  An explosion on comet 17P/Holmes occurred on 2007 Oct 23, projecting
particulate debris of a wide range of sizes into the interplanetary medium. We
observed the comet using the Spitzer spectrograph on 2007 Nov 10 and 2008 Feb
27, and the photometer, on 2008 Mar 13. The fresh ejecta have detailed
mineralogical features from small crystalline silicate grains. The 2008 Feb 27
spectra, and the central core of the 2007 Nov 10 spectral map, reveal nearly
featureless spectra, due to much larger grains that were ejected from the
nucleus more slowly. We break the infrared image into three components (size,
speed) that also explain the temporal evolution of the mm-wave flux. Optical
images were obtained on multiple dates spanning 2007 Oct 27 to 2008 Mar 10 at
the Holloway Comet Observatory and 1.5-m telescope at Palomar Observatory. The
orientation 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 initial
explosion. The kinetic energy of the ejecta >1e21 erg is greater than the
gravitational binding energy of the nucleus. We model the explosion as being
due to crystallization and release of volatiles from interior amorphous ice
within a subsurface cavity; once the pressure in the cavity exceeded the
surface 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 the
nucleus are constrained by the observed properties of the ejecta; tensile
strengths on >10 m scale must be greater than 10 kPa. The appearance of the
2007 outburst is similar to that witnessed in 1892, but the 1892 explosion was
less energetic by a factor of about 20.
