Although Mars is much smaller than Earth, it has two moons. Deimos and Phobos were probably once asteroids that were captured by the gravity of Mars. The red planet has also captured nine other small bodies. These asteroids don’t orbit Mars directly, but instead, orbit gravitationally stable points on either side of the planet known as Lagrange points. They are known as trojans, and they move along the Martian orbit about 60° ahead or behind Mars. Most of these trojans seem to be of Martian origin and formed from asteroid impacts with 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 coming from their surface. Since each type of molecule has a unique spectrum, we can determine the chemical fingerprint of each body. For example, the Martian trojans all contain a mineral known as olivine. Olivine is rare among asteroids but relatively common on Mars. So it’s likely that the trojans originate from Mars.
The spectrum of 101429 and the lunar surface compared. Credit: Armagh Observatory
But one of the trojans, named simply 101429, has a different spectrum. When a team recently observed the infrared spectrum of 101429, they found it has traces of a mineral known as pyroxene. This mineral is also found on the surface of the Moon. Given the similarity of its spectrum with that of the Moon, it is quite possible that it has a lunar origin.
While that might seem a bit far-fetched, it’s completely possible. We know, for example, that fragments of Mars created by impacts have drifted to Earth over time. There are a handful of meteorites known to have a Martian origin. During the early period of our solar system, the Moon was bombarded, and fragments of the Moon could have reached Mars.
Most trojans orbit Jupiter. Credit: ESA/Hubble, M. Kornmesser
That said, we should be cautious about jumping to conclusions. The spectrum comparison is not precise 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 an origin different from the other Martian trojans.
Trojan asteroids exist near other worlds, such as Jupiter and Neptune. Together with the Martian trojans, these bodies can provide clues about the origin and history of the solar system. With this latest study, we are learning that even after the planets formed there was still a dynamic exchange of material between 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 L5 Trojan asteroids of Mars: Insights from spectroscopy.” Icarus 354 (2020): 113994.