Water in comet C/2003 K4 (LINEAR) with Spitzer

We present sensitive 5.5-7.6 μm spectra of comet C/2003 K4 (LINEAR) obtained on 2004 July 16 (r_h = 1.760 AU, Δ_Spitzer = 1.409 AU, phase angle = 35.4°) with the Spitzer Space Telescope. The v ₂ vibrational band of water is detected with a high signal-to-noise ratio (≳50). Model fitting to the best spectrum yields a water ortho-to-para ratio of 2.47±0.27, which corresponds to a spin temperature of 28.5_-3.5^+6.5 K. Spectra acquired at different offset positions show that the rotational temperature decreases with increasing distance from the nucleus, which is consistent with evolution from thermal to fluorescence equilibrium. The inferred water production rate is (2.43±0.25) × 10²⁹ molecules s^-1. The spectra do not show any evidence for emission from PAHs and carbonate minerals, in contrast to results reported for comets 9P/Tempel 1 and C/1995 O1 (Hale-Bopp). However, residual emission is observed near 7.3 μm, the origin of which remains unidentified.