Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p13g..01d&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P13G-01
Other
5410 Composition (1060, 3672), 5422 Ices, 5464 Remote Sensing, 5470 Surface Materials And Properties
Scientific paper
The bulk of our knowledge regarding icy satellite surface composition is derived from visible to near-infrared (VNIR) reflectance spectroscopy, much of it from spacecraft observations. Spectra of planetary surfaces can be modeled either as linear (areal) mixtures, or as nonlinear (intimate) mixtures, to yield estimates of relative abundance of surface compounds. Linear mixture analysis of planetary surface composition requires access to reflectance spectra of the candidate compounds. Nonlinear mixture analysis requires the real and imaginary indices of refraction (optical constants), which may be estimated from reflectance spectra, or derived from a combination of reflectance and transmittance measurements. To date, most of the candidate species proposed as icy satellite surface constituents have not yet been sufficiently characterized to enable such models. Most infrared spectra of candidate icy satellite surface materials published to date were measured in the mid-infrared (MIR) for purposes of understanding the interstellar medium. In order to constrain abundances of surface materials from spectral observations of icy bodies, cryogenic laboratory measurements for all candidate materials will be required, having the following characteristics: First, they must be in either reflectance or optical constants. These are the quantities which enable quantitative abundance modeling. Transmittance, absorbance, absorption coefficient, line strength, or other quantities which cannot be converted to reflectance (primarily due to poorly constrained scattering processes) are of limited usefulness. Second, measurements are needed across the full spectral range of typical spacecraft instruments (298 to 5500 nm would cover the Galileo and Cassini cameras and spectrometers). While a compound may only have strong absorption features in part of the wavelength range, it still contributes to the continuum everywhere, including the vicinity of diagnostic features of other compounds. Deconvolving the observations requires laboratory measurements across the full range, for all proposed constituents, particularly where they may occur together. Third, measurements must be conducted with samples sufficiently thick to yield useful absorption features, shapes and strengths. The overtones and combinations which make up most of the VNIR spectral signatures are far weaker than the MIR fundamentals. However, reflected sunlight from cold icy bodies in the outer solar system exhibits insufficient spectral contrast and inadequate signal to enable the identification of surface materials in the MIR so remote-sensing instrumentation for icy bodies concentrates upon the VNIR, where there is more available signal. Yet, a thin film (<~10 microns) in the laboratory does not engender sufficient path length for the weak VNIR absorptions to manifest. This is not a problem for a planetary regolith several meters to kilometers thick, but does present a challenge for laboratory work. Fourth, measurements must be temperature-appropriate to the bodies of interest. Most of the candidate compounds (especially ices) display marked spectral changes with temperature. Differences of as little as 5-10 K can be distinguished in laboratory spectra of many materials. In order to explain planetary observations, laboratory measurements in the 50-150 K range will be critical. Scientific return from spacecraft- and ground-based observations of planetary surfaces will be significantly enhanced by the proper application of cryogenic laboratory spectroscopy. With these measurements in hand, investigators may identify materials, derive their abundances, map their distributions, and infer their roles in the evolution of these enigmatic bodies.
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