We present Spitzer IRAC images, with representative 5.27 to 38.5 micron IRS
spectra, of the Cassiopeia A supernova remnant. Where each IRAC channel is
dominant over the others, it illuminates different regions related to the
nucleosynthetic layers of the progenitor star, echoing the inhomogeneities
seen in the X-ray and optical. The Channel 1 (3.19 to 3.94
microns) emission mechanism is synchrotron, but spectra towards Channel 1
bright patches show a broad featureless continuum peaking around 26
microns. We suggest that this is due to un-enriched circumstellar dust
from the progenitor behind the outer shock and heated by the photons andelectrons from the shock as well as potentially processed (shattered,sputtered) by the shock. Where Channel 4 (6.45 to 9.38 microns) isdominant compared to the other IRAC channels, the spectra show a
strong, 2-3 micron-wide peak at 21 microns, in addition to ionic lines of
[ArII], [ArIII], [SIV] and [NeII], probably indicating where the shock has
penetrated into the oxygen- and silicon-burning layers. Thelong-wavelength continuum emission where Channel 3 (5.02 to 6.44
microns) is dominant over the other channels rises gradually to 21
microns, with a plateau to longer wavelengths. Channel 2 (4.02 to 5.03
microns) depicts, in part, H recombination matching a Paschen beta emission
seen in the image obtained at Palomar, but spectra of strong Channel 2
knots show a variety of broadband shapes. Where Channel 2 is very strong
compared to Channel 4, Channel 2 isolates regions where [ArII] is weak
compared to [NeII] in the spectra. In particular, Channels 2 and 3 are
consistent with line and dust emission reflecting only carbon- and
neon-burning material. We suggest that all of these findings are
consistent with both the ionic and dust components delineating the distance
the reverse shockhas penetrated into the nucleosynthetic layers of the ejecta. Thepresence of Si and S in the remnant$apos;s interior, avoiding the Channel 4[ArII] regions, shows the distribution of the once-shocked (by the initial
forward shock) material that has yet to be re-shocked by the reverse shock
that will heat it to X-ray temperatures. The patchy distribution of
different elements at the reverse shock, in X-rays, optical, and
infrared, would then be due to hydrodynamic inhomogeneities deep in the
progenitor core, as opposed to major azimuthal variations in explosive
nucleosynthesis.