Uncovering Signatures of Refractory Materials on KBOs and Centaurs by Reflectance Spectroscopy

In reflectance spectra of minor planets such as Kuiper Belt Objects (KBOs) and centaurs, absorption features related to components of their surface composition reveal details of their likely formation environment and their subsequent dynamical and thermal evolution. Their NIR reflectance spectra often exhibit water ice absorption features and for some objects methanol ice has also been observed. Their optical spectra, however, have mostly consisted of only a red featureless slope. The presence of suspected absorption features in this range that might be associated with silicate and organic materials, while reported in the spectra of some KBOs, have so far eluded confirmation. We report three new results that have arisen from a study into the surface compositions of KBOs and centaurs through optical reflectance spectroscopy. First, we report the discovery that the plutino 2004 EW95 exhibits optical reflectance properties that are very similar to those of a hydrated carbonaceous asteroid, bearing features associated with ferric oxides and phyllosilicates. These surface properties indicate that this object has been heated at some point during its history, potentially pointing to its origins in the early Solar System's giant planet region. Second, we report spectroscopic comparisons of the surface of active centaur 174P/Echeclus before and after its 2016 cometary outburst along with the observation of its unusually blue dust coma. Third, we report preliminary results on the spectrum of an extreme centaur which exhibits a curious drop in reflectance towards the near-UV. This spectrally confirmed feature is similar to that observed in laboratory reflectance spectra of complex aromatic hydrocarbons. These results constitute the first spectroscopically confirmed features observed at visual wavelengths in KBO and centaur spectra, and are the first attributable to refractory materials.