We surveyed 23 comets using the Infrared Array Camera on the Spitzer Space Telescope in wide filters centered at 3.6 and 4.5 \mu m. Emission in the 3.6 \mu m filter arises from sunlight scattered by dust grains ; the 3.6 \mu m 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 \mu m filter contains scattered sunlight by , and thermal emission from , the same dust grains , as well as strong emission lines from CO _ { 2 } and CO gas .
The 4.5 \mu m images are often much brighter than could be explained by dust grains , and they show sometimes distinct morphologies , in which cases we infer they are dominated by gas .
Based on the ratio of 4.5 to 3.6 \mu m brightness , we classify the survey comets as CO _ { 2 } +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 CO _ { 2 } +CO .
The gas emission in the IRAC 4.5 \mu m 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 H _ { 2 } O or CO _ { 2 } .
Notable features in the images include the following .
There is a prominent loop of gas emission from 103P/Hartley 2 , offset toward the sunward direction ; the loop could be due to an outburst of CO _ { 2 } before the Spitzer image .
Prominent , double jets are present in the image of 88P/Howell , with one directed nearly toward the Sun and the other closer to the terminator ( but still on the daytime hemisphere ) .
A prominent single jet is evident for C/2002 T7 ( LINEAR ) , 22P/Kopff and 81P/Wild 2 .
Spirals are apparent in 29P/Schwassmann-Wachmann 1 and C/2006 W3 ( Christensen ) ; 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 .