzene and examine their spectral evolution with time and temperature. Laboratory data were then incorporated in radiative transfer modelling to match CIRS observations. We present a summary of all these results and show how they are determinant for understanding better the processes of planetary organic vapors condensation, ice particles aggregates formation as well as ice cloud formation, not only in Titan but as well on other planetary icy objects (e.g. Pluto, other Kuiper belt objects, and comets).