06 November 2020 News

The Moon could have a long-lost "twin" in orbit around Mars

Depiction of the planet Mars and its retinue of Trojans circling around the L4 and L5 Lagrange points. At L5, asteroid 101429 is represented by the blue point, the asteroid Eureka and its family are represented in red and amber respectively. Image: AOP
Depiction of the planet Mars and its retinue of Trojans circling around the L4 and L5 Lagrange points. At L5, asteroid 101429 is represented by the blue point, the asteroid Eureka and its family are represented in red and amber respectively. Image: AOP

Scientists studying an asteroid trailing behind Mars have found that the sizeable space rock has the same chemical signature as the Moon, suggesting that this relic fragment could be a long-lost sub kilometre-sized "twin" of our nearest celestial neighbour.

Loitering 60 degrees ahead and behind Mars along its orbit is a family of asteroids known as Trojans.

Along with Mars, these small rocky worlds can also be found in their thousands flocking around Jupiter and Neptune within gravitational “safe havens” known as Lagrange points.

Scientists think that the Trojans may have attached themselves to the Martian orbit just as the planets ended up in their final configuration, so their primitive nature makes them ideal objects for studying the early evolution of the Solar System.

Whats more, all of the Trojans in a Lagrange 5 (L5) orbit around Mars have the same ancestry and belong to the same group known as the Eureka family.

Eureka family members contain lots of olivine; a magnesium-iron silicate mineral that is common in Earth's subsurface. That is, all except for one rogue rock.

Known as 101429, this particular object has come to the attention of scientists as its mineralogical makeup is not the same as the other L5 Trojans.

After using spectroscopy to study the chemical contents of 101429, a team headed by Christou Apostolos at the Armagh Observatory and Planetarium (AOP), found that this imposter is packed with orthopyroxene – a mineral that ranges from pure magnesium silicate (MgSiO3) to pure ferrous iron silicate (FeSiO3) – and has more in common with areas of the lunar surface than its fellow L5 inhabitants.

“The early Solar System was very different from the place we see today,” says Apostolos. “The space between the newly-formed planets was full of debris and collisions were commonplace. Large asteroids – we call these planetesimals – were constantly hitting the Moon and the other planets.”

How the Moon formed is a matter of debate, but many scientists think that a Mars-sized object collided with a proto-Earth ejecting substantial material that eventually collected and coalesced in an orbit around our planet to form the Moon.

This is known as the giant-impact hypothesis and a shard from such a collision could have reached the orbit of Mars when the planet was still forming and was trapped in its Trojan clouds, says Apostolos.

However, say Apostolos and team, there is a chance that 101429 is just another asteroid, similar perhaps to ordinary chondrite meteorites, that became lunar-like in composition through millions or billions of years of space weathering.

Alternatively, the object came from Mars itself. “The shape of the 101429 spectrum tells us that it is rich in pyroxene, a mineral found in the outer layer or crust of planet-sized bodies,” says the team leader.

“Mars, like the Moon and the Earth, was pummelled by impacts early in its history, one of these was responsible for the gigantic Borealis basin, a crater as wide as the planet itself. Such a colossal impact could easily have sent 101429 on its way to the planet’s L5 Lagrangian point,” Apostolos points out.

The true origins of 101429 is still undetermined, but its identification is important as it could teach scientists about finding Earth Trojans, if they exist.

So far, Mars is the only terrestrial planet known to harbour Trojans, but if more were found in the inner Solar System, and if they did once belong to Earth, then they should stand out because of their uncommon composition.

These yet to be discovered small asteroids might have to wait until the Vera C. Rubin Observatory begins operations in 2022.

Although the Rubin Observatory is set to probe the nature of dark matter and dark energy, it will also conduct the most ambitious survey of the solar system to-date.

Rubin is expected to discover roughly ten times as many asteroids as currently known to help scientists gain new insight into how planets form and how our Solar System evolved over its history.

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