ABSTRACT
Future spacecraft missions to planetary systems, Trans-Neptunian objects, and cometary bodies could implement far-infrared surveys to confirm the presence of condensed-phase species via their unique lattice features. For composite molecular ices of astrophysical significance, laboratory reference spectra are required to provide absorption coefficients used to quantify solid-state abundances. However, due to strong intermolecular interactions in polar ice systems, laboratory data of mixed-phase ices are difficult to interpret. In this study we have applied periodic density functional theory code to model bulk molecular crystals. This method allows for more accurate simulation of thin-film spectra than approaches simulating small clusters. For this proof-of-principle study on a series of pure nitrile ices of planetary interest, our simulated far-infrared spectra show excellent agreement to data from thin film studies performed at the Australian Synchrotron (crystalline acetonitrile and propionitrile) and to previously published spectra (hydrogen cyanide, acrylonitrile, cyanoacetylene, and cyanogen). The combined theoretical and experimental approach has provided a new explanation for the asymmetric profile of the hydrogen cyanide lattice feature and a more systematic assignment of nitrile ice absorption bands to low-frequency lattice modes. We nominate prominent absorption features for the detection of crystalline nitrile carriers located on planetary surfaces.
ABSTRACT
Pure, crystalline acetonitrile (CH3CN) and propionitrile (CH3CH2CN) particles were formed in a collisional cooling cell allowing for infrared (IR) signatures to be compiled from 50 to 5000 cm-1. The cell temperature and pressure conditions were controlled to simulate Titan's lower atmosphere (80-130 K and 1-100 mbar), allowing for the comparison of laboratory data to the spectra obtained from the Cassini-Huygens mission. The far-IR features confirmed the morphology of CH3CN aerosols as the metastable ß-phase (monoclinic) ice, however, a specific crystalline phase for CH3CH2CN could not be verified. Mie theory and the literature complex refractive indices enabled of the experimental spectra to be modelled. The procedure yielded size distributions for CH3CN (55-140 nm) and CH3CH2CN (140-160 nm) particles. Effective kinetic profiles, tracing the evolution of aerosol band intensities, showed that condensation of CH3CH2CN proceeded at twice the rate of CH3CN aerosols. In addition, the rate of CH3CH2CN aerosol depletion via lateral diffusion of the particles from the interrogation volume was approximately 50% faster than that of CH3CN. The far-IR spectra recorded for both nitrile aerosols did not display absorption profiles that could be attributed to the unassigned 220 cm-1 feature, which has been observed to fluctuate seasonally in the spectra obtained from Titan's atmosphere.
