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

¹ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
² Department of Astronomy, University of Florida at Gainesville, Gainesville, FL 326112055, USA
³ Division of Science, New York University Abu Dhabi, Abu Dhabi, UAE
⁴ Center for Astro, Particle and Planetary Physics (CAP 3), New York University Abu Dhabi, UAE
⁵ Department of Physics and Astronomy, Queen’s University Belfast, University Road, Belfast, BT7 1NN, Northern Ireland, UK
⁶ SETI Institute, Mountain View, CA 94043, USA
⁷ INAF, Osservatorio Astrofisico di Arcetri, 50125 Firenze, Italy

^★ Corresponding author: apostolos.christou@armagh.ac.uk

Received: 18 January 2025
Accepted: 8 April 2025

Abstract

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 L₅ 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 L₅ and 1 L₄ 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 ∼10⁸ 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.