Mars's Peculiar Moon Deimos Was Shaped by a Single Catastrophic Impact
Of all the moons in our Solar System, few are as enigmatic as the two tiny satellites orbiting Mars. Both Phobos and Deimos are small, irregularly shaped bodies — lumpy and potato-like — that have puzzled astronomers for generations. For decades, the prevailing hypothesis was that they were captured asteroids, gravitationally snagged by Mars in the distant past. However, a newer and increasingly compelling explanation suggests they may instead be the remnants of a massive ancient impact on Mars' own surface, flinging debris into orbit that eventually coalesced into the two moons we see today.
But regardless of their ultimate origins, both moons bear the scars of a violent Solar System history. They have been battered, cratered, and reshaped by billions of years of collisions. Now, groundbreaking new research — aided by observations from ESA's Hera spacecraft — is revealing that a single, ancient impact fundamentally transformed the surface of Deimos, the smaller and more distant of the two moons, covering it in a thick, globally distributed blanket of fine dust and regolith.
Hera's Unexpected Encounter with Deimos
In March 2025, the ESA's Hera spacecraft performed a critical gravity-assist maneuver at Mars, using the planet's gravitational field to redirect itself toward its primary destination: the asteroid Didymos and its moonlet Dimorphos. But the flyby was far more than a navigational formality. Scientists had carefully planned the trajectory to bring Hera within close proximity of Deimos, turning the maneuver into a rare and valuable scientific opportunity.
During the flyby, Hera tested its autonomous navigation system by locking onto surface features of the lumpy moon, including impact craters. This was the closest any spacecraft had approached Deimos in decades, and the imagery and data returned were revelatory. Deimos has long been known for its strikingly smooth, regolith-covered surface — a stark contrast to the heavily cratered and deeply grooved terrain of its sibling, Phobos. But Hera's observations revealed something hidden beneath that deceptively tranquil exterior: ancient craters buried under layers of fine dust, craters so thoroughly blanketed that they had been invisible to previous missions.
"Small planetary bodies record the processes that shaped the Solar System, but their surfaces and interiors often evolve in unexpected ways. Deimos, the potato-shaped, 12-km-diameter outer satellite of Mars, has a strikingly smooth surface covered by fine regolith, in contrast to its heavily cratered and grooved sibling, 22-km-diameter Phobos."
— Raducan et al., Nature Astronomy, 2025
A Single Impact to Explain It All
The new research, published in the prestigious journal Nature Astronomy under the title "Deimos's shape and geology explained by a subcatastrophic impact," offers a bold and elegant explanation for Deimos' smooth appearance. The lead author is Dr. Sabina Raducan from the Space Research and Planetary Sciences department at the University of Bern, who also serves as co-chair of the Hera Impact Physics Working Group for ESA's Hera mission.
The key to the mystery lies at Deimos' south pole, where a massive, ancient depression stretches approximately 10 kilometers wide — nearly the entire diameter of the 12-km moon itself. This enormous crater dominates the moon's southern hemisphere and has long raised questions about its origin and its relationship to the moon's overall geology. Raducan and her colleagues propose a striking hypothesis: that a single, large-scale impact that excavated this south polar depression is also directly responsible for the thick, globally distributed regolith that blankets Deimos today.
To understand why this is so significant, consider the scale of such an impact relative to the size of Deimos. A crater that is nearly as wide as the body it struck represents a subcatastrophic impact — one powerful enough to reshape the entire surface of the moon without completely shattering it. The energy released would have been enormous, ejecting vast quantities of material into space, much of which subsequently fell back onto Deimos' surface in the form of fine dust and rubble, burying older craters and creating the smooth, regolith-rich terrain we observe today.
