Infrared spectral observations of comet 103 P / Hartley 2 by ISOPHOT ?

Spectral observations in the 2.5–12 μm range of comet 103P/Hartley 2 were performed on 1 Jan. 1998 by the ISOPHOT instrument, on board the Infrared Space Observatory. The obtained spectrum evidences intense bands at 2.66 and 4.26 μm, attributable to H2O and CO2, respectively. The derived molecular production rates are Q(H2O) = (3.1 ± 0.2) × 1028 molec. s−1 and Q(CO2) = (2.5 ± 0.3) × 1027 molec. s−1. This leads to a CO2/H2O abundance ratio of ∼ 8%, showing that P/Hartley 2 is a relatively CO2 abundant comet, with respect to 1P/Halley and several other comets. A feature has been detected at 3.69 μm, whose attribution is rather puzzling. The long wavelength part of the spectrum is well fitted by a black– body profile at 285 K colour temperature. No significant silicate emission feature around 10 μm is observed, within observation errors.