Origin Of Mars’ Moons Finally Established: It Was A Large Asteroid That Did It, According To New Study

The research reveals Mars’ two moons, Phobos And Deimos, were created after a large asteroid impact, and that they weren't alone.

3D illustration of Mars and its two Moons.
Vadim Sadovski / Shuterstock

The research reveals Mars’ two moons, Phobos And Deimos, were created after a large asteroid impact, and that they weren't alone.

Scientists have been trying for decades to unravel how Mars’ moons came to be.

Since their discovery in 1877, two main hypotheses have been largely circulated: one suggests the two moons (Phobos in particular) are in fact tiny asteroids trapped by the planet’s gravitational force; the other says they were born from a cloud of debris that formed around Mars after a giant asteroid impact.

A new study, however, offers evidence that the asteroid which set everything in motion was not actually giant but only large, Space.com reports.

Moreover, it seems the Red Planet originally had more than two moons to begin with and that these were eventually “assimilated into Mars,” leaving just Phobos and Deimos behind.

The research, conducted by scientists at the Southwest Research Institute (SwRI), reveals that the two tiny Mars moons were formed much like our own, but the asteroid impact that led to their creation was of a significantly smaller scale than in the case of Earth.

“Ours is the first self-consistent model to identify the type of impact needed to lead to the formation of Mars’ two small moons,” lead author Dr. Robin Canup said in a SwRI news release.

What separates this study from other investigations that also supported the asteroid impact theory is that it considerably scales down the size of the impactor.

In fact, according to the paper, the space rock that crashed into Mars kicking up debris that later clumped into the planet’s two moons was no bigger than Ceres or Vesta — the two largest inhabitants of the asteroid belt, notes Space.com.

“We find that a large impactor — similar in size to the largest asteroids Vesta and Ceres — is needed, rather than a giant impactor,” Canup pointed out in the news release.

The scientists base their conclusions on impact simulations and “state-of-the-art models” that compare Mars’ diameter with that of Earth and the sizes of Phobos and Deimos with that of our moon.

Their results show that Mars’ two moons were formed after “an oblique impact” with an object 1,000 times smaller than the Red Planet.

This means that Phobos and Deimos were created by “a much less massive impactor than previously considered,” the authors wrote in their paper, published this week in the journal Science Advances.

By comparison, the object that rammed into Earth and gave rise to our moon was a Mars-sized giant.

After this large asteroid impact, the debris that floated around the planet had two different fates, depending on how far from Mars it was projected, explained SwRI researcher Julien Salmon, another author of the paper.

“The outer portions of the disk accumulate into Phobos and Deimos, while the inner portions of the disk accumulate into large moons that eventually spiral inward and are assimilated into Mars.”

The team debunked the theory that Phobos and Deimos are actually captured asteroids that were drawn out of the asteroid belt by Mars’ gravitation by showing that the two moons are made of the same material as the Red Planet.

“The model also predicts that the two moons are derived primarily from material originating in Mars, so their bulk compositions should be similar to that of Mars for most elements,” Canup explained.

Other studies have also indicated that the Mars moons had a violent beginning and were smacked into existence by an asteroid collision. But, according to this latest model, they got the perpetrator wrong.

“Larger impacts advocated in prior works produce massive disks and more massive inner moons that prevent the survival of tiny moons like Phobos and Deimos,” Salmon clarified matters once and for all.