Mars has never had an easy time as a planet, constantly getting bombarded with asteroids throughout its history as its atmosphere was stripped away to nearly nothing. A study published in Nature Astronomy today further highlights Mars's struggles by highlighting an enormous and violent impact in the Red Planet's recent past.

The study looks at a group of Mars trojan asteroids, which trail along behind Mars at a stable point between the planet and the sun called the 5th Lagrange point, or L5. While trojan asteroids of other planets seem to be captured space rocks along for the ride, there's something different about this grouping, called the Eureka cluster after the largest asteroid in the group.

According to spectral analysis, the asteroids in the Eureka cluster are likely made of olivine, a magnesium and iron compound. The high olivine content makes the Eureka cluster among a group of rare A-type asteroids, of which only about 17 had been discovered previously. Most other asteroids in the vicinity are more common iron heavy asteroids.

Finding olivine on asteroids might be rare, but finding olivine on Mars is not.

"The chances that 1 out of 3 asteroids captured by Mars are olivine-rich asteroids is extremely low," lead author David Polishook of the Weizman Institute of Science says. "On the other hand, Mars has enough olivine in its mantle that can be blown away by an impact. Since we know such impacts have happened, we have a source for the trojans that is much more available."

Polishook and his team developed a theory to explain how a series of asteroids could have been flung out to a stable orbit around the sun, trailing behind Mars for more than a billion years. According to the study, a 60 to 120 mile asteroid smashed into the surface of Mars anywhere from 1 billion to 4.5 billion years ago. The team is trying to narrow down where, exactly, on Mars the impactor struck, but leading candidates include the enormous Borealis Basin in the northern hemisphere as well as the Hellas Planitia in the south.

"The biggest issues facing this model are whether the timing of the large impact corresponds well with when Mars settled into its current orbit, and how formation of the trojan asteroids compares compositionally and temporally to the formation of Mars' two moons, Phobos and Deimos," says Nadine Barlow, an expert in Mars' impact history at Northern Arizona University who did not participate in the study. "Details of the impact itself, such as impact angle, direction of impact, and velocity, also need to be more carefully investigated."

Barlow also says that the team does make a convincing case for a Mars impact leading to the Eureka cluster, pointing especially to the Borealis Basin as the possible site for a violent impact.

Polishook says he and his team didn't include Phobos and Deimos in in their simulations. Polishook says that unlike the Eureka group, Phobos and Deimos don't appear to be in stable orbits. In a few million years, Phobos will reach such a point in its declining orbit around Mars that it will be ripped apart into a ring around the Red Planet before fully plunging to the surface.

More modeling is needed to piece together the whole history of Mars' multiple craters, and to explain the debris in the vicinity of the planet as well as its two small moons. The work could also could help explain other trojans in the solar system. Venus, Earth, and other rocky objects (including some moons and dwarf planets) all appear to have trojans, and some of those could have originated from a large impact into their host planet.

"This is definitely an interesting idea to explain the compositionally unusual Mars trojan asteroids," Barlow says. "We certainly know that material can be ejected off planetary surfaces by impact events, as is the case for the Martian and lunar meteorites in our collections."

It may be that heavy asteroid bombardments did more to form our solar system than planetary scientists originally thought. Fortunately such events are not so common anymore.

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