Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p32c..03c&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P32C-03
Other
[5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6008] Planetary Sciences: Comets And Small Bodies / Composition, [6055] Planetary Sciences: Comets And Small Bodies / Surfaces, [6281] Planetary Sciences: Solar System Objects / Titan
Scientific paper
Determining the surface composition of Titan has been inhibited by the lack of spectral properties of potential compounds. We have measured the 0.35 to 5-micron spectral reflectance of a wide range of compounds that might be relevant to Titan and trends are now coming to light with possible spectral matches for classes of materials. While some compounds have been identified and mapped on Titan's surface, such as liquid ethane + methane lakes and benzene, the compounds responsible for the main spectral properties have remained elusive (Clark et al, JGR 2010). Titan's surface is seen in the near infrared in only a few spectral windows, near 0.94, 1.1, 1.3, 1.6, 2.0, 2.68-2.78, and 4.9-5.1 microns in the Cassini Visual and Infrared Mapping Spectrometer (VIMS) spectral range. At shorter wavelengths, UV absorption in the spectra of Titan's haze constrains the surface composition because haze particles settle onto Titan's surface. The average apparent reflectance in the IR windows generally decreases with increasing wavelength except for the 2.7 and 5-micron windows which are at similar levels. The decrease has led researchers to infer a number of compounds responsible for the observed decreasing spectral shape; the most common being water ice. But ice is incompatible with the 2.78/2.68 micron I/F ratio. Many organic compounds have absorptions that are not seen in spectra of Titan, eliminating them as possible major components at the surface, including many polycyclic aromatic hydrocarbons (PAH) previously thought to be compatible with parts of Titan's spectrum. We find that ring compounds similar to benzene rings, but with some C-H bonds replaced by NH have a closer match to Titan's overall spectrum and can explain the relative intensities observed in the spectral windows, including the 2.68 and 2.78-micron double window, the low 3-5 micron reflectance, and increased absorption near 2.1-microns. Key among these compounds that show general properties that match Titan are Cytosine (C4H5N3O), Uracil (C4H4N2O2), Guanine (C5H5N5O), and Adenine (C5H5N5). These compounds are the four nucleobases in the nucleic acid of RNA. If these compounds can be confirmed to be on Titan, their formation pathways may have implications for the formation of life. Other compounds that match features in Titan's spectra include the polycyclic aromatic hydrocarbon (PAH) coronene, consisting of 6 benzene rings. Coronene is also a naturally occurring mineral on Earth, known as karpatite. Combinations of coronene, phenanthene (C14H12), pentacene (C22H14), indole (C8H7N), Cytosine, Uracil, Guanine, and Adenine match the overall spectral structure of Titan spectra. Indole, Cytosine, and Uracil, have 1.5-micron bands that can explain the feature observed in DISR spectra of Titan's surface. These compounds can also help explain the pyrolysis results from the Huygens probe.
Baines Kevin Hays
Barnes Jason W.
Brown Harvey R.
Buratti Bonnie Jean
Clark Roger Nelson
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