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An explosion on comet 17P/Holmes occurred on 2007 Oct 23, projectingparticulate debris of a wide range of sizes into the interplanetary medium. Weobserved the comet using the Spitzer spectrograph on 2007 Nov 10 and 2008 Feb27, and the photometer, on 2008 Mar 13. The fresh ejecta have detailedmineralogical features from small crystalline silicate grains. The 2008 Feb 27spectra, and the central core of the 2007 Nov 10 spectral map, reveal nearlyfeatureless spectra, due to much larger grains that were ejected from thenucleus more slowly. We break the infrared image into three components (size,speed) that also explain the temporal evolution of the mm-wave flux. Opticalimages were obtained on multiple dates spanning 2007 Oct 27 to 2008 Mar 10 atthe Holloway Comet Observatory and 1.5-m telescope at Palomar Observatory. Theorientation of the leading edge of the ejecta shell and the ejecta blob,relative to the nucleus, do not change as the orientation of the Sun changes;instead, the configuration was imprinted by the orientation of the initialexplosion. The kinetic energy of the ejecta >1e21 erg is greater than thegravitational binding energy of the nucleus. We model the explosion as beingdue to crystallization and release of volatiles from interior amorphous icewithin a subsurface cavity; once the pressure in the cavity exceeded thesurface strength, the material above the cavity was propelled from the comet.The size of the cavity and the tensile strength of the upper layer of thenucleus are constrained by the observed properties of the ejecta; tensilestrengths on >10 m scale must be greater than 10 kPa. The appearance of the2007 outburst is similar to that witnessed in 1892, but the 1892 explosion wasless energetic by a factor of about 20.

Explosion of Comet 17P/Holmes as revealed by the Spitzer Space Telescope