Mars, our planetary neighbor, has two small, irregularly shaped natural satellites: Phobos and Deimos. Studying these moons offers clues to the Red Planet’s distant past and long-term future.
Naming and Discovery History
The two Martian moons were officially discovered in August 1877 by American astronomer Asaph Hall. Hall spotted Deimos first on August 12, followed by Phobos six days later on August 18. Finding them was challenging due to their diminutive size and proximity to the glare of Mars.
The names Phobos and Deimos are drawn from Greek mythology, specifically the twin sons of Ares, the Greek god of war (known to the Romans as Mars). Phobos personifies fear, and Deimos represents dread or panic, fitting companions for the god of war.
Phobos: The Doomed Inner Moon
Phobos is the larger and innermost of the two satellites, characterized by its non-spherical, potato-like shape. Its dimensions are roughly 27 by 22 by 18 kilometers. Phobos is locked in an extremely close orbit, circling Mars at an altitude of only about 6,000 kilometers above the surface.
This close proximity gives Phobos one of the fastest orbits in the solar system. It completes a lap around Mars in just 7 hours and 39 minutes, significantly faster than the planet’s 24.6-hour rotation period. Consequently, an observer on the Martian surface would see Phobos rise in the west and set in the east several times a day.
The tidal forces exerted by Mars are relentlessly pulling Phobos inward, causing its orbit to decay. Scientists predict that within 30 to 50 million years, this orbital spiral will lead to a catastrophic end. Phobos is expected to either crash into the Martian surface or, more likely, be torn apart by gravity once it crosses the planet’s Roche limit. The resulting debris would then form a temporary ring system around Mars.
Deimos: The Distant Outer Moon
Deimos is the smaller and more distant of the two moons, measuring only about 12.5 kilometers across its longest dimension. It maintains a wider orbit, located approximately 23,460 kilometers from the Martian surface. This distance results in a slower orbital period of about 30.3 hours, which is just slightly longer than a Martian day.
Unlike Phobos, Deimos is slowly spiraling outward, moving away from Mars due to tidal acceleration. Its surface is noticeably smoother than Phobos’s, a characteristic attributed to a thick layer of regolith that has partially filled its impact craters. Due to its small size and distance, Deimos would appear from the Martian surface as a bright, star-like object.
Scientific Puzzles: Origins and Future Missions
The origins of Phobos and Deimos remain a significant scientific puzzle. Two main hypotheses attempt to explain their existence. The first, older theory suggests they are captured asteroids, supported by their composition resembling carbonaceous C-type asteroids.
However, this capture theory is difficult to reconcile with the fact that both moons have highly circular orbits that lie almost perfectly in Mars’s equatorial plane. The newer, more widely accepted theory posits that the moons formed from a debris disk created after a massive impact struck early Mars. This model is similar to the formation of Earth’s Moon and better explains the regularity of their orbits.
Scientists hope to resolve this debate with direct sample analysis through upcoming exploration. Japan’s space agency, JAXA, is planning the Martian Moons Exploration (MMX) mission to visit both Phobos and Deimos. The mission’s primary goal is to land on Phobos, collect a sample of its surface material, and return it to Earth for detailed study. This returned sample is expected to provide the definitive chemical signature needed to confirm whether the moons are captured asteroids or material born from Mars itself.