Laboratory Infrared Spectra (2.3-23 μm) of SO 2 Phases: Applications to Io Surface Analysis

Abstract

Mid-infrared spectra (∼2-25 μm) of various SO 2 phases (some known to be present on Jupiter's satellite Io) contain diagnostic band features that can be used with observational data of Io to learn about its surface and atmospheric composition. A key portion of this spectral range (2-5 μm) will be accessible to the Near Infrared Mapping Spectrometer (NIMS) on the Galileo spacecraft now headed for Jupiter. Our lab work provides reflectance spectra of SO 2 phases for: (1) comparison with IR spectra of Io, (2) identification of compositional species on Io, and (3) mapping the spatial distribution of SO 2 phases on Io. Spectra at 4 cm -1 resolution were measured for a variety of phase states of SO 2 including SO 2 gas, SO 2 frost, SO 2 slab ice, SO 2 liquid, and adsorbed SO 2. Detailed lab spectra and band identifications including some new identifications are given for several varieties of each of these phases with particular emphasis on the 2- to 6-μm regions. We also produced a variety of surface textures for condensed SO 2, ranging from frosts with many peculiar (reticulated) textures to solid slab or glaze ice and liquid. The different frost textures result from different growth conditions (such as temperature, pressure, gas-supply rate, and substrate composition). All the optically thick frosts and ices that we produced have reflectance spectra that are essentially identical, regardless of texture. Previously reported IR spectra of solid SO 2 in other investigations were usually produced as transmission spectra of films of ice, not reflectance spectra of frost or ice surfaces. Our work shows that transmission spectra of thin films are not adequate to represent all features that may be present in reflectance spectra of macroscopic frost or ice on planetary bodies like Io; this is because thin-film samples provide optical pathlength insufficient to produce detectable absorption bands at wavelengths where bands are strong in reflectance spectra of optically thick deposits (or in transmission spectra of thick (few millimeters) monocrystals). Reflection spectra, as opposed to transmission spectra, also allow easier simulation of frost thickness effects and better reproduction of hand shapes that will actually be observed in planetary spectral observations. Our results indicate that one can estimate phase state and concentration level for condensed SO 2 on Io using the principal band ( v1 + v3) in the 4-μm region; for example, we found that the v1 + v3 band-minimum position is dependent on: (a) phase state of the SO 2, (b) thickness of frost or ice, and (c) instrument resolution. SO 2 ice or frost thickness (on a millimeter scale or less) can also be assessed by the 4-μm band shape characteristics and by the presence or absence of several other weaker bands; we have done this for In spectra and find that significant fractions of In must be covered by thick (>2 mm) frost. We have successfully used our lab spectra to identify and assign new hands in Io observational data reported previously by other researchers. We developed NIMS-equivalent spectra of various SO 2 phases showing what NIMS should "see" on Io at various spectral sampling strategies (resolutions): our lab data will provide a basis for NIMS to distinguish and map the distribution of various SO 2 phases on Io's surface and also identify such phases on other Galilean satellites if detectable quantities exist there.