The moons of Mars, Phobos and Deimos, have intrigued astronomers for decades with their strange shapes and surprisingly regular orbits. While they resemble captured asteroids, their orbital trajectories contradict this classic explanation. A new study now proposes an unprecedented origin: a gravitational cataclysm caused by an asteroid dangerously approaching the red planet.
Simulation of the trajectories of fragments from an asteroid broken apart by Mars' gravity.
Credit: NASA Ames/Durham University
Researchers used advanced computer simulations to test an intriguing hypothesis. When a giant asteroid crosses Mars' "Roche limit," where tidal forces are so intense that they can disintegrate an object, it gets pulverized. The debris then forms a disk around the planet, from which moons like Phobos and Deimos could gradually assemble.
This explanation resolves several mysteries. Unlike other captured moons, such as Triton around Neptune, Phobos and Deimos follow circular orbits aligned with Mars' equator. These characteristics would be consistent with local formation from a debris disk rather than a captured asteroid.
Previously, another theory suggested that these moons originated from an impact on Mars similar to the collision that formed Earth's Moon. However, notable differences between Phobos and Deimos, particularly in their respective altitudes, pose challenges to this model. The new scenario proposed by Jacob Kegerreis and his colleagues may thus provide a coherent answer.
The simulations, conducted on supercomputers at Durham University, explored various sizes, speeds, and approach angles for the asteroid. These calculations show that a significant portion of the debris remained in orbit, gradually forming smaller particles that coalesced into two moons.
Kegerreis' model also predicts a different distribution of materials, allowing Deimos to form farther away from Mars. Jack Lissauer of NASA notes that this implies a more modest "parent" asteroid, yet one large enough to explain the genesis of the two moons.
Mars' two moons, Phobos and Deimos, depicted in orbit around the red planet.
Credit: NASA
The hypothesis will be tested as early as 2026 with the Japanese Space Agency's (JAXA) Martian Moons eXploration (MMX) mission. The goal: to bring back samples from Phobos for analysis. NASA's MEGANE instrument will play a key role in identifying the chemical elements present on the moon's surface.
The data collected might help settle the debate among these different theories. If the samples reveal materials similar to those of Mars, it would support the impact hypothesis. Conversely, a composition closer to that of asteroids would bolster the debris disk theory.
Meanwhile, the simulations continue to open new perspectives. Researchers hope to apply these models to other phenomena, such as the formation of Saturn's rings or the origins of other moons in the Solar System.