Artist impression of JAXA’s MMX spacecraft around Mars (JAXA).

Japan in planning to launch a mission to visit the two moons of Mars in 2024. The spacecraft will touchdown on the surface of Phobos, gathering a sample to bring back to Earth. But what is so important about a moon the size of a city?

Unlike the spherical shape of the Earth’s moon, the Martian moons resemble asteroids, with an asymmetric lumpy potato structure. This highlights one of the first mysteries about the pair: how did they form?

Light reflected from the moons’ surface gives clues to their composition, as different minerals absorb particular wavelengths of radiation. If an object reflects more light at longer wavelengths, it is said to have a spectra with a red slope. This is true of both Phobos and Deimos, which appear very dark in visible light but reflect more strongly in longer near-infrared wavelengths. It is also true of D-type asteroids, which orbit the sun in the outer edge of the asteroid belt that sits between Mars and Jupiter.

The similarities between both their lumpy shape and reflected light has led to speculation that the two moons are captured asteroids, snagged by Mars’s gravity after a collision in the asteroid belt scattered them towards the sun.

How did the martian moons form? Were they asteroids captured by Mars’s gravity or formed during a giant impact event? (Elizabeth Tasker)

However, such a gravitational lasso would typically move the captured object onto an inclined or highly elliptical orbit. Neptune’s moon, Triton, is suspected to be captured as it orbits in the reverse direction to Neptune’s own spin and on a path tilted from the ice giant’s equator by 157 degrees.

Yet both Phobos and Deimos sit on near-circular orbits in the equatorial plane of the planet. This configuration suggests the moons may have been formed in a giant impact with Mars, which threw debris into orbit and this coalesced into the two moons.

This mystery will be one of the first tackled by Japan’s planned Martian Moons eXploration (MMX) mission, that is due to launch in the fiscal year of 2024. Onboard are multiple instruments designed to unpick the moons’ composition from close quarters, providing far more detailed information than that from distant reflected light.

If these moons are impact debris, their composition should be similar to Mars. Captured asteroids would show a more unique rocky formula. The spacecraft will also gather a sample from one of the moons, digging below the surface layer to scoop up at least 10 grams (.35 ounces) of material from a minimum of 2 centimeters (.78 inches) deep. The moon selected for landing is Phobos, for a series of exciting reasons.

Artist impression of the MMX spacecraft (JAXA).

Initially, Phobos seems like the more difficult choice. The inner of the two moons sits just 5,826 miles (9,380 km) from the Martian surface, compared to our moon’s average distance of 238,855 miles (384,400 km) from the Earth. This tucks Phobos deep into Mars’s gravitational well, meaning that more propellant is needed to overcome the attraction of the planet as the spacecraft makes observations in the moon’s vicinity.

Mars’s presence also changes the gravitational pull on the moon’s surface, in the same way that standing on a slope on Earth can cause you to lose your balance. This results in areas on the tiny moon where the slightest wobble could send you skittering off the moon’s surface entirely.

Yet the proximity to Mars is also an advantage. In the same way that meteorites from Mars occasionally hit the Earth, material from Mars can also be thrown onto its moons. As Phobos is vastly closer to Mars than the Earth, much more fragile material can survive the journey to its surface, including sedimentary rocks such as clays that are formed in reactions with water.

ESA’s Mars Express tracks Phobos as the moon passes in front of the spacecraft’s camera (ESA/DLR/FU Berlin).

The surface of Mars is currently cold and dry. But in the past, Mars may have had an environment similar to the Earth, with liquid seas and a possibly habitable environment. Through its history, large and small impacts with the Martian surface have been kicking up material to coat its moons, making the pair time capsules for Mars’s own history. Simulations suggest that the MMX sample from Phobos could contain over 100 grains from Mars, significantly more than that from the more distant Deimos. This is not enough to mask the moon’s own composition, but is sufficient to delve into Mars’s past.

Phobos’s own composition may also be more interesting than its sibling. While the surface of both moons have asteroid-like red-slope spectra, Phobos also has regions which reflect less strongly at longer wavelengths. These ‘bluer’ regions may indicate different minerals, revealing more about how the moon formed.

While neither Martian moon has been previously explored in detail, the existing data is more detailed for Phobos ( approximately 23 km or 13 miles in diameter) than Deimos (about 12.6 km or 8 miles in diameter.) This is invaluable in selecting a landing site for the spacecraft.

Image of Phobos captured using the optical and near infrared filters of the HiRISE camera on the NASA Mars Reconnaissance Orbiter, showing two distinct regions that may have different compositions (NASA / JPL / University of Arizona).

Japanese scientists and engineers are expert in collecting samples from small celestial objects, with Hayabusa and —just last year— Hayabusa2 gathering material from asteroids. But while these spacecraft only briefly touched the surface to gather their treasure, the MMX spacecraft is expected to be on the surface of Phobos for several hours and operate a corer to dig into the moon’s top layers.

The spacecraft will also carry a rover designed by the French (CNES) and German (DLR) space agencies, the same group behind the MASCOT lander on Hayabusa2 and the Philae lander on the European Rosetta comet mission. The perfect landing site will need to consider factors such as the gravitational environment (remember the bully, Mars), local obstacles such boulders or steep slopes and scientific interest.

There is also the fact that Phobos is on an unstable orbit which will eventually spiral into the red planet within the next 50 million years. So… we better go now.

The MMX spacecraft will explore the moons and environment around Mars for three years before returning to Earth in 2029. Regardless of which of the moons’ origin theories proves to be true, the sample will be extremely exciting to understanding a topic close to everyone’s hearts: us.

Past the ice line, conditions were cold enough for water to solidify into ice and form part of the planets, asteroids and comets. This water-rich material was then scattered inwards, passing the outermost terrestrial planet, Mars. (Elizabeth Tasker)

Orbiting further from the sun than the Earth, Mars sits at a gateway in our solar system. Just beyond its orbit is the so-called ice line where temperatures were cold enough for ice to solidify and become part of the forming planets. The terrestrial planets such as the Earth and Mars may therefore have formed from dry rocky silicates, without the water that gives Earth its life today.

One theory as to how life began on Earth is that water and early organics was delivered via asteroids and comets that formed beyond the ice line. Phobos and Deimos may two of these objects, grabbed by Mars before they could strike one of the planets, and are now evidence of this process in action. If the moons were carved from Mars itself, their composition will preserve conditions on the early Mars, revealing a glimpse of what might be an early habitable world.

JAXA’s MMX mission to the Martian moons (JAXA).

The exploration by MMX may also play a role in making the planet habitable once again. Phobos has been considered for the first human base in the Mars system, avoiding the challenging descent and ascent from the Martian surface. To investigate the possible health implications for a Phobos-based crew, the MMX spacecraft will carry a radiation detector to measure the environment, as well as provide invaluable data on surface operations on a low-gravity object.

Last week, the Japan Aerospace Exploration Agency (JAXA) approved the mission transition from the pre-project phase to the project phase; an important milestone towards launch. The mission focus will now switch from predominantly research and analysis to developing software and hardware development for the mission.

In addition to the European-built rover, MMX will also carry instruments developed by NASA for examining the chemical elements of the moons (the gamma ray and neutron spectrometer, MEGANE) and a pneumatic sampler. It is a true international team for a mission whose results relate to us all.

The development of the MMX mission can be followed both from their website and twitter feeds.