Investigating Centaur surface colors: connecting surface transformation to thermal and dynamical history

Centaurs are small bodies residing on unstable orbits between Jupiter and Neptune. As a transition population between the Trans- Neptunian Objects (TNOs) and Jupiter family comets (JFCs) they carry the information on the composition of their parent populations into the realm of giant planets. Centaurs therefore offer a unique opportunity to study the small-size regime of TNOs up close, which can be used to constrain current models of Solar system formation and the physical evolution of small bodies beyond Neptune. The most interesting feature of Centaurs is the striking bimodal distribution of their surface colors. This color disparity either showcases two different intrinsic compositions among Centaurs, and therefore also their formation locations, or if all Centaurs have formed in the same region, they must have taken different physical evolution pathways. If the first scenario is correct, the color bimodality will be equally observed among the population members of all sizes, and across the whole Centaur region. In the second scenario, cometary activity has been proposed as a process transforming the Centaur surface properties by blanketing the primordial, extremely red material, with a fresh neutral ejecta. In this case we would observe a gradual transition of surface colors from mostly red to neutral with decreasing perihelion distance, because the observational evidence suggests that the cometary activity in Centaurs is limited by the distance from the Sun. Furthermore, Centaur activity appears to by induced by rapid orbital changes pushing distant dynamically new objects inwards, exposing them to the warmer environment in the inner Solar system, where the thermal processing of the nucleus propels the loss of volatiles and subsequent possible surface changes.

The primary goal of this effort is to detangle the origins of the Centaur color bimodality by investigating whether the complex thermo- dynamical history of Centaurs could have a transformative effect on their surface properties. To achieve our objective, we will (1) conduct new extensive multi-filter photometric survey of Centaurs in order to collect surface colors of targets with a wide variety of sizes and perihelion distances, and (2) use numerical integrations to search for recent abrupt orbital changes in order to explore the correlation of the orbital evolution with the current observed surface colors of our targets. This work will also capitalize on the advanced capabilities of current all-sky surveys to detect more small Centaurs than ever before, allowing us to obtain photometry of targets in the same size regime as the typical JFCs, and therefore extend the current sample of Centaurs with measured photometric color down to the km-regime. Our dataset could therefore be used as an input for the development and improvement of current and new thermo-dynamical models of small body interiors.

This proposal is highly relevant to the Solar System Observations Program, as it includes a primary element of new observations of an under-studied Centaur population that cannot be carried out by current spacecraft, complemented by archival observation data. The observations and subsequent theoretical analysis will offer a broad insight into the origin and evolution of the Centaur population, its source regions, and subsequently the Solar system as a whole.