A Mars Trojan asteroid has the same chemical signature as Earth’s moon

Although Mars is much smaller than Earth, it has two moons. Deimos and Phobos were probably once asteroids captured by the gravity of Mars. The red planet also captured nine other small bodies. These asteroids do not orbit Mars directly, but instead orbit gravitationally stable points on both sides of the planet known as Lagrange points. They are known as Trojans and they move along the Martian orbit approximately 60 ° ahead or behind Mars. Most of these Trojans appear to be of Martian origin and formed from the impact of asteroids with Mars. But one of the Trojans appears to have a different origin.

We can learn the origin of small bodies by observing the spectrum of light coming from their surface. Since each type of molecule has a unique spectrum, we can determine the chemical footprint of each body. For example, Martian Trojans all contain a mineral known as olivine. Olivine is rare among asteroids but relatively common on Mars. So the Trojans are likely to come from Mars.

But one of the Trojans, simply named 101429, has a different spectrum. When a team recently looked at the infrared spectrum of 101429, they found that it contains traces of a mineral known as pyroxene. This mineral is also found on the surface of the Moon. Given the similarity of its spectrum to that of the Moon, it is entirely possible that it has a lunar origin.

While it might sound a bit far-fetched, it’s completely possible. We know, for example, that the fragments of Mars created by the impacts have moved to Earth over time. There are a handful of meteorites known to have a Martian origin. During the first period of our solar system, the Moon was bombed and fragments of the Moon could have reached Mars.

That said, we should be wary of jumping to conclusions. The spectrum comparison is not accurate enough to confirm a lunar origin. Other bodies are also known to have pyroxene on their surface. All we know for sure is that 101429 has a different origin from other Martian Trojans.

Trojan asteroids exist near other worlds, such as Jupiter and Neptune. Together with the Martian Trojans, these bodies can provide clues to the origin and history of the solar system. With this latest study, we are learning that even after the formation of planets there was still a dynamic exchange of material between the worlds.

Reference: Polishook, David, et al. “A Martian origin for the Trojan Mars asteroids.” Nature Astronomy 1.8 (2017): 1-5.

Reference: Christou, Apostolos A., et al. “Composition and origin of the Trojan asteroids L5 of Mars: insights from spectroscopy.” Icarus 354 (2020): 113994.