  In this era of spatially resolved observations of planet forming disks with
ALMA and large ground-based telescopes such as the VLT, Keck and Subaru, we
still lack statistically relevant information on the quantity and composition
of the material that is building the planets, such as the total disk gas mass,
the ice content of dust, and the state of water in planetesimals. SPICA is an
infrared space mission concept developed jointly by JAXA and ESA to address
these questions. The key unique capabilities of SPICA that enable this research
are (1) the wide spectral coverage 10-220 micron, (2) the high line detection
sensitivity of (1-2) 10-19 W m-2 with R~2000-5000 in the far-IR (SAFARI) and
10-20 W m-2 with R~29000 in the mid-IR (SMI, spectrally resolving line
profiles), (3) the high far-IR continuum sensitivity of 0.45 mJy (SAFARI), and
(4) the observing efficiency for point source surveys. This paper details how
mid- to far-IR infrared spectra will be unique in measuring the gas masses and
water/ice content of disks and how these quantities evolve during the planet
forming period. These observations will clarify the crucial transition when
disks exhaust their primordial gas and further planet formation requires
secondary gas produced from planetesimals. The high spectral resolution mid-IR
is also unique for determining the location of the snowline dividing the rocky
and icy mass reservoirs within the disk and how the divide evolves during the
build-up of planetary systems. Infrared spectroscopy (mid- to far-IR) of key
solid state bands is crucial for assessing whether extensive radial mixing,
which is part of our Solar System history, is a general process occurring in
most planetary systems and whether extrasolar planetesimals are similar to our
Solar System comets/asteroids. ... (abbreviated)
