New asteroid clusters and evidence of collisional fragmentation in the L₅ Trojan cloud of Mars

Context . Trojan asteroids of Mars date from an early phase of Solar System evolution. The Mars Trojan (MT) distribution has been previously shown to be highly asymmetric and inhomogeneous. Remarkably, a single asteroid family associated with (5261) Eureka ( H ∼ 16) at L 5 contains all stable Trojans fainter than H = 18. A possible culprit is the action of thermal radiation forces on the orbits and rotation states of these small asteroids. Aims . Using a larger MT sample than previously available, we took a fresh look at this population to re-evaluate these earlier conclusions. We also searched for additional features diagnostic of MT evolutionary history and of the Eureka family in particular. Methods . We performed harmonic analysis on numerical time series of the osculating elements to compile a new proper element catalogue comprising 16 L 5 and 1 L 4 MT asteroids. We then combined sample variance analysis with statistical hypothesis testing to identify clusters in the distribution of orbits and assess their significance. Results . We identify two small clusterings significant at 95% confidence of three H =20−21 asteroids each and investigate their likely origin. One of the clusters is probably the result of rotational breakup of a Eureka family asteroid ∼10 8 yr ago. The significantly higher tadpole libration width of asteroids in the other cluster is more consistent with an origin as impact ejecta from Eureka itself on a timescale comparable to the ∼1 gigayear age of its family. We further confirm the previously reported correlations in Eureka family orbital distribution attributed to the long-term action of radiation-driven forces and torques on the asteroids.