P13G-01
Mission-Critical Laboratory Spectroscopy for Modeling Surface Abundances on Outer Solar System Satellites
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.
P13G-02
Laboratory Studies of Organic Compounds With Reflectance Spectroscopy
In order to properly interpret reflectance spectra of any solar system surface from the earth to the Oort cloud, laboratory spectra of candidate materials for comparative analysis are needed. Although the common cosmochemical species (H2O, CO2, CO, NH3, and CH4) are well represented in the spectroscopic literature, comparatively little reflectance work has been done on organics from room to cryogenic temperatures at visible to near infrared wavelengths. Reflectance spectra not only enhance weak or unseen transmission features, they are also more analogous to spectra obtained by spacecraft that are imaging such bodies as giant planet moons, kuiper belt objects, centaurs, comets and asteroids, as well as remote sensing of the earth. The USGS Spectroscopy Laboratory is measuring reflectance spectra of organic compounds from room to cryogenic temperatures over the spectral range of 0.35 to 15.5 microns. This region encompasses the fundamental absorptions and many overtones and combinations of C, H, O, and N molecular bonds. Because most organic compounds belong to families whose members have similar structure and composition, individual species identification within a narrow wavelength range may be ambiguous. By measuring spectral reflectance of the pure laboratory samples from the visible through the near and mid-infrared, absorption bands unique to each can be observed, cataloged, and compared to planetary reflectance data. We present here spectra of organic compounds belonging to five families: the alkanes, alkenes, alkynes, aromatics, and cyanides. Common to all of these are the deep C-H stretch fundamental absorptions, which shift shortward from 3.35+ microns in alkanes to 3.25+ microns in aromatics, to 3.2+ microns in alkenes, and down to 3.0+ microns in alkynes. Mid-IR absorptions due to C-H bending deformations at 6.8+ and 7.2+ microns are also identified. In the near infrared these stretching and bending fundamentals yield a diagnostic set of combination absorptions at approximately 2.3 microns, as well as the first C-H stretching overtones at 1.6 to 1.7 microns, and even the second stretching overtones at 1.2+ microns. Additionally, the spectral properties of these organic materials have applications to remote sensing of terrestrial environments, including hazardous waste and disaster site characterization. http://speclab.cr.usgs.gov/
P13G-03
Surface Binding Energies Of N2, CH4, And H2O-ice Systems
We will present measurements of binding energies of the following systems: CH4-CH4, CH4- H2O, N2-N2, and N2-H2O determined from measurements made in a cryo-vacuum system following the methods of Sandford and Allamandola (1, 2). Briefly, for the N2-N2 and CH4-CH4 measurements, we monitored the area of relevant infrared features with time at three different temperatures (<50 K) and use the results to estimate sublimation rates. For the mixtures H2O/CH4 = 20 and H2O/N2= 20, we carried out a series of experiments in which we deposited the mixtures at increasingly higher temperatures (starting at 15 K) until the infrared features of the more volatile component are no longer seen (usually at > 50 K). We then use the deposition rate of H2O to estimate the residence time of the more volatile species (N2 or CH4). In previous measurements (3), binding energies of several volatile molecules (CO, CO2, H2) to H2O-ice were found to be considerably higher than the binding energies of these molecules to themselves. For example, the binding energy of CO on CO is ΔHs/k = 960 K, while that of CO on H2O is ΔHs/k = 1740 K. These results have implications for the condensation, residence time, and sublimation of volatile species on icy surfaces inside and outside of the Solar System. 1. S. A. Sandford, L. Allamandola, Icarus 87 (1990). 2. S. A. Sandford, L. Allamandola, Icarus 76, 201 (1998). 3. S. A. Sandford, L. J. Allamandola, Astrophysical Journal 417, 815 (1993).
P13G-04
Laboratory Investigations into the Physisorption of Volatiles in Nonice Materials Relevant to the Outer Solar System
We propose that physisorption may be responsible for the prsence of CO2, and potentially the other volatiles, detected in the surfaces of the icy Galilean and Saturnian satellites. Physisorption is caused by weak inter- molecular van der Waals forces between the adsorbate molecules and areas of asymmetric charge distribution within the adsorbing host material. The van der Waals force can exist between CO2 and an adsorbant when a charge asymmetry in the adsorbent induces a weak dipole within the otherwise symmetric CO2 molecule. This partial electronic polarization of the CO2 is thus a function of the structure of the host molecule and the charge distribution within the host molecule. Complex silicates and potentially other materials offer excellent structures for the adsorption of CO2 by this mechanism because of their significant asymmetric distribution of charges within a unit cell as well as by their large microporosity. However, at room temperature, the strength of the van der Waals bond is insufficient to keep the CO2 adsorbed when the host material is exposed to vacuum. We have found that CO2 can remain physisorb onto some clays when the CO2 partial pressure is effectively zero under ultra-high vacuum (UHV) if the adsorbant is cooled to the surface temperatures of the icy satellites of Jupiter and Saturn. CO2 remains adsorbed onto the clay mineral montmorillonite for 10s of minutes when exposed to a vacuum of approx. 1E-8 Torr at approx. 125K. However, CO2 does not adsorb onto serpentine, goethite, or palagonite under these conditions. A small amount may adsorb onto kaolinite. These are materials materials that may have similar microporosities, but whose unit cells possess much less charge asymmetry than smectites. When heated above 150K under vacuum, the CO2 desorbs from the montmorillonite within a few minutes. We infer that the presence of a strong charge asymmetry which can induce a strong dipole in the CO2 molecule is the most important requirement for physisorption at cryogenic temperatures.