  We present initial results from a JWST survey of the youngest Galactic
core-collapse supernova remnant Cassiopeia A (Cas A), made up of NIRCam and
MIRI imaging mosaics that map emission from the main shell, interior, and
surrounding circumstellar/interstellar material (CSM/ISM). We also present four
exploratory positions of MIRI/MRS IFU spectroscopy that sample ejecta, CSM, and
associated dust from representative shocked and unshocked regions. Surprising
discoveries include: 1) a web-like network of unshocked ejecta filaments
resolved to 0.01 pc scales exhibiting an overall morphology consistent with
turbulent mixing of cool, low-entropy matter from the progenitor's oxygen layer
with hot, neutrino and radioactively heated high-entropy matter, 2) a thick
sheet of dust-dominated emission from shocked CSM seen in projection toward the
remnant's interior pockmarked with small (approximately one arcsecond) round
holes formed by knots of high-velocity ejecta that have pierced through the CSM
and driven expanding tangential shocks, 3) dozens of light echoes with angular
sizes between 0.1 arcsecond to 1 arcminute reflecting previously unseen
fine-scale structure in the ISM. NIRCam observations place new upper limits on
infrared emission from the neutron star in Cas A's center and tightly constrain
scenarios involving a possible fallback disk. These JWST survey data and
initial findings help address unresolved questions about massive star
explosions that have broad implications for the formation and evolution of
stellar populations, the metal and dust enrichment of galaxies, and the origin
of compact remnant objects.

High Energy Astrophysical Phenomena