Although Mars is much smaller than Earth, it has two moons. Deimos and Phobos were likely once asteroids captured by the gravity of Mars. The red planet also trapped nine other small bodies. These asteroids do not orbit Mars directly, but rather orbit gravity stable points on either side of the planet known as Lagrangian points. Known as Trojans, they move about 60 degrees in front of or behind Mars along Mars orbit. Most of these Trojans appear to be of Martian origin and were created from asteroid impacts on Mars. But one of the Trojans seems to have a different origin.
We can learn the origin of small bodies by looking at the spectrum of light that comes from their surface. Since each type of molecule has a unique spectrum, we can determine the chemical fingerprint of each body. For example, the Mars Trojans all contain a mineral known as olivine. Olivine is rare among asteroids, but relatively common on Mars. So it is likely that the Trojans came from Mars.
The spectrum of 101429 and the surface of the moon compared. Photo credit: Armagh Observatory
But one of the Trojans, simply called 101429, has a different spectrum. When a team recently observed the infrared spectrum of 101429, they found it had traces of a mineral known as pyroxene. This mineral is also found on the surface of the moon. Because of the similarity of its spectrum to that of the moon, it is entirely possible that it has a lunar origin.
That seems a bit far-fetched, but it is entirely possible. We know, for example, that fragments of Mars caused by impacts migrated to Earth over time. There are a handful of meteorites known to be of Martian origin. In the early stages of our solar system, the moon was bombed and fragments of the moon may have reached Mars.
Most Trojans orbit Jupiter. Photo credits: ESA / Hubble, M. Kornmesser
However, we should be careful when reaching conclusions. The spectrum comparison is not precise enough to confirm a moon origin. Other bodies are also known to have pyroxene on their surface. We only know that 101429 has a different origin than the other Mars Trojans.
Trojan asteroids exist near other worlds such as Jupiter and Neptune. Together with the Mars Trojans, these bodies can provide clues as to the origin and history of the solar system. With this latest study we learn that even after the formation of the planets, there was still a dynamic exchange of materials between the worlds.
Reference: Polishook, David et al. "A Martian origin for the Mars Trojan asteroids." Nature Astronomy 1.8 (2017): 1-5.
Reference: Christou, Apostolos A. et al. "Composition and Origin of the L5 Trojan Asteroids of Mars: Findings from Spectroscopy." Icarus 354 (2020): 113994.