In a Nutshell
The Kuiper belt is widely thought to be the source of the short-period (specifically "Jupiter Family") comets. If so, one might expect the physical properties of the latter to be like those of the former. To test this expectation, I measured and compared the optical colors of a set of cometary nuclei with the colors of KBOs measured in the same way. The result, shown in the Figure below, is that the two types of object have color distributions which are NOT the same. The most important difference is that the "ultrared matter" that is a distinctive component of the surface materials on many KBOs is rare or absent on the surfaces of cometary nuclei, hence the title to this paper.

(Click on the figure to enlarge it)

How Could This Be?
There are several possibilities. The most likely is that the observed cometary nuclei are highly modified with respect to their Kuiper belt counterparts. There is more than enough time for various modification processes to occur in the 10 Myr journey from Kuiper Belt to comet, and there are several possible processes capable of causing the color change. The one I prefer is simple resurfacing of the nucleus, by suborbital debris launched from the nuclei once they begin to sublimate inside the orbit of Jupiter. The timescale for this process is miniscule, and we know that it happens. Essentially, the nuclei are covered in a cloak of fall-back debris that obliterates the more primordial surface.

So What?
This observations shows that the cometary nuclei cannot be considered as pristine samples of the Kuiper Belt. They are modified samples, perhaps greatly modified by thermal and physical effects. Spacecraft launched to study the nuclei as carriers of primitive matter will in fact be measuring objects that are quite different from in their source regions.

The paper itself as a PDF file.

Last updated April 2003

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