The Power of Smoothed Particle Hydrodynamics Simulations
To rigorously test this hypothesis, Raducan and her team employed one of the most sophisticated computational tools available in planetary science: the Bern Smoothed Particle Hydrodynamics (SPH) code. SPH simulations are a class of computational fluid dynamics method particularly well-suited for modeling violent astrophysical events involving large surface deformations, fragmentation, and complex material interactions. Unlike grid-based methods, SPH represents matter as a collection of discrete particles, making it ideal for simulating the chaotic dynamics of a massive impact on a small, irregularly shaped body.
"The code runs on a high-performance computing cluster here at the University of Bern and is one of the few codes capable of performing this type of simulation."
— Dr. Sabina Raducan, Lead Author
The team constructed a detailed shape model of Deimos from millions of SPH particles, carefully reconstructing its three-dimensional form — including artificially filling in the south polar depression to simulate the pre-impact moon. They then ran approximately one hundred individual simulations, each requiring approximately one week of continuous computation on a high-performance computing cluster. This painstaking process allowed the researchers to systematically vary the key parameters of the impact: the impactor's mass, size, and angle of approach.
- Impactor size: Between 300 and 360 meters in diameter
- Impact angles tested: Between 0° and 60° from vertical
- Impact velocity: Fixed at 8.2 km/s, based on previous research into Deimos' cratering history
- Number of simulations run: Approximately 100
- Computation time per simulation: Approximately one week on a high-performance cluster
What the Simulations Revealed
The results of this exhaustive simulation campaign were striking in their clarity. The models converged on a best-fit scenario: a small asteroid approximately 320 meters wide struck Deimos at an oblique angle of approximately 45 degrees, traveling at 8.2 kilometers per second. The impact was violent enough to excavate the enormous south polar depression and loft vast quantities of material across the entire surface of the moon, but not so catastrophic as to shatter Deimos entirely — hence the term subcatastrophic.
According to the simulations, the impact generated an enormous plume of ejecta — material blasted from the impact site — most of which was traveling too slowly to escape Deimos' weak gravitational field and instead fell back across the moon's surface. This process, known as ejecta redeposition, blanketed Deimos in a layer of fine regolith that reaches depths of up to 200 meters in some locations. This is the debris blanket that gives Deimos its characteristically smooth appearance and that has buried so many of its older impact craters from view.
The simulations also beautifully reproduce the observed brightness patterns across Deimos' surface, which are linked to the gradual migration of this loose, dust-like regolith across the moon's terrain. The models predict that the regolith behaves more like loosely packed dust than a cohesive rocky material — consistent with what remote sensing observations of the moon have long suggested.
"Our simulation is consistent with patterns of brightness observed across the moon's surface, linked to the gradual migration of regolith that behaves more like loosely packed dust than anything more cohesive. The same is true of the way the southern depression subsequently smoothed out, rather than retaining a sharp crater."
— Dr. Sabina Raducan
A Rubble-Pile Moon: Implications for Deimos' Interior Structure
Perhaps the most profound implication of this research concerns not Deimos' surface, but its interior structure. The simulations required specific physical properties for Deimos' interior in order to reproduce the observed crater morphology and regolith distribution. In particular, the south polar depression's rounded, smoothed-out appearance — rather than a sharp, well-preserved crater rim — required the moon's interior to be relatively weak and porous.
Specifically, the models demanded a low surface cohesion of less than 100 Pascals (about 1/1000th of atmospheric pressure at sea level on Earth) and a high-porosity, low crushing-strength interior. These properties allowed the impact shock wave to be rapidly dissipated within the body rather than propagating efficiently throughout it. This is the signature of what planetary scientists call a rubble-pile body — an object that is not a single coherent rock, but rather a loose aggregate of fragments held together primarily by their own gravity rather than material strength.
This places Deimos in the same structural category as several well-studied small Solar System bodies, including Dimorphos (the target of NASA's DART mission), Ryugu (visited by JAXA's Hayabusa2), and Bennu (the target of NASA's OSIRIS-REx mission). All three have been confirmed or strongly inferred to be rubble piles. This finding has significant implications for our understanding of how Deimos formed and how it has evolved over time.
