Remains of ancient “mini planets” in Mars’s orbit

The planet Mars shares its orbit with a few small asteroids called “Trojans”. Recently, an international team of astronomers have found that most of these objects share a common composition and are likely the remains of a mini-planet that was destroyed by a collision long ago.

Trojan asteroids move in orbits with the same average distance from the Sun as a planet, trapped within gravitational “safe havens” 60 degrees in front of and behind the planet. The significance of these locations was worked out by 18th-century French mathematician Joseph-Louis Lagrange. In his honour, these locations are now known as “Lagrange points”. The point leading the planet is L4; that trailing the planet is L5.

About 6000 such Trojans have been found at the orbit of Jupiter and about ten have been found at Neptune’s. They are believed to date from the solar system’s earliest times when the distribution of planets, asteroids, and comets was very different than the one we observe today.

Although many Trojans are known to exist, Mars is currently the only terrestrial planet known to have Trojan asteroids in stable orbits. The first Mars Trojan was discovered over 25 years ago and named “Eureka” in reference to Archimedes’ famous exclamation. So far, the number of known Trojans in Mars’s orbit is nine, a factor of 600 fewer than Jupiter Trojans; however, this relatively small sample reveals an interesting structure not seen elsewhere in the solar system.

All the Trojans, save one, are trailing Mars at its L5 Lagrange point. What’s more, the orbits of all but one of the eight L5 Trojans group up around Eureka itself. The reason for the uneven distribution of these objects is unclear, though there are a couple of possibilities. In one scenario, a collision broke up a precursor asteroid at the L5 point, the fragments making up the group we observe today. Another possibility is that a process called “rotational fission” caused Eureka to spin up, eventually spawning off small pieces of itself in heliocentric orbit. Whatever the cause, the grouping strongly suggests that the asteroids in this “Eureka family” were once part of a single object or a progenitor body, and circumstantial evidence for this hypothesis is strong. By obtaining the colour spectrum of sunlight reflected off the asteroid’s surface, this hypothesis can be tested.

For this purpose, an international team of astronomers, led by Apostolos Christou and Galin Borisov, recorded the spectra of two asteroids that belong to the Eureka family using the X-SHOOTER spectrograph mounted on the Unit 2 telescope of the European Southern Observatory’s Very Large Telescope facility in Chile. Analysing the spectra, they found that both objects had very similar compositions to that of Eureka, thus confirming the genetic relationship between family asteroids. The finding also marks a significant “first” for asteroid studies; the spectra show that these asteroids are predominantly composed of olivine, a mineral that typically forms within much larger objects under conditions of high pressure and temperature. The implication is that these asteroids are likely relict mantle material from within mini-planets or “planetesimals” which, like the Earth, developed a crust, mantle, and core through the process of differentiation but have long since been destroyed by collisions.

Many other families exist in the Asteroid Belt between Mars and Jupiter, but such olivine-dominated asteroids are unique. This is related to the so-called missing-mantle problem: if one adds up the mass of different minerals in the asteroid belt and particularly those thought to be pieces of broken up, differentiated asteroids, there is a deficit of mantle material compared to rocky crust and metallic core material.

Although the discovery of this olivine-dominated family does not provide a final solution to the missing mantle problem, it does show that mantle material was present near Mars early in the solar system’s history. This suggests that such material has participated in the formation of Mars and perhaps its planetary neighbour, our own Earth.

Featured image credit: ‘mars-land-planet-cosmos-stars’ by Aynur_zakirov. CC0 Public Domain via Pixabay.