The infrared (3.2-3.6 m) spectrum of comet P/Swift-Tuttle: detection of methanol and other organics

We present an IR spectrum of Comet P/Swift–Tuttle which exhibits broad emission features at 3.4 and 3.52 µm with fluxes of $$(5.7 \pm 0.25) \times {10}^{-15} \enspace \text and\enspace (0.8 \pm 0.14) \times {10}^{-15}\enspace \text {Wm}^{-2}$$ respectively, but has no pronounced feature near 3.28 µm. We show that the 3.52-micron feature and much of the 3.4-micron emission can be attributed to fluorescence from methanol, but that some other source is required to explain residual emission centred on 3.43 µm. The derived methanol production rate is $$(1.6 \pm 0.3) \times 10^{28}$$ molecules s^–1, which implies a ratio of methanol to water of approximately 0.05, a value more typical of dynamically new comets than of periodic comets.