Abstract
Titan’s stratospheric ice clouds are by far the most complex of any observed in the solar system, with over a dozen organic vapors condensing out to form a suite of pure and co-condensed ices, typically observed at high winter polar latitudes. Once these stratospheric ices are formed, they will diffuse throughout Titan’s lower atmosphere and most will eventually precipitate to the surface, where they are expected to contribute to Titan’s regolith.Early and important contributions were first made by the InfraRed Interferometer Spectrometer (IRIS) on Voyager 1, followed by notable contributions from IRIS’ successor, the Cassini Composite InfraRed Spectrometer (CIRS), and to a lesser extent, from Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem (ISS) instruments. All three remote sensing instruments made new ice cloud discoveries, combined with monitoring the seasonal behaviors and time evolution throughout Cassini’s 13-year mission tenure.A significant advance by CIRS was the realization that co-condensing chemical compounds can account for many of the CIRS-observed stratospheric ice cloud spectral features, especially for some that were previously puzzling, even though some of the observed spectral features are still not well understood. Relevant laboratory transmission spectroscopy efforts began just after the Voyager encounters, and have accelerated in the last few years due to new experimental efforts aimed at simulating co-condensed ices in Titan’s stratosphere. This review details the current state of knowledge regarding the organic ice clouds in Titan’s stratosphere, with perspectives from both observational and experimental standpoints.
Highlights
A significant advance by Composite InfraRed Spectrometer (CIRS) was the realization that co-condensing chemical compounds can account for many of the CIRS-observed stratospheric ice cloud spectral features, especially for some that were previously puzzling, even though some of the observed spectral features are still not well understood
These investigators analyzed a Visible and Infrared Mapping Spectrometer (VIMS) image cube acquired in June 2012 and identified the spectral feature at 3.21 μm to be that of hydrogen cyanide (HCN) ice, with a derived ice cloud altitude of 300 ± 70 km, somewhat similar to the Imaging Science Subsystem (ISS)-determined altitude location from May 2012 (West et al 2016)
The extension of the CIRS far-IR spectral range down to 10 cm−1 (1000 μm; InfraRed Interferometer Spectrometer (IRIS) cut-off at 200 cm−1 [50 μm]) opened up a previously unexplored far-IR spectral region containing a wealth of nitrile ice signatures, many of which are expected to comprise the chemical compositions of Titan’s stratospheric ice clouds
Summary
It comes as no surprise that ices thrive on the surfaces of many airless bodies in the outer solar system, e.g., comets, gas and ice giant planet satellites, Kuiper Belt objects, etc. Methane vapor plays a non-negligible role in radiatively heating Titan’s stratosphere From their formation altitudes, Titan’s aerosol particles eventually drift downward into the stratosphere, providing heterogeneous nucleation sites for vapor condensation. The specific vapor compound and altitude location estimate of condensation depends on the vapor abundances and the temperatures at which saturation for each vapor occurs during its downward journey—cooling as it descends—through Titan’s stratosphere Once formed, these ice cloud particles will precipitate through the mid and lower regions of Titan’s stratosphere, continually adding layers as they pass through altitude regions where the individual organic vapors successively become saturated. Some consideration of solid-state photochemistry as a stratospheric ice cloud formation mechanism will be discussed
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