Observational confirmation and characterization of the Ennomos collisional family

The Jupiter Trojans hold the key to understanding fundamental aspects of solar system evolution. Recent models posit that the Trojans initially formed in the outer solar system before being scattered inward during a period of dynamical instability. Constraining the surface composition of Trojans serves as a powerful empirical test of these theories. Studying collisional families in particular and comparing their properties to the background population can provide crucial insights into the bulk composition of these objects and the effects of solar irradiation on their pristine surfaces. While the overall Trojan population is known to contain two distinct color classes – the red and less-red subpopulations – analyses of the color distribution of the Eurybates collisional family have revealed significantly bluer colors than the background population, suggesting that the present-day space weathering environment of the Trojans is markedly different than that of their primordial formation region. While these observations offer tentative support to dynamical instability models, more families must be studied before concrete conclusions about the surface colors of Trojan collisional fragments can be made. To this end, we carried out a photometric campaign to image members of the purported Ennomos family. By combining multiband observations from 2015 and 2021, we measured high-precision visible colors of more than 70 out of the 104 candidate family members. Our analysis of the color distribution reveals a small cluster of objects with significantly bluer characteristic spectral slope values than the background population, similar to the Eurybates family members. Having confirmed the Ennomos collisional family, we place our findings in the broader context of Trojan formation and composition. Further study of this and other potential collisional families will greatly impact the interpretation of data from the upcoming NASA Lucy mission, which is slated to fly by Eurybates in 2027.