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

We present 1.2-18.5 μm infrared (IR) photometric observations of comet Hyakutake (C/1996 B2) that define the temporal development of the spectral energy 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 comet was at a heliocentric distance r ≥ 0.69 AU were consistent with the interpretation that the optically important grains dominating the IR emission from the coma were larger than 1 μm. The sudden appearance of a strong 10 μm silicate emission feature and several other abrupt changes in the fundamental characteristics of the SED by 1996 April 22.1 UT suggest that thermal emission from grains with radii ≲0.5 μm began to dominate the IR emission from the coma for r ≲0.39 AU. We discuss the implications of these findings in terms of the structure of the comet nucleus and suggest that they imply that Sun-activated jets may be involved in the observed temporal changes of the IR SED.