Researchers model the violent birth of Martian moons


Study stirs up debate on whether Phobos and Deimos are captured asteroids or the aftermath of a planetary collision. Lauren Fuge reports.


An image of Phobos, captured in 1976 by one of the Viking orbiters. Is it a captured asteroid, or a bit of Mars itself?
An image of Phobos, captured in 1976 by one of the Viking orbiters. Is it a captured asteroid, or a bit of Mars itself?
SSPL/Getty Images

The two moons of Mars may have formed after a violent crash early in the solar system’s history, according to a new model.

Roughly 4.5 billion years ago, the solar system was in disarray as gravity sculpted planets, moons and asteroids from the swirling debris generated by the birth of the Sun. In this tumultuous period, a Mars-sized object is thought to have slammed into Earth — and when the dust settled millions of years later, a moon had formed in orbit around our planet.

Now, US researchers think the Martian moons formed in a similar way, just on a smaller scale.

In a paper published in Science Advances, astrophysicists Robin Canup and Julien Salmon from the Southwest Research Institute in Colorado, US, explore a new model showing that the two moons, Phobos and Deimos, could have been created when an object the size of a dwarf planet collided with the planet. The energetic impact would have boiled off any water vapour present, leaving the ejected satellites dry.

The origin of Phobos and Deimos has been debated for decades. They look more like asteroids than moons — indeed, they’re among the smallest in the solar system, at just 22 and 13 kilometres across respectively — leading many researchers to believe they were indeed originally asteroids, nudged into orbit around Mars by Jupiter’s influential gravity.

But unlike most captured bodies, they have relatively smooth and stable orbits. Other astronomers think they formed from a disc of debris left over from Mars’ birth — or from a massive impact.

Canup and Salmon’s new model is the first to determine what kind of cosmic clash was necessary to create the moons.

It demonstrates that when an object slammed into Mars, it would have created either the Borealis, Utopia or Hellas basin on the surface and produced a disc of debris around the planet. In order for Phobos and Deimos to form, the disc must have been fairly small, which then limits the size of the impacting body.

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

This puts the object at between 500 and 950 kilometres across.

Co-author Salmon explains that in the model, “the outer portions of the disc accumulate into Phobos and Deimos, while the inner portions accumulate into larger moons that eventually spiral inward and are assimilated into Mars.”

If the impactor — and therefore the disc — had been any larger, it would have created massive inner moons and prevented the survival of tinier ones like the pair in existence.

Jonti Horner, an astronomer at the University of Southern Queensland, Australia, says that the argument made by the research is compelling, as it is able to reasonably explain away some of the problems with earlier theories.

“The model they propose is really elegant and it makes a lot of sense,” Horner says. “The idea of these two moons being potentially the last survivors of a larger number of tiny moons left over after a large impact is kind of cool — and fits with the fact we know that, eventually, Phobos will spiral in far enough to be destroyed too, leaving Deimos alone.”

Andrew Prentice, astronomer at Australia’s Monash University, disagrees, favouring the captured asteroid theory. He notes that in the case of the Earth-Moon system, increasing complexity from real data means that “the validity of the impact model has become quite muddied”.

“If Canup is correct then the composition of [the moons] should closely resemble that of Mars,” Prentice says. “But if the asteroid capture hypothesis is correct, then Phobos and Deimos should have chemical compositions very similar to that of the rocky asteroids that orbit between Mars and Jupiter.”

So, who is right?

Luckily, the theories will be “testable with the missions scheduled to visit Mars’ moons in coming years,” says Horner.

In 2024, the Japan Aerospace Exploration Agency (JAXA) will launch the Mars Moons eXploration (MMX) mission. This robotic space probe aims to land on the surface of Phobos and do a spot of rock-collecting, then return its samples to Earth by 2029.

Then the mystery of this duo’s formation should finally be answered.

“The nice thing about this Mars paper is that [Canup] is sticking her neck out before the arrival of the MMX data,” Prentice concludes. “That is how science should be done.”

  1. http://dx.doi.org/10.1126/sciadv.aar6887
  2. http://dx.doi.org/10.1126/sciadv.aar6887
  3. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/98JE02248
  4. https://www.nature.com/articles/ngeo2742
  5. https://www.sciencedirect.com/science/article/pii/S0019103515000639
  6. http://global.jaxa.jp/
  7. http://mmx.isas.jaxa.jp/en/
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