We present an analysis of the 4–2600 μ m spectral energy distributions (SEDs) of the west and east nuclei and the diffuse infrared (IR) region of the merger-driven starburst Arp?220. We examine several possible source morphologies and dust temperature distributions using a mixture of silicate and carbonaceous grains. From fits to the SEDs we derive dust masses, temperatures, luminosities, and dust-inferred gas masses. We show that the west and east nuclei are powered by central sources deeply enshrouded behind cm~(?2) column densities of hydrogen with an exponential density distribution, and that the west and east nuclei are optically thick out to wavelengths of ～1900 and ～770?μ m, respectively. The nature of the central sources cannot be determined from our analysis. We derive star formation rates (SFRs) or black hole masses needed to power the IR emission, and show that the [C ii] 158 μ m line cannot be used as a tracer of the SFR in heavily obscured systems. Gas masses inferred from dust are larger than those inferred from CO observations, suggesting either larger dust-to-H mass ratios or the presence of hidden atomic?H that cannot be inferred from CO observations. The luminosities per unit mass in the nuclei are ～450 in solar units, smaller than the Eddington limit of ～1000–3000 for an optically thick star-forming region, suggesting that the observed gas outflows are primarily driven by stellar winds and supernova shock waves instead of radiation pressure on the dust.
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