We present 5.5–7.6 µm spectra of comets 71P/Clark ( 2006 May 27.56 UT , r _ { h } = 1.57 AU pre-perihelion ) and C/2004 B1 ( LINEAR ) ( 2005 October 15.22 UT , r _ { h } = 2.21 AU pre-perihelion and 2006 May 16.22 UT , r _ { h } = 2.06 AU post-perihelion ) obtained with the Spitzer Space Telescope . The \nu _ { 2 } vibrational band of water is detected with a signal-to-noise ratio of 11 to 50 .
Fitting the spectra using a fluorescence model of water emission yields a water rotational temperature of < 18 K for 71P/Clark and \simeq 14 \pm 2 K ( pre-perihelion ) and 23 \pm 4 K ( post-perihelion ) for C/2004 B1 ( LINEAR ) .
The water ortho-to-para ratio in C/2004 B1 ( LINEAR ) is measured to be 2.31 \pm 0.18 , which corresponds to a spin temperature of 26 ^ { +3 } _ { -2 } K. Water production rates are derived .
The agreement between the water model and the measurements is good , as previously found for Spitzer spectra of C/2003 K4 ( LINEAR ) .
The Spitzer spectra of these three comets 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 ) .