(Left panel) SPIRE 350 μm image of the Horsehead PDR. The white circle shows the FWHM SPIRE beam (25.2″). Black circles show the locations at which 557 GHz o-H₂O spectra were obtained, with the circle size corresponding to the FWHM HIFI beam (38.1″). (Right panel) Spectra of the 557 GHz o-H₂O and CO (2–1) lines across the PDR (black and blue curves, respectively), labeled by the right ascension offsets with respect to the reference position. The CO 2–1 spectra have been scaled down by a factor of 200. The bottom-right panel shows spectra of the ground state o– and p–H₂O lines at the central position.