A new study proposes a source for the mysterious Mars Trojans: Mars itself.

An artist’s conception of a Mars Trojan, ejected from the Red Planet.
Polishook / Weizmann Institute of Science

It’s one of the major mysteries of the inner solar system: How did Mars — a tiny world only a tenth the mass of Earth — capture its cluster of orbit-sharing Trojan asteroids?

Trojans are asteroids that co-orbit either ahead of a planet, at the L₄Lagrangian point, or behind it at the L₅ point. These regions are stable because the gravitational pull of the planet balances that of the Sun. Trojan asteroids have been discovered around Jupiter, Uranus, Neptune, Venus, and Mars. (Only one Trojan (2010 TK₇) has been discovered related to the Earth, though the Osiris-REX mission bound for 101955 Bennu is currently on the hunt for more.)

Many studies have suggested that the asteroid belt, which lies just outside the orbit of Mars, may have been the source of the Mars Trojans. Now, a study published in the July 17th Nature Astronomy points to a new possible source: Mars itself.

NASA’s Infrared Telescope Facility
NASA

The study used observations from NASA’s Infrared Telescope Facility, based at the Mauna Kea Observatory in Hawai’i, to look at the spectra of two Mars Trojans: 311999 (initially desgnated 2007 NS₂) and 385250 (2001 DH₄₇. The light reflected off these asteroids shows a characteristic broad absorption band around 1 micron, consistent with the presence of olivine — a mineral rare in asteroids but common in the crust of Mars.

“Asteroids like this are very rare in the main belt of asteroids (0.4%),” says David Polishook (Weizmann Institute of Science, Israel). “Therefore, the chances that the few asteroids captured by Mars are olivine-rich asteroids is extremely low.” But Martian rovers and orbiters, and even Martian meteorites recovered on Earth, have shown that Mars itself offers an ample supply of olivine.