Mars' innermost moon of Phobos has long puzzled planetary scientists who have continually debated whether it's a captured asteroid or formed from debris after a giant impactor struck the Martian surface. The key to solving the mystery mainly rests with a better understanding of Phobos’ internal structure which unfortunately remains a ‘known unknown.’
But a presentation given at the recent European Geosciences Union general assembly in Vienna, tries addressing these questions by modeling slight variations in Phobos’ so-called geophysical observables, specifically at the site of the moon’s Stickney Crater.
In the giant-impact hypothesis, the impact that formed Phobos’ 9km-diameter Stickney Crater could be about 4.2 billion years old. In the asteroid-capture hypothesis, the Stickney-forming event could be significantly younger, around 2.6 billion years old.
Current estimates suggest a porous interior with possible water–ice content, note Haser and co-author Thomas Andert, in a 2026 paper appearing in the journal The Monthly Notices of the Royal Astronomical Society (MNRAS). Detailed gravitational field mapping emerges as a crucial method to address these open questions motivated by the hypothesis that the Stickney impact produced a localized zone of densified material, the authors note.
The Stickney event is one of the most important events in Phobos’ history and understanding it better might help to resolve its origin, Benjamin Haser, a doctoral student in planetary science at Germany’s Universität der Bundeswehr München, told me in Vienna.
Phobos is small and irregular, but it is not just a simple ‘rock in orbit,’ says Haser.
Even so, with a mean diameter of only 22.2km and a Mars orbital period of only 7 hours and 39 minutes, Phobos is tiny.
Two theories of Phobos’ origin have emerged.
The first theory suggests a giant impact onto Mars, causing the fragments to bounce back into orbit, creating a debris disc which finally results in the two moons Deimos and Phobos, Haser and Andert write in their MNRAS paper. In contrast, spectroscopic properties and asteroid capture models suggest that both moons originated from asteroids and were captured by Mars’s gravity field, the authors write.
Determining and understanding Phobos’s gravitational field is a fundamental step toward constraining its interior and, consequently, its origin, Haser noted in his EGU 26 paper. Current estimates suggest a porous interior with possible water-ice content and a denser mass concentration in its equatorial region, he noted.
You would assume that such an impact would have shattered Phobos, unless it has a very low homogeneous density, like a sponge that can absorb that kind of impact, says Haser. And at that impact region, there must be very high temperature that melted and compressed the stone beneath it, he says.
Haser says Phobos aligns well with the captured asteroid scenario. Its irregular shape looks very much like a rubble pile asteroid, he says.
But Haser notes that it’s difficult to connect Phobos’ present-day gravity field, shape, density, spectral characteristics, and orbital evolution into one consistent geophysical picture. At the same time, its shape, and proximity to Mars make the interpretation of its gravity field and internal structure quite challenging, he says.
In the paper, we investigate how a compressed mass beneath Stickney crater affects the tiny moon’s gravitational signal, moments of inertia, and libration amplitude (essentially how Phobos wobbles and oscillates), says Haser.
*This is an image of the 2024 version of the MMX spacecraft during MOI (Martian Orbit Insertion). Thrusters are turned off. Credit: JAXA*
Phobos' orbit is dynamically very special; it is very close to Mars, slowly spiraling inward, and will eventually be disrupted or impact Mars, says Haser. This means that Phobos is not only a record of the past, but also an actively evolving geophysical system, he says.
The upcoming Japanese Martian Moons Exploration (MMX) Phobos sample return mission, targeted to launch in late 2026, will attempt a quasi-stable orbit around the tiny moon. This is a difficult task because as Haser points out there truly is no stable orbit around Phobos.
Phobos’ gravity field is strongly overshadowed by the Mars’ gravity field, says Haser.
Even so, the MMX’s main spacecraft will use two sampling mechanisms to collect material from Phobos’ surface. One core sampler will collect matter down to 2cm, while a pneumatic sampler (being contributed by NASA) will use pressurized gas to “loft material into a sample container,” says the Japanese Space Agency (JAXA).
All samples will subsequently be sent back to Earth by mid-2031 via a sample return capsule constructed to withstand re-entry into our atmosphere.
As for what Haser finds most puzzling about Phobos?
The main puzzle, says Haser, is not just what is Phobos made of, but what kind of interior structure can explain all its characteristics simultaneously. Understanding this is essential to distinguish between formation scenarios such as capture, formation from impact-generated debris, or a more complex mixed origin, he says.
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