Mars has two small moons, Phobos and Deimos, but their origins remain uncertain. Were they asteroids captured by Mars’ gravity, or did they form from debris following a giant impact on the planet?

Japan’s Martian Moons Exploration (MMX) mission aims to solve this mystery by collecting samples of soil from Phobos, the largest of the two moons. Laboratory experiments in the UK are helping prepare scientists to interpret its findings.

MMX launched successfully on October 19 atop an H3 rocket from Tanegashima Space Center in southern Japan. About two weeks ago, ground controllers received the first signal from the spacecraft to check it is communicating normally.

Studying small bodies and comparing them with meteorites – natural samples of space rocks that reach Earth – helps scientists understand the formation and evolution of the Solar System.

Observations from the Soviet Union’s Phobos 2 spacecraft, which gathered data from its position in Martian orbit in 1989, and by researchers using Nasa’s Infrared Telescope Facility in the 2010s, have provided important hints about Phobos’ origins.

They showed that the Martian moon has spectral properties (the way a planetary surface absorbs and reflects light, providing information about its composition) resembling those of dark, primitive asteroids known as D-types. These are thought to have originated in the outer Solar System before some migrated towards the Sun.

For Mars to have captured two separate asteroids would be difficult to explain. An alternative proposal suggests that Mars captured fragments of an asteroid torn apart by its gravity. These fragments subsequently formed a disc of material from which the moons evolved.

However, this raises the question of how an object from the distant outer Solar System reached Mars. Another theory proposes that Phobos and Deimos formed from debris ejected into orbit when a large object struck Mars.

Although this scenario can explain aspects of their orbits, questions remain about whether their composition matches that expected from such an impact. Distinguishing between these possibilities is a major goal of the MMX mission, which will investigate both moons and return samples from Phobos to Earth.

What if Phobos contains material from elsewhere? Mars’ largest moon may hold another important clue: material delivered from another part of the Solar System.

Large impacts on Mars can eject rocks into space, some of which could eventually land on Phobos. Asteroids striking the moon directly could also leave fragments behind. Scientists call this exogenous material.

Preparing for MMX

About ten years ago, I investigated this process through laboratory experiments and numerical simulations. My research demonstrated that objects colliding with another planetary body can leave material on a target’s surface even at relatively low-impact velocities.

Years later, Nasa’s Osiris-Rex mission to the asteroid Bennu and Japan’s Hayabusa2 mission to the asteroid Ryugu identified foreign material associated with impacts on their target asteroids. Finding fragments of rock from Mars or asteroid fragments on Phobos could similarly help reconstruct the moon’s impact history and shed light on its origin.

In 2023, I was selected as a participating scientist in the MMX mission and joined its surface science and geology SST (science strategy team). My research focuses on the processes shaping the moons’ surfaces, particularly impacts.

By combining observations from different spacecraft instruments, I aim to study crater numbers and sizes to understand the moons’ impact histories. I also want to establish whether foreign material can be identified on Phobos.

If the captured-asteroid theory is correct, MMX will be investigating a primitive object that formed in the outer Solar System. If the moons formed following a giant impact on Mars, they could still contain fragments of rock from Mars and the object that collided with it. This may be evident in the distinct compositions of material collected by MMX from Phobos.

Recreating space impacts

Laboratory experiments have already been helping scientists predict what the MMX spacecraft’s instruments might detect. With financial support from Europlanet, a society that supports planetary scientists, I launched an experimental campaign at the University of Kent.

The tests used a light-gas gun (an apparatus that propels objects at high velocities) to fire projectiles at material designed to simulate the surface of Phobos under both proposed formation scenarios.

We investigated how crater formation depends on impact velocity and the size of the object. We also tested whether material ejected from Mars could contaminate Phobos.

Our experiments demonstrated that this process is feasible under the conditions that applied during the experiment. These tests can replicate the natural process and impact speeds. This work is being submitted to the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

Together with early-career scientists Dr Emelia Branagan-Harris of the University of Oxford, Dr Ben Rider-Stokes from The Open University and Dr Vassi Spathis of the Institute for Space in Leicester, we also investigated whether MMX’s instruments could detect grains of material from Mars within Phobos’ surface layer before the spacecraft collects samples. This work is currently under revision in MNRAS.

Future tests

If foreign material can be identified remotely, scientists will be better equipped to interpret observations and select soil samples that will provide the most useful information. We presented our findings at the Europlanet Science Congress in The Hague, Netherlands, in September.

What comes next? At the University of Leicester, our team of scientists – called Hard Rocks – is designing experiments to investigate the potential presence of organic matter on Phobos.

Organic compounds are not necessarily evidence of life, they can also form through non-biological processes or be delivered by asteroids. But they can tell us much about the history of Phobos, adding to our understanding of how the Martian moons formed and evolved.

Whether they are captured asteroids or remnants of a giant impact, their surfaces and returned samples could reveal important clues about their formation. If Phobos also preserves material delivered from Mars or distant asteroids, these tiny moons could tell a much bigger story about the collisions and movements of material that shaped our Solar System.