Infrared observations of the dusty , massive Homunculus Nebula around the luminous blue variable \eta Carinae are crucial to characterize the mass-loss history and help constrain the mechanisms leading to the Great Eruption .
We present the 2.4 - 670 \mu m spectral energy distribution , constructed from legacy ISO observations and new spectroscopy obtained with the Herschel Space Observatory .
Using radiative transfer modeling , we find that the two best-fit dust models yield compositions which are consistent with CNO-processed material , with iron , pyroxene and other metal-rich silicates , corundum , and magnesium-iron sulfide in common .
Spherical corundum grains are supported by the good match to a narrow 20.2 \mu m feature .
Our preferred model contains nitrides AlN and Si _ { 3 } N _ { 4 } in low abundances .
Dust masses range from 0.25 to 0.44 M _ { \odot } but M _ { tot } \geq 45 M _ { \odot } in both cases due to an expected high Fe gas-to-dust ratio .
The bulk of dust is within a 5 ^ { \prime \prime } \times 7 ^ { \prime \prime } central region .
An additional compact feature is detected at 390 \mu m. We obtain L _ { IR } = 2.96 \times 10 ^ { 6 } L _ { \odot } , a 25 % decline from an average of mid-IR photometric levels observed in 1971-1977 .
This indicates a reduction in circumstellar extinction in conjunction with an increase in visual brightness , allowing 25-40 % of optical and UV radiation to escape from the central source .
We also present an analysis of ^ { 12 } CO and ^ { 13 } CO J = 5 - 4 through 9 - 8 lines , showing that the abundances are consistent with expectations for CNO-processed material .
The [ ^ { 12 } C ii ] line is detected in absorption , which we suspect originates in foreground material at very low excitation temperatures .