  We present mid-infrared (5-40 micron) spectra of shocked ejecta in the
Galactic oxygen-rich supernova remnant G292.0+1.8, acquired with the IRS
spectrograph on board the Spitzer Space Telescope. The observations targeted
two positions within the brightest oxygen-rich feature in G292.0+1.8. Emission
lines of [Ne II] 12.8, [Ne III] 15.5, 36.0, [Ne V] 24.3 and [O IV] 25.9 are
detected from the shocked ejecta. No discernible mid-IR emission from heavier
species such as Mg, Si, S, Ar or Fe is detected in G292.0+1.8. We also detect a
broad emission bump between 15 and 28 microns in spectra of the radiatively
shocked O-rich ejecta in G292.0+1.8. We suggest that this feature arises from
either shock-heated Mg2SiO4 (forsterite) dust in the radiatively shocked O-rich
ejecta, or collisional excitation of PAHs in the blast wave of the SNR. If the
former interpretation is correct, this would be the first mid-IR detection of
ejecta dust in G292.0+1.8. A featureless dust continuum is also detected from
non-radiative shocks in the circumstellar medium around G292.0+1.8. The mid-IR
continuum from these structures is well described by a two-component dust
model. The temperature of the hot dust component (mass ~ 0.002 Solar masses) is
~ 115 K, while that of the cold component (> = 3 Solar masses) is ~ 35 K. We
attribute the hot component to collisionally heated dust in the circumstellar
shocks in G292.0+1.8, and attribute the cold component to dust heated by the
hard FUV radiation from the circumstellar shocks. Our models yield mid-IR line
strengths consistent with M(O)/M(Ne) ~ 3, M(O)/M(Si) >~ 61 and M(O)/M(S) ~ 50.
These ratios are difficult to reproduce with standard nucleosynthesis models of
well-mixed SN ejecta (abridged).