Crucially, the discovery of buried ancient craters by Hera — craters that predate the south polar impact — provides independent support for this rubble-pile interpretation. If Deimos had a stronger, more cohesive interior, the powerful shock waves from the south polar impact would have propagated efficiently through the body and erased these older craters. Instead, the porous, fractured interior dampened the shock waves before they could do so, allowing the older craters to survive — buried but preserved — beneath the regolith blanket.
- Deimos' interior porosity is comparable to known rubble-pile asteroids such as Ryugu and Bennu
- Surface cohesion is estimated at less than 100 Pascals — extremely low by Solar System standards
- The porous interior acts as a shock absorber, dampening impact waves before they can propagate globally
- This structure may be consistent with either a captured asteroid origin or formation from Mars impact ejecta
Brian May's Unexpected Contribution
In a fascinating and distinctly unusual footnote to this already compelling scientific story, the research team includes Sir Brian May — legendary guitarist of the rock band Queen, but also a serious astrophysicist who completed his PhD in astrophysics from Imperial College London in 2007, some 36 years after beginning his doctoral studies. May is a member of the Hera science team, and his contribution to this particular research was far from ceremonial.
It was May who first identified the buried craters in Hera's imagery of Deimos — not through conventional image analysis, but through stereoscopic depictions of the Hera images, exploiting the same principles of depth perception that underpin 3D photography, a subject on which May is a recognized expert. His trained eye for stereoscopic imagery allowed him to perceive subtle topographic features that were not immediately obvious in standard two-dimensional representations of the data.
"These were really the last pieces of the puzzle I needed – although they weren't immediately obvious to the naked eye. Instead I first made them out via stereoscopic depictions of the Hera images made immediately afterwards by Sir Brian May, who is part of the Hera science team."
— Dr. Sabina Raducan
What This Means for the Origins of Mars' Moons
The finding that Deimos is likely a rubble-pile body carries important implications for the longstanding debate about the origins of Mars' two moons. The captured asteroid hypothesis has long struggled to explain the near-circular, near-equatorial orbits of both Phobos and Deimos — a dynamical configuration that is difficult to achieve through gravitational capture. The competing giant impact hypothesis, which proposes that a large body struck early Mars and flung debris into orbit that later assembled into the two moons, naturally produces satellites in low-inclination, near-circular orbits.
The rubble-pile structure of Deimos is consistent with both hypotheses. As ESA's Hera project scientist Michael Kueppers noted, a rubble-pile structure does not necessarily indicate an asteroid origin; it may simply reflect how the body formed and evolved, whether from captured asteroid material or from impact-generated debris. What it does strongly suggest is that Deimos was never significantly heated or processed in a way that would have compacted or melted its interior — which in turn constrains the conditions under which it formed.
"This simulation therefore implies that Deimos is a rubble-pile body, akin to many asteroids. This does not necessarily mean the moon is in fact a captured asteroid – it might well have formed out of material kicked up from Mars by surface impacts – but more that it may have formed in a similar way and therefore shares comparable properties."
— Dr. Michael Kueppers, ESA Hera Project Scientist
Looking Ahead: The MMX Mission and Future Observations
The researchers are clear that while their impact hypothesis is compelling and well-supported by current evidence, it remains one of potentially several viable explanations for Deimos' morphology. Distinguishing between these possibilities will require higher-resolution data than currently available. Fortunately, that data may not be far away.
The Japan Aerospace Exploration Agency (JAXA) is preparing to launch the Martian Moons eXploration (MMX) mission, with a planned launch in late 2026. MMX's primary scientific objective is ambitious: to land on Phobos and return a physical sample to Earth — the first sample return from a Martian moon. However, the mission profile also includes multiple flybys of Deimos, offering a new and unprecedented opportunity to study the smaller moon in detail.
While MMX will not
