How did Mars get its moons? Japan’s MMX space mission aims to solve the puzzle

The Earth got its moon when a small planet collided with our planet more than 4 billion years ago. This impact hurled molten rock into space, which eventually coalesced to form the satellite we can see every night from across our planet.

But what of our neighbor, Mars? The red planet has two small, irregularly shaped moons called Phobos and Deimos. There are several theories on how these natural satellites formed, but no clear front-runner. Soon, a Japanese-led space mission called Martian Moons Exploration (MMX) will begin its hunt for the answer.

On Monday, Oct. 19, MMX is due to lift off on an H3 rocket from Tanegashima Space Center in southern Japan. The mission will aim to land on Phobos and collect a sample of soil. The spacecraft will then send this payload to Earth for study in laboratories.

Of the two Martian moons, Phobos is the larger and orbits closer to the planet. It has a mean diameter of 22.2km (13.8 miles) and a mass of 10,637 million million kg. Deimos has a mean diameter of 12.5km (7.8 miles) and a mass of 1,480 million million kg.

The moons were discovered during a six-day window in 1877 by the American astronomer Asaph Hall, who was working at the U.S. Naval Observatory in Washington, D.C.

Two main hypotheses dominate discussions of how Mars got these moons. The giant impact theory proposes that a celestial object a third of Mars’ size hit the planet early in its history, ejecting material into space.

The general mechanism is similar to how scientists think Earth’s natural satellite formed. But in the Martian impact theory, a single “giant” moon was created by the ejected material, along with a host of smaller satellites.

Over time, each one broke up and fell to Mars’ surface. Simulations by Pascal Rosenblatt, from the Laboratoire de Planétologie et Géodynamique (LPG) in Nantes, France, and his colleagues show that Phobos and Deimos would have been the sole moons to survive.

Gravitational capture

The other main idea is known as the captured asteroid theory, which goes back to the 1960s. This suggests that Phobos and Deimos were originally asteroids that wandered so close to Mars they were captured by the planet’s gravity.

According to this idea, the moons may have originated from within or just beyond the asteroid belt that lies between Mars and Jupiter. The moons might even have been what’s known as a binary—two asteroids that orbit each other.

The moons are characterized by their low density, which implies they have porous interiors.

The captured asteroid hypothesis, however, requires an extremely dense Martian atmosphere to explain how the asteroids were trapped by tidal forces. Mars today has a thin atmosphere mainly composed of carbon dioxide. Also, asteroids captured by planets tend to have highly eccentric orbits. The orbits of Phobos and Deimos around Mars are, by contrast, almost circular.

Scientists have also proposed other theories for the origins of these moons, but they command lower levels of support among the scientific community. One visualizes the origin of these moons as debris from the destruction of a single “proto-moon.”

Another lesser-known idea proposes that Mars undergoes billion-year cycles where rings of rocky material orbiting Mars coalesce over time into moons.

Once the moons are formed, they are eventually ripped apart by gravitational (tidal) forces to form new rings. Under this scenario, we just happen to live in an era where the debris is in the form of moons and not as rings.

Solving the mystery

Amid this ongoing uncertainty surrounding the moons’ origins, MMX aims to collect more than 10g of material from Phobos—marking the world’s first sample return mission from a Martian moon.

If the samples contain a large amount of material from Mars itself, along with material from the asteroid that collided with the red planet, then it would favor the giant impact theory. If the composition of Phobos is similar to that of meteorites from known categories of asteroid, it would favor the asteroid capture theory.

MMX’s round trip to Mars will take five years in total. It will arrive at the red planet in late 2027 and return to Earth in 2031.

The spacecraft consists of three modules, one to get to Mars, one to land on Phobos and another to return the sample to Earth. It houses 11 scientific instruments, including a gamma-ray and neutron spectrometer for measuring the chemical composition of Phobos, two cameras and a near-infrared instrument for characterizing minerals. MMX will also deploy a 25kg (55-pound) rover called Idefix on Phobos’ surface.

For decades, the origin of Mars’ unusual moons has been subject to intense debate. Now, with the launch of this space mission, scientists could be on the verge of a breakthrough.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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