Research by Max P. Bernstein, Dale P. Cruikshank and Scott A. Sandford, NASA Ames Research Center, MS 245-6, Moffett Field, CA

Triton: a moon of ice.

Infrared (IR) spectra have demonstrated that solid methane (CH4) is present on a number of outer solar system objects including Triton, where methane is thought to be frozen into nitrogen (N2) ice. CH4 is known to be present in a number of comets.

Since water (H2O) is nearly ubiquitous in the outer solar system, CH4 on icy planetesimals is likely to come into contact with H2O, potentially changing its spectral properties. Since mathematical addition of spectra of pure materials is not equivalent to the spectra of actual mixtures, fitting CH4 profiles in spectra of outer solar system bodies will require lab spectra of solid CH4 mixed at a molecular level with H2O at differing relevant temperatures.

Max Bernstein, Dale Cruikshank and Scott Sandford measured near-infrared (IR) spectra of water-methane (H2O-CH4) ice mixtures at various concentrations and temperatures from 15 to 150 Kelvin (K) and documented how peaks shift and broaden, both as a result of interactions between the molecules of H2O and CH4 and as a result of changes in temperature.

Since an interaction with H2O on a molecular level has been shown to cause significant changes in the position and profile of CH4 absorptions in the mid-IR, the presence of H2O could change peaks in the near-IR as well. This would complicate the interpretation of reflection spectra of outer solar system bodies.

The scientists presented the first near-IR spectra of H2O/CH4 solid mixtures, finding that the absorptions of CH4 in H2O are broader and shifted to higher frequency compared to those of pure solid CH4 at the same temperature. Further, the absorptions of CH4 in H2O become broader, shift to higher frequency, and display changes in relative and absolute intensity with increasing temperature from 15 to 150 K. These changes were reversible on re-cooling, despite the fact the phase changes of the H2O (into which the CH4 is frozen) are irreversible.

Implications

The experiments were conducted in order to get a more firm grasp on the state of solid methane (CH4) on the surface of outer solar system bodies, such as Quaoar. The scientists hoped that these observations of changes in the positions, profiles, and relative intensities of CH4 absorptions with concentration and temperature will be of use in understanding spectra of other icy outer solar system bodies, both existing in and around the Kuiper Belt and beyond.

An artist's concept of Kuiper Belt Object 2002 LM60, also known as Quaoar. Image credit NASA/STScI.

Significance to Solar System Exploration

Ices of many kinds are essential and fascinating components of bodies in the outer solar system including comets, the moons of the planets, Pluto, and thousands of objects in the Kuiper Belt and beyond. The study of the properties of ices in the lab gives scientists the tools needed to identify them on other worlds and to determine their compositions, structures, and other physical properties. This in turn gives us new insight into the chemical processes by which our solar system may have formed.

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Last Updated: 23 February 2011