Ultraviolet spectral reflectance of carbonaceous materials

1Daniel M. Applin, 1,2Matthew R.M. Izawa, 1Edward A. Cloutis, 3Jeffrey J. Gillis-Davis, 4Karly M. Pitman, 5Ted L. Roush, 6Amanda R. Hendrix, 3Paul G. Lucey
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.02.012]
1Dept. of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, Canada R3B 2E9
2Institute for Planetary Materials, Okayama University, 827 Yamada, Misasa, Tottori 682-0193 Japan
3 Hawaii Institute of Geophysics and Planetology, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822
4Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301 USA
5NASA Ames Research Center, Moffett Field, California, 94035-0001 USA
6Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ, USA 85719-2395
Copyright Elsevier

A number of planetary spacecraft missions carried instruments with sensors covering the ultraviolet (UV) wavelength range. However, there exists a general lack of relevant UV laboratory data to compare against these planetary surface remote sensing observations in order to make confident material identifications. To address this need, we have systematically analyzed reflectance spectra of carbonaceous materials in the 200-500 nm spectral range, and found spectral-compositional-structural relationships that suggest this wavelength region could distinguish between otherwise difficult-to-identify phases. In particular (and by analogy with the infrared spectral region), large changes over short wavelength intervals in the refractive indices associated with the trigonal sp2 π-π* transition of carbon can lead to Fresnel peaks and Christiansen-like features in reflectance. Previous studies extending to shorter wavelengths also show that anomalous dispersion caused by the σ-σ* transition associated with both the trigonal sp2 and tetrahedral sp3 sites causes these features below λ = 200 nm. The peak wavelength positions and shapes of π-π* and σ-σ* features contain information on sp3/sp2, structure, crystallinity, and powder grain size. A brief comparison with existing observational data indicates that the carbon fraction of the surface of Mercury is likely amorphous and submicroscopic, as is that on the surface of the martian satellites Phobos and Deimos, and possibly comet 67P/Churyumov-Gerasimenko, while further coordinated observations and laboratory experiments should refine these feature assignments and compositional hypotheses. The new laboratory diffuse reflectance data reported here provide an important new resource for interpreting UV measurements from planetary surfaces throughout the solar system, and confirm that the UV can be rich in important spectral information.

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