Matija Cuk, Apostolos A. Christou, Douglas P. Hamilton

Yarkovsky-Driven Spreading of the Eureka Family of Mars Trojans

Figure 1. Correlations between eccentricities and inclinations (top left), eccentricities and libration amplitudes (bottom) and inclinations and libration amplitudes (top right) at the end of a 1 Gyr simulation incorporating the Yarkovsky effect. The elements have been averaged over the last 10 Myr. Test particles with inward-type Yarkovsky drift are plotted as red pluses, those with outward-type drift are plotted as blue x’s. Known family members are plotted as open squares, with Eureka shown as the solid square.


Out of nine known stable Mars Trojans, seven appear to be members of an orbital grouping including the largest Trojan, Eureka. In order to test if this could be a genetic family, we simulated the long term evolution of a tight orbital cluster centred on Eureka. We explored two cases: cluster dispersal through planetary gravity alone over 1 Gyr, and a 1 Gyr evolution due to both gravity and the Yarkovsky effect. We find that the dispersal of the cluster in eccentricity is primarily due to dynamical chaos, while the inclinations and libration amplitudes are primarily changed by the Yarkovsky effect. Current distribution of the cluster members orbits are indicative of an initially tight orbital grouping that was affected by a negative acceleration (i.e. one against the orbital motion) consistent with the thermal Yarkovsky effect. We conclude that the cluster is a genetic family formed either in a collision or through multiple rotational fissions. The cluster’s age is on the order of 1 Gyr, and its long-term orbital evolution is likely dominated by the seasonal, rather than diurnal, Yarkovsky effect. If confirmed, Gyr-scale dominance of the seasonal Yarkovsky effect may indicate suppression of the diurnal Yarkovsky drift by the related YORP effect. Further study of Mars Trojans is essential for understanding the long-term orbital and rotational dynamics of small bodies in the absence of frequent collisions.

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Last Revised: 2015 July 22nd