We present Spitzer observations of the unusual variable V838 Monocerotis. Extended emission is detected around the object at 24, 70 and 160um. The extended infrared emission is strongly correlated spatially with the HST optical light echo images taken at a similar epoch. We attribute this diffuse nebulosity to be from an infrared light echo caused by reprocessed thermal emission from dust heated by the outward-propagating radiation from the 2002 eruption. The detection of an IR light echo provides an opportunity to estimate the mass in dust of the echo material and hence constrain its origin. We estimate the dust mass of the light echo to be on the order of a solar mass - thereby implying the total gas plus dust mass to be considerably more - too massive for the echo material to be the ejecta from previous outburst/mass-losing events. This is therefore suggestive that a significant fraction of the matter seen through the light echo is interstellar in origin. Unresolved emission at 24 and 70um is also seen at the position of the central star possibly indicating the presence of hot dust freshly condensed in the outburst ejecta.
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