  We used the Spitzer Space Telescope's Infrared Spectrograph to create a high
resolution spectral map of the central region of the Cassiopeia A supernova
remnant, allowing us to make a Doppler reconstruction of its 3D structure. The
ejecta responsible for this emission have not yet encountered the remnant's
reverse shock or the circumstellar medium, making it an ideal laboratory for
exploring the dynamics of the supernova explosion itself. We observe that the
O, Si, and S ejecta can form both sheet-like structures as well as filaments.
Si and O, which come from different nucleosynthetic layers of the star, are
observed to be coincident in velocity space in some regions, and separated by
500 km/s or more in others. Ejecta traveling toward us are, on average, ~900
km/s slower than the material traveling away from us. We compare our
observations to recent supernova explosion models and find that no single model
can simultaneously reproduce all the observed features. However, models of
different supernova explosions can collectively produce the observed geometries
and structures of the interior emission. We use the results from the models to
address the conditions during the supernova explosion, concentrating on
asymmetries in the shock structure. We also predict that the back surface of
Cassiopeia A will begin brightening in ~30 years, and the front surface in ~100
years.
