LOW TEMPERATURE VISIBLE AND INFRARED REFLECTANCE SPECTRA OF SOLID AND LIQUID METHANE WITH APPLICATIONS TO THE OUTER SOLAR SYSTEM ICY BODIES

Reflectance spectroscopy is used to determine the composition of the surfaces of outer solar system bodies by comparing observed spectra to experimental laboratory data of proposed materials in the visible through mid-IR. The acquisition of observational reflectance spectra has surpassed laboratory studies at outer solar system temperatures. To fully understand the composition and phase of materials composing the icy outer solar system surfaces, it is essential to have low temperature reflectance spectra of candidate materials to compare to observed reflectance data. Methane is an abundant molecule, existing in several forms in the outer solar system, which has yet to be studied with low temperature reflectance. We present low temperature (50-100K) methane reflectance for both solid and liquid phases in the visible through mid-IR, 300 to 3300 nm.

Diamond anvil cells were loaded with methane gas at about 180-200 MPa at the Geophysical Laboratory at the Carnegie Institution of Washington. The initial state of methane in the diamond cell at room temperature was gas. The loaded diamond anvil cell was analyzed at U2A beamline, National Synchrotron Light Source at Brookhaven National Laboratory where synchrotron light was used as an infrared source. Liquid helium was used to obtain the low temperatures needed. Reflectance spectra were obtained at about 5-10K intervals between 100 and 50 K.

We compare our methane reflectance spectra to the observed spectra of Pluto and Triton. Pluto, and its close cousin Triton, formed approximately 40 AU from the sun at temperatures less than 50K. We find good agreement between the three main absorption bands in the 1.6-1.8 μm range, and the three main bands in the 2.2-2.4 μm range, characteristic of methane. Our laboratory spectra show little temperature dependence of the absorption band peak widths or positions in the 100-50 K range. Ground based telescopes have been used to determine the surface composition of both of these icy bodies; however the New Horizons mission to Pluto will obtain a more detailed spectral map of Pluto's surface.