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We present far-infrared spectroscopic observations, taken with thePhotodetector Array Camera and Spectrometer (PACS) on the Herschel SpaceObservatory, of the protoplanetary disk around the pre-main-sequence star HD100546. These observations are the first within the DIGIT Herschel key program,which aims to follow the evolution of dust, ice, and gas from young stellarobjects still embedded in their parental molecular cloud core, through thefinal pre-main-sequence phases when the circumstellar disks are dissipated.  Our aim is to improve the constraints on temperature and chemical compositionof the crystalline olivines in the disk of HD 100546 and to give an inventoryof the gas lines present in its far-infrared spectrum. The 69 \mu\m feature isanalyzed in terms of position and shape to derive the dust temperature andcomposition. Furthermore, we detected 32 emission lines from five gaseousspecies and measured their line fluxes. The 69 \mu\m emission comes either fromdust grains with ~70 K at radii larger than 50 AU, as suggested by blackbodyfitting, or it arises from ~200 K dust at ~13 AU, close to the midplane, assupported by radiative transfer models. We also conclude that the forsteritecrystals have few defects and contain at most a few percent iron by mass.Forbidden line emission from [CII] at 157 \mu\m and [OI] at 63 and 145 \mu\m,most likely due to photodissociation by stellar photons, is detected.Furthermore, five H2O and several OH lines are detected. We also found high-Jrotational transition lines of CO, with rotational temperatures of ~300 K forthe transitions up to J=22-21 and T~800 K for higher transitions.

First results of the Herschel Key Program 'Dust, Ice and Gas in Time': Dust and Gas Spectroscopy of HD 100546