The temporal development of the pre-perihelion infrared spectral energy distribution of comet Hyakutake (C/1996 B2)

We present 1.2-18.5 mu m infrared (IR) photometric observations of comet Hy akutake (C/1996 B2) that define the temporal development of the spectral en ergy distribution (SED) of the comet during its passage toward the Sun. The characteristics of the SED on/and prior to 1996 April 10.1 UT when the com et was at a heliocentric distance r greater than or equal to 0.69 AU were c onsistent with the interpretation that the optically important grains domin ating the IR emission from the coma were larger than 1 mu m. The sudden app earance of a strong 10 mu m silicate emission feature and several other abr upt changes in the fundamental characteristics of the SED by 1996 April 22. 1 UT suggest that thermal emission from grains with radii less than or simi lar to 0.5 mu m began to dominate the IR emission from the coma for r less than or similar to 0.39 AU. We discuss the implications of these findings i n terms of the structure of the comet nucleus and suggest that they imply t hat Sun-activated jets may be involved in the observed temporal changes of the IR SED.