The Mysterious Origins of Mars’s Trailing Asteroids

A cluster of space rocks orbiting the planet may have come from within the planet itself.

A rocky, dusty, pale brown and gray landscape with no signs of life
NASA / JPL-Caltech / University of Arizona / Texas A&M University

In 1990, astronomers detected an asteroid, about one mile wide, trailing Mars. In the spirit of discovery, they named it Eureka. Over the next few years, they found a few more, six near Eureka, and two in other locations in Mars’s orbit. The rocky bodies are known as Trojans, the name given to asteroids that exist in stable spots around their parent planet and share its orbit around the sun.

The origin of the Mars Trojans has puzzled asteroid researchers since. Mars is the only terrestrial planet in the solar system to have Trojans, which number in the thousands around Jupiter and about 17 around Neptune, perhaps more. One theory suggests the rocks came from material that was floating around near Mars when the planet was young and accumulating matter. Another says they arrived later in Mars’s life from elsewhere, and became trapped by the planet’s gravity. A third theory suggests the asteroids actually came from Mars itself, perhaps even from its below its crust, and were thrown out during a violent collision with another object.

That last theory is perhaps the most interesting, because it means that future Mars astronomers may not even need to step foot on the planet to study its history. They could just swing by the Trojans, shave off a few pieces, and fly them back to Earth’s laboratories.

“To tie them back to Mars really makes them much more intriguing,” says Andy Rivkin, a planetary astronomer at Johns Hopkins University who studies asteroids. “It means that there are these pieces of Mars that are billions of years old, that are potentially as scientifically interesting—or could tell us as much about the history of Mars—as almost anything we could pick up on Mars today.”

While humanity is a ways off from a potential sample return mission to Mars, the prospects of finding chunks of the planet’s interior floating around are looking good. The latest research on the Eureka cluster of Mars Trojans, published Monday in the journal Nature Astronomy, provides more evidence for the theory that at least some of the space rocks came from within the planet.

The researchers studied two asteroids in the Eureka cluster and found that they contain the mineral olivine, which scientists believe is the main component of Earth’s mantle, and, it would follow, of other rocky planets, like Mars. Studies of asteroid populations show that olivine-rich asteroids in other parts of the solar system are rare.

The research adds to a growing body of research on olivine’s presence on Mars. A similar study in January found the same results for the same pair of asteroids, and previous studies have also detected olivine in Eureka. Researchers previously have created models of Mars’s interior that suggest olivine could exist there in large amounts, and some Mars meteorites have been found to contain olivine. Studies have also found olivine-rich bedrock around some craters on Mars, the products of enormous impacts.

Researchers investigate the composition of asteroids by studying the sunlight they reflect. Like a red apple reflects a certain kind of wavelength (red), olivine-rich asteroids reflect certain wavelengths. Scientists can observe these wavelengths with sensitive Earth-based instruments and then use them to determine the asteroids’ chemical composition.

The researchers in the Nature Astronomy paper also used dozens of previously published simulations of rocky planet formation to further investigate how the Mars Trojans got to where they are. They found that impacts that create massive craters, like the Borealis Basin in Mars’s northern region, which covers more than 40 percent of the planet’s surface, may be powerful enough to eject debris that could then become trapped in orbit.

The researchers say more members of the Eureka cluster likely have the same composition as the two they studied. They plan to further investigate this possibility next March and April, when the asteroids’ orbit brings them to a spot with ample sunlight. The more light they reflect and the brighter they appear, the better scientists are able to investigate them for clues about their origins.