  HD 181327 is a young F5/F6V star belonging to the Beta Pictoris moving group
(12 Myr). It harbors an optically thin belt of circumstellar material at 90 AU.
We aim to study the dust properties in the belt in details, and to constrain
the gas-to-dust ratio. We obtained far-IR observations with the Herschel/PACS
instrument, and 3.2 mm observations with the ATCA array. The geometry of the
belt is constrained with newly reduced HST/NICMOS images that break the
degeneracy between the disk geometry and the dust properties. We use the
radiative transfer code GRaTer to compute a large grid of models, and we
identify the grain models that best reproduce the Spectral Energy Distribution
through a Bayesian analysis. We attempt to detect the [OI] and [CII] lines with
PACS spectroscopy, providing observables to our photochemical code ProDiMo. The
HST observations confirm that the dust is confined in a narrow belt. The
continuum is detected in the far-IR with PACS and the disk is resolved with
both PACS and ATCA. A medium integration of the gas spectral lines only
provides upper limits on the line fluxes. We show that the HD 181327 dust disk
consists of micron-sized grains of porous amorphous silicates and carbonaceous
material surrounded by an important layer of ice, for a total dust mass of 0.05
M\oplus (up to 1 mm). We discuss evidences that the grains are fluffy
aggregates. The upper limits on the gas lines do not provide unambiguous
constraints: only if the PAH abundance is high, the gas mass must be lower than
17 M\oplus. Despite the weak constraints on the gas disk, the age of HD 181327
and the properties of the dust disk suggest that it has passed the stage of
gaseous planets formation. The dust reveals a population of icy planetesimals,
similar to the primitive Kuiper Belt, that may be a source for the future
delivery of water and volatiles onto forming terrestrial planets.
