Trojans are distinct from the asteroids found in the asteroid belt, which begins about 101 million kilometres past Mars and stretches toward Jupiter. While Trojans orbit the sun, like the asteroid belt, they are trapped in gravitational sweet spots that ensure they will permanently trail or precede the planet in its trip around our star. These are known as Lagrange points.

Mars is the only rocky planet accompanied by Trojan asteroids in stable orbits. Occasionally, a passing asteroid can latch on to Earth’s gravity and temporarily trail the planet for a few thousand years, but Earth doesn’t have any permanent Trojans. They are common around Jupiter, which has about 6000. Neptune has 10.

True Trojans come from the material that was present when the planets were formed, says Apostolos Christou at the Armagh Observatory and Planetarium in the UK. “The Trojans are really a relic from the early life of the solar system, when the planets were still forming.”

Trojan Eureka

In 2013, he was studying a Mars Trojan nicknamed Eureka when he spotted a group of asteroids accompanying it. They all seemed to occupy the same area in Mars’s orbital path, which was strong circumstantial evidence that they shared a common ancestor.

Previous research on Eureka showed it is rich in a mineral called olivine, which typically forms within the mantles of large rocky bodies, such as Earth and the other terrestrial planets. This implies the asteroid was once part of a bigger object, maybe even a planet, that has long since been destroyed. But asteroids like this are quite rare.

To find out if Eureka’s fellow Trojans also contained olivine, Christou studied the spectrum of sunlight that two of them reflected and found they did. This suggests that mantle material existed near Mars when it was forming.

“These asteroids might well be samples of the original building blocks that came together to form Mars and the other terrestrial planets,” Christou and his co-authors say.

Shaved planet

It is also possible that the Trojans are pieces of Mars itself, shaved off the planet in its youth and still following in its wake, Christou says.

“If it turns out that some of the Trojans are pieces of Mars’s mantle, then presumably we can have sample return missions. You can go get a piece,” he says. “The number of Mars-related science questions you can ask, if you have a piece of its mantle in the lab, would be amazing.”

The Eureka Trojans could shed light on Mars’s past whether the asteroids originated in the planet or not, says Andrew Rivkin at Johns Hopkins University in Maryland, who studies asteroids. If they did come from Mars, their make-up would give clues about the planet’s interior and formation.

If not, they could still provide clues about where Mars might have been located in the solar system in the past; about collisions that carved out its giant craters; and about the natural history of Mars compared to that of Earth, Venus and Mercury.

“Then solar system dynamics researchers would have to look at it and say, ‘Here’s why Mars has this group of asteroids, and here’s why Earth doesn’t’,” says Rivkin. He adds that interest in Mars Trojans seems to spike every few years. “Hopefully in the next 10 years, we’ll get to have a spacecraft visit them.”