Phobos has orbited Mars for billions of years, yet scientists have not determined exactly how the small moon formed. One theory suggests that Mars captured Phobos as an asteroid, while another proposes that it formed from debris thrown into space when a massive object slammed into the Martian surface. Solving the mystery may depend on uncovering something researchers still cannot observe directly: Phobos’ internal structure.
The moon’s hidden interior remains a major “known unknown” for planetary scientists. At a recent European Geosciences Union General Assembly in Vienna, researchers examined the question by modeling subtle variations in Phobos’ geophysical measurements, particularly around the Stickney Crater.
Stickney Crater May Hold Clues to Phobos’ Origins
Stretching roughly 9 kilometers across, Stickney Crater could provide important clues about Phobos’ origins. If the moon formed from material ejected by a massive impact on Mars, the event that created Stickney may have occurred around 4.2 billion years ago. If Phobos is instead a captured asteroid, the crater-forming collision could have taken place much later, around 2.6 billion years ago.
In a 2026 study published in Monthly Notices of the Royal Astronomical Society (MNRAS), Benjamin Haser and his co-author, Thomas Andert, report that current estimates suggest Phobos has a porous interior that could contain water ice. They propose that detailed measurements of the moon’s gravitational field could help test these theories, particularly if the impact that formed Stickney Crater left behind a concentrated zone of denser, compacted material beneath the surface.
“The Stickney event is one of the most important events in Phobos’ history,” Haser, a doctoral student in planetary science at Germany’s Universität der Bundeswehr München, told me in Vienna. He added that studying the impact in greater detail could help scientists determine how Phobos itself originated.
More Than Just an Ordinary Rock
Although Phobos is small and irregularly shaped, it is far more complex than simply being a “rock in orbit,” Haser says.
Despite its relatively complex history, Phobos remains remarkably small, measuring just 22.2 kilometers in mean diameter and completing an orbit around Mars in only 7 hours and 39 minutes.
Scientists have proposed two leading explanations for how the moon formed.
The first suggests that a massive impact struck Mars and sent fragments back into orbit. The debris eventually formed a disk around the planet, which later gave rise to Phobos and Deimos, Haser and Andert explain in their MNRAS study. The second theory points to asteroid capture. Phobos’ gravitational field and internal structure remain difficult to reconcile with a single formation theory.
Mapping and analyzing Phobos’ gravitational field is an essential step toward revealing the moon’s internal structure and, ultimately, determining how it formed, Haser explained in his EGU 26 paper. Current estimates indicate that Phobos may have a porous interior that could contain water ice, along with a denser concentration of material around its equatorial region.
Is Phobos a Giant Cosmic Sponge?
“Haser says an impact that powerful would normally shatter Phobos unless its extremely low, uniform density allowed it to absorb the collision like a sponge.” He adds that the impact zone would have reached extremely high temperatures, enough to melt and compress the rock beneath the crater.
Could Phobos Be a Loose Pile of Rubble?
Haser says Phobos fits well with the idea that it was once a captured asteroid. Its irregular shape closely resembles that of a rubble-pile asteroid, he explains.
However, he points out that scientists still struggle to reconcile Phobos’ current gravitational field, shape, density, spectral properties, and orbital evolution into a single, consistent geophysical model. Its unusual shape and close proximity to Mars make it even more difficult to interpret the moon’s gravity field and determine what lies beneath its surface.
“In the paper, we examine how a compressed mass beneath Stickney Crater influences Phobos’ gravitational signal, moments of inertia, and libration amplitude,” Haser says. Libration refers to the way Phobos subtly wobbles and oscillates as it orbits Mars.
An Unusual Orbital Path
“Phobos follows a remarkably unusual orbit. It circles very close to Mars, gradually spiraling toward the planet, and will eventually either break apart or collide with Mars,” Haser says. He explains that this makes Phobos more than a remnant of the distant past—it is also a geophysical system that continues to evolve today.
Japan’s upcoming Martian Moons Exploration (MMX) mission, which is scheduled to launch in late 2026, aims to place its spacecraft in a quasi-stable orbit around Phobos. The maneuver will be challenging because, as Haser explains, Phobos does not have a truly stable orbit of its own.
“Phobos’ gravitational field is strongly dominated by Mars’ gravity,” Haser says.
Despite this difficulty, the MMX spacecraft will employ two sampling systems to gather material from Phobos’ surface. A core sampler will extract material from depths of up to 2 centimeters, while a pneumatic sampler contributed by NASA will use pressurized gas to lift surface particles into a collection container, according to the Japanese space agency JAXA.
The collected samples are expected to return to Earth by mid-2031 inside a specially designed capsule capable of surviving atmospheric re-entry.
What does Haser believe is the greatest mystery surrounding Phobos?
According to Haser, the central mystery is not simply what Phobos is made of, but what type of internal structure could account for all of its observed characteristics at once. Solving that puzzle could help scientists determine whether Phobos was captured as an asteroid, formed from debris produced by a massive impact, or arose through a more complicated combination of processes.
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