Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p23e0112s&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P23E-0112
Other
5422 Ices, 5460 Physical Properties Of Materials, 5464 Remote Sensing, 5470 Surface Materials And Properties
Scientific paper
Water ice is the dominant surface component on many surfaces in the solar system. Understanding the interaction of light with ice and solutes in ice is of prime importance to understanding many planetary objects include Mars, where the polar caps ice and solutes at depth; Titan, which appears to have organics in abundance and may have water ice; and comets, where the ice appears to be mixed with primordial organics. Light and other radiation impinging on these surfaces serves both as a probe of the surface composition (as measured with remote sensing and in situ instruments) and as a source of energy that modifies the ice matrix, solutes, and inclusions. Models do not yet reliably calculate the detailed radiation field within icy surfaces due to complex scattering and absorption issues. Scattering properties within an icy mixture impose a spectral shape different from that found from (for instance) transmission measurements on thin films. The spectral shape also depends with viewing geometry. Present scattering models use parameters that are difficult to tie to physical properties of the mixture, such as particle shape and absorption path length. As a consequence, it is not yet possible to accurately determine the composition of icy mixtures from remote sensing measurements, nor to set reliable detection limits for solute molecules that may reside within icy surfaces. The size and shape of crystals within the ice matrix can change over time at a rate that is strongly controlled by temperature. Size and shape affect the depth of penetration of radiation and the absorption path length within the ice. The ice matrix itself can be modified by radiation which affects the transport of energy within the ice, the shape of ice spectra, and the amount of reactive species available for modifying solutes. Amorphous ice crystals formed by deposition from the vapor phase on borosilicate substrates have particle sizes, as measured directly by environmental scanning electron microscopy measurements and by surface area (BET) measurements that are related to formation temperature. These sizes are constant at time scales accessible in the laboratory. The particle sizes do increase when the samples are annealed. The laboratory observations that amorphous ice is deposited at temperatures below 140K and that that the mean crystal diameter is sub-micron to microns at the temperatures found on outer planet surfaces appear to be at variance with mean particle size, packing density, and crystal form derived from remote sensing observations of the outer satellites. The disparity in observations could arise from the very different time scales accessible in the laboratory relative to those relevant to the surfaces of the moons of the outer planets. This work was performed at JPL under contract to NASA.
Bodsgard B. R.
Boxe C. S.
Leu M.
Smythe William D.
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