A projectile with a diameter of 320 meters created the dominant south-polar depression on Mars’ moon Deimos, new simulations reveal. The impact not only excavated this large cavity but also explains the globally distributed fine regolith layer covering the small moon’s surface. Researchers found Deimos’s upper layers are extremely weak and its interior highly porous, more closely resembling recently visited “rubble-pile” asteroids than previously assumed lunar regolith. This suggests that small dark moons and asteroids may converge on similar physical states, despite differing histories.
Deimos’s Smooth Surface Contrasts with Phobos’s Terrain
The unexpectedly smooth surface of Mars’ moon Deimos originates from a single, massive impact, new simulations reveal, an event that also explains the globally distributed layer of fine regolith blanketing the small body. Unlike its heavily cratered companion, Phobos, Deimos exhibits a subdued topography, a characteristic now linked to a collision with a 320-meter-diameter projectile. This impact not only excavated the dominant south-polar depression but also created the fine debris layer observed across the moon’s surface, resolving a long-standing question about its unusual appearance.
Detailed smoothed-particle hydrodynamics simulations, accounting for material cohesion, porosity, and internal friction, pinpointed the impact parameters that best replicate Deimos’s current state. Researchers iteratively adjusted the model, using a detailed shape model of Deimos resolving topography to approximately 100 meters, to achieve a best-fit solution.
The simulations converged on an impactor striking at an angle between -60° and +60°, with a lateral offset between 0 and 2.5 kilometers, producing a cavity consistent with the observed south-polar depression. “The depression on Deimos does not have a unique solution, but for a fixed impact velocity, our simulations converged towards a best-fitting scenario,” the study finds.
With a bulk density of only 1.5 grams per cubic centimeter, Deimos is estimated to be 40-50% porous. This low density, combined with a friction angle comparable to that observed on small asteroids like Dimorphos, Bennu and Ryugu, suggests a loosely consolidated body.
The simulations indicate a relatively low surface cohesion, allowing for the formation of bright downslope streaks observed on Deimos, features indicative of regolith transport. Researchers note that these features indicate regolith transport across the surface, which would be unlikely if it were bound by substantial cohesion, highlighting the interplay between impact history and surface processes.
Meter Impact Excavated South-Polar Depression
Researchers utilized smoothed-particle hydrodynamics simulations, incorporating material cohesion, porosity, and crushing behavior, to reconstruct the impact event and assess its effects on Deimos’s structure. The simulations were configured 0 ± 0.5 kilometers relative to the pole, consistently producing a crater morphology matching the observed south-polar depression, as determined by comparing simulated and observed topographic profiles. The resulting crater size and shape were assessed using rim-to-rim diameter measurements, further refining the impact parameters.
The simulations demonstrate that Deimos’s upper layers possess remarkably low strength, while its interior is highly porous and dissipative. This internal structure closely resembles that of recently visited “rubble-pile” asteroids, such as Dimorphos, Bennu, and Ryugu, rather than the more consolidated regolith typically associated with larger moons. The best-fitting impact scenario indicates that the collision was substantial, yet avoided complete fragmentation of Deimos.
Regolith Distribution Explained by Single Impact
Researchers employed detailed smoothed-particle hydrodynamics simulations to model the impact event and constrain Deimos’s internal properties. One method compared the post-impact shape to current topography, while the other measured the crater diameter against observations from the SPC shape model. The team initially assumed an unconsolidated body with a friction angle of 35 degrees and a cohesion of Y 0 = 1 Pa, finding this sufficient to prevent large-scale slope failure.
Deimos, Mars’ smaller moon, possesses a surprisingly low density, approximately 1.5 grams per cubic centimeter, revealing an internal structure far more open than previously assumed for such bodies. This finding challenges earlier expectations that Deimos’s internal composition would more closely resemble the compacted regolith of our own Moon.
Material Properties Constrain Impact Modeling
The work establishes constraints on the moon’s internal structure, suggesting a surprisingly weak and porous composition. The simulations explored a range of impact scenarios, varying projectile size, velocity, and angle, to determine the best fit with Deimos’s observed morphology. Researchers found that an impact velocity of 8.2 km s −1 most accurately reproduced the 10-kilometer-wide depression. This specific configuration minimized energy transfer that would have resulted in complete fragmentation of Deimos, indicating a collision.
Crucially, the simulations required specific material properties for Deimos to achieve the observed result. This convergence on a single impact event as the origin of both the depression and the global regolith layer offers a compelling explanation for Deimos’s unusual characteristics.
Impact Angle and Offset Affect Crater Formation
The unusual smoothness of Deimos, Mars’ outer moon, stems from a single, oblique impact by a 320-meter-diameter projectile, according to new simulations detailing the formation of the moon’s dominant south-polar depression. These high-resolution smoothed-particle hydrodynamics (SPH) simulations reveal how a specific impact angle and lateral offset were critical in creating the 10-kilometer-wide depression and simultaneously distributing a globally consistent layer of fine regolith across the moon’s surface.
Initial assumptions about Deimos’s composition, including a bulk density of only 1, were used in the simulations. Simulations explored a range of impact parameters, including velocities of 8.2 km s −1, but consistently converged on a specific configuration for the most accurate reproduction of observed features. The preferred scenario involved an impact angle ranging from -60° to +60°, combined with a lateral offset ranging from 0 to 2.5 km. The simulations also revealed constraints on the moon’s surface cohesion, explaining the presence of bright downslope streaks observed by spacecraft.
Deimos Resembles Rubble-Pile Asteroids Post-Impact
This impact event, the team found, didn’t shatter Deimos, but instead left it with an unexpectedly porous interior. Evaluating the accuracy of these reconstructions involved comparing the simulated post-impact shape to Deimos’s current topography. The best-fitting scenario demonstrated a strong correlation between the simulations and actual surface features, with values of f = 0.65 and Y 0 = 1 Pa found to be sufficient to explain the observed surface stability.
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