  Hubble Space Telescope images of the young Galactic supernova remnant
Cassiopeia A reveal a far larger population of outlying, high-velocity knots of
ejecta with a broader range of chemical properties than previously suspected.
We identify three main classes of outer ejecta: 1) Knots dominated by [N II]
6548,6583 emission; 2) Knots dominated by oxygen emission lines especially [O
II] 7319,7330; and 3) Knots with emission line strengths similar to the [S II]
strong FMK ejecta commonly seen in the main emission shell. The discovery of a
significant population of O-rich ejecta situated in between the suspected
N-rich outer photospheric layer and S-rich FMK-like ejecta suggests that the
Cas A progenitor's chemical layers were not completely disrupted by the
supernova explosion outside of the remnant's NE and SW high velocity `jet'
regions. In addition, we find the majority of O-rich outer ejecta at projected
locations out beyond (v = 6500 - 9000 km/s) the remnant's fastest moving
Fe-rich X-ray emission material (6000 km/s) seen in Chandra and XMM data along
the eastern limb. This suggests that penetration of Fe-rich material up through
the S and Si-rich mantle did not extend past the progenitor's N or O-rich outer
layers for at least this section of the remnant.
