Water frost has been identified on the surfaces of the satellites of Jupiter1, Saturn2 and Uranus3,4; methane frost is dominant on Pluto5-7. However, although the IR spectrum of Triton, the largest satellite of Neptune, is similar to that of methane or methane frost8,this identification is not consistent with all the available data9,10. We have now used the Multiple Mirror Telescope (MMT) to obtain an improved IR spectrum of Triton. Our observations show features that are not in detailed agreement with the identification of methane, although the general spectral behaviour that led to this identification is confirmed. A satisfactory identification of the surface and/or atmospheric composition on Triton does not yet seem possible.
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