Asteroid Trailing Mars May Be Our Moon’s Long-Lost Twin

Depiction of the planet Mars and its retinue of Trojans circling around the L4 and L5 Lagrange points. The dashed curve traces the planet’s orbit. At L5, asteroid 101429 is represented by the blue point, the asteroid Eureka and its family are represented in red and amber respectively.
Credit: Armagh Observatory

An international team of planetary scientists led by astronomers at AOP have found an asteroid trailing behind Mars with a composition very similar to the Moon’s. The asteroid could be an ancient piece of debris, dating back to the gigantic impacts that formed the Moon and the other rocky planets in our solar system like Mars and the Earth. The research, which was published in the journal Icarus, also has implications for finding such primordial objects associated with our own planet.

Trojans are a class of asteroid that follows the planets in their orbits as a flock of sheep might follow a shepherd, trapped within gravitational “safe havens” 60 degrees in front of, and behind, the planet (Figure 1). They are of great interest to scientists as they represent leftover material from the formation and early evolution of the solar system. Several thousands of those Trojans exist along the orbit of the giant planet Jupiter. Closer to the Sun, astronomers have so far discovered only a handful of Trojans of Mars, the planet next door to Earth.

Where could such an unusual object have come from? One possibility is that 101429 is just another asteroid, similar perhaps to ordinary chondrite meteorites, that acquired its lunar-like appearance through eons of exposure to solar radiation, a process called space weathering.

Alternatively, the asteroid may look like the Moon because it does come from the Moon. Dr. Apostolos Christou, AOP astronomer and lead author of the paper explains: “The early solar system was very different from the place we see today. 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. 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.”

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