We present sensitive 5.5–7.6 µm spectra of comet C/2003 K4 ( LINEAR ) obtained on 16 July 2004 ( r _ { h } = 1.760 AU , \Delta _ { Spitzer } = 1.409 AU , phase angle 35.4° ) with the Spitzer Space Telescope .
The \nu _ { 2 } vibrational band of water is detected with a high signal-to-noise ratio ( \stackrel { \raisebox { -0.86 pt } { $ \textstyle > $ } } { \sim } 50 ) .
Model fitting to the best spectrum yields a water ortho-to-para ratio of 2.47 \pm 0.27 , which corresponds to a spin temperature of 28.5 ^ { +6.5 } _ { -3.5 } 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 \pm 0.25 ) \times 10 ^ { 29 } molec .
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 .