Simulated collisions show how Phobos could preserve Martian rocks

The case for looking for foreign grains

About 10 years ago, the researcher behind the work used laboratory experiments and numerical simulations to show that collisions can leave material on a target surface even at relatively low impact velocities.

Later, NASA’s OSIRIS-REx mission to asteroid Bennu and Japan’s Hayabusa2 mission to asteroid Ryugu identified foreign material associated with impacts on their targets. That precedent helps, but proves nothing about Phobos.

In 2023, the researcher became an MMX participating scientist and joined the mission’s surface science and geology science strategy team, abbreviated SST. The proposed work combines observations from different spacecraft instruments to count craters, measure their sizes and reconstruct impact histories. It also seeks to establish whether foreign material can be identified on Phobos.

The collected material could separate origin stories:

• If the captured-asteroid theory is correct, MMX will be studying a primitive object that formed in the outer solar system.
• If the moons formed after a giant impact on Mars, Phobos could contain Martian rock and fragments from the object that struck Mars. Distinct compositions may reveal the difference.

Recreating impacts in the laboratory

At the University of Kent, the team used a light-gas gun, an apparatus that propels objects at high velocity, to fire projectiles at materials designed to simulate Phobos’s surface under both proposed formation scenarios. The experiments tested how crater formation changes with impact velocity and projectile size. They also tested whether material ejected from Mars could contaminate Phobos.

The researcher writes: “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.”

That supports the proposed delivery mechanism under the tested conditions, not every possible scenario. Only the experimental conditions apply.

The work is being submitted to Monthly Notices of the Royal Astronomical Society, or MNRAS.

With early-career scientists Dr. Emelia Branagan-Harris of the University of Oxford, Dr. Ben Rider-Stokes of The Open University and Dr. Vassi Spathis of the Institute for Space in Leicester, the researcher also tested whether MMX instruments could detect grains of Martian material within Phobos’s surface layer before sampling.

That work remains under revision at MNRAS. The team presented findings at the Europlanet Science Congress in The Hague, Netherlands, in September.

What does this change for MMX sample selection?

If scientists can identify foreign grains remotely, they can better interpret observations and select soil samples that provide the most useful information. Phobos’s samples and crater record could then help distinguish captured asteroids from giant-impact debris and trace how material moved through the solar system.

The next step looks beyond rock composition. At the University of Leicester, a team called Hard Rocks is designing experiments to investigate the possible presence of organic matter on Phobos.

The researcher cautions: “Organic compounds are not necessarily evidence of life; they can also form through nonbiological processes or be delivered by asteroids.” Such compounds could add information about Phobos’s history and how its surface evolved.

Neither laboratory result settles the moons’ origins, and neither MNRAS paper is published; one is being submitted and the other is under revision. The claim is narrower, and more defensible: remote detection and careful sampling could turn Phobos into a recorder of collisions involving Mars, Phobos itself and more distant bodies.