Introduction
Spectra can reveal the chemical composition of solar system bodies. However, most solar system objects are too faint for their spectra to be measured. Instead, we use broadband colors from filters as a proxy. Even though compositions cannot be uniquely determined from colors alone, they are useful for comparing different types of object. We can hope, for example, to use colors to test dynamical models that purport to show members of one population displaced to form another.

I measured and compiled optical colors for a large number of solar system "small body" populations, all thought to be related in some way to parent reservoirs in the Kuiper belt and Oort cloud. The results are summarized in the Figure below.

CAPTION Color-color diagram (in which bigger numbers along the axes mean 'redder') showing the locations of various small-body populations, as labelled. Dynamically distinct subsets of the Kuiper belt are shown as red circles while measurements of comet-related bodies are shown as blue circles. Black circled letters denote asteroid spectral types in the Tholen classification system, from Dandy et al.~(2003). The solid line shows the locus of points for reflection spectra of constant gradient; numbers give the slope in units of percent per 1000A. The large yellow circle shows the color of the Sun. Some overlapping points have been displaced (by 0.005 magnitudes) for clarity. Error bars show the uncertainties on the respective means. From Jewitt 2015 . (Click graph for a full size version).

This work is described in full in a paper here, The figures to this paper are linked here in PDF format:

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14

Discussion
The colors of all populations are splayed out along the reddening line (diagonal black arc). This line shows the locus of points in the color-color diagram for reflection spectra that are linear with wavelength, from neutral (at the yellow circle, representing the color of the Sun) to increasingly red at the upper right. Several ramifications of this plot are discussed in the paper. One is that the reddest colors (so-called ultrared matter, which is probably irradiated organic material) are found only in the Kuiper belt and Centaur (escaped Kuiper belt) populations. The ultrared matter somehow does not survive the journey inwards to the Sun, dissappearing at about 10 AU, the orbit of Saturn. Another is that the inactive and cometary Centaurs have quite different mean colors. In fact, the cometary activity begins, and the ultrared matter vanishes, at about the same distance from the Sun. I conclude that cometary activity destroys ultrared matter, probably by blanketing the surface in low-velocity, fallback debris (for which the timescale is very short). This means that the most primitive organic matter in the solar system cannot be found inside the orbit of Saturn. It also means that broadband colors do not provide a simple test of dynamical models describing possible routes of transfer from the Kuiper belt and Oort cloud inwards, once the objects have dipped inside the orbit of Saturn.

Contact David Jewitt [jewitt@ucla.edu 301-825-2521]

Comet	Jewitt	Kuiper