Linear Continuum Modelling to Explain The Majority of Bulk Features of Kuiper Belt Object Spectra

Abstract

The first analyses of the James Webb Space Telescope spectra of trans-Neptunian Objects (TNOs) revealed three discrete types of surfaces. This seems to contradict ground-based spectro-photometric datasets, which suggest a continuum of colors with only two surface types. Here we present linear spectral modelling that reconciles these two results. In our model, the sole parameter is the object's optical slope, and the reflectance spectrum at all wavelengths is linearly proportional to that color, with the slope of that function evaluated from the spectra themselves. When applied to small (H>5 for H2O-types and H>4 for the merged sample of organic-rich and CO2-types) and distant (q>18 au) objects, we find that this model does a reasonable job of reproducing the overall spectral behavior of both samples. Bootstrapping simulations show that if the optical slope were not a good predictor of an object's spectrum, then finding an explained variance of the model that is better than observed occurred in 2.3% of realizations for the H2O-types and 0% of realizations for the organics sample. In a chi^2 sense, the optical color is a better predictor of most spectra as compared to the mean spectrum of a class. The trends of optical color and spectra band-areas exhibited for many key compositional materials are well reproduced, and demonstrate that those materials govern the overall spectral shape within a class. Importantly, these results require that within a given class, the band-areas of those key materials are predictable given only its optical color and its surface type. Unsurprisingly, our simple one-parameter model does not account for the full spectral diversity of TNOs. We speculate that albedo encapsulates much of the remaining diversity.