  We surveyed 23 comets using the Infrared Array Camera on the Spitzer Space
Telescope in wide filters centered at 3.6 and 4.5 microns. Emission in the 3.6
micron filter arises from sunlight scattered by dust grains; these images
generally have a coma near the nucleus and a tail in the antisolar direction
due to dust grains swept back by solar radiation pressure. The 4.5 micron
filter contains the same dust grains, as well as strong emission lines from CO2
and CO gas; these show distinct morphologies, in which cases we infer they are
dominated by gas. Based on the ratio of 4.5 to 3.6 micron brightness, we
classify the survey comets as CO2+CO "rich" and "poor." This classification is
correlated with previous classifications by A'Hearn based on carbon-chain
molecule abundance, in the sense that comets classified as "depleted" in
carbon-chain molecules are also "poor" in CO2+CO. The gas emission in the IRAC
4.5 micron images is characterized by a smooth morphology, typically a fan in
the sunward hemisphere with a radial profile that varies approximately as the
inverse of projected distance from the nucleus, as would apply for constant
production and free expansion. There are very significant radial and azimuthal
enhancements in many of the comets, and these are often distinct between the
gas and dust, indicating that ejection of solid material may be driven either
by H2O or CO2. Notable features in the images include the following. There is a
prominent loop of gas emission from 103P/Hartley 2, possible due to an outburst
of CO2 before the Spitzer image. Prominent, double jets are present in the
image of 88P/Howell. A prominent single jet is evident for 3 comets. Spirals
are apparent in 29P and C/2006 W3; we measure a rotation rate of 21 hr for the
latter comet. Arcs (possibly parts of a spiral) are apparent in the images of
10P/Tempel 2, and 2P/Encke.

Earth and Planetary Astrophysics