Mars is the fourth world from the Sun and Earth’s nearest planetary neighbor. Like Earth, Mars is a rocky planet, but its environment is vastly different, featuring a thin atmosphere and a surface marked by ancient volcanoes and vast canyons. The presence of natural satellites, or moons, which orbit the primary body, provides important clues about the history and evolution of the Martian system and the solar system as a whole.
Mars’ Satellite Count
Mars possesses two small natural satellites, Phobos and Deimos, discovered in 1877 by American astronomer Asaph Hall. Their names come from the Greek mythological sons of Ares, the Greek counterpart to the Roman god Mars. Phobos translates to “fear,” and Deimos means “dread,” fitting names for the companions of the god of war. Both moons are significantly smaller and less spherical than Earth’s Moon. They are characterized by irregular, potato-like shapes, resembling captured asteroids.
Characteristics of Phobos and Deimos
Phobos is the larger moon, measuring approximately 22 kilometers across at its widest point, and it orbits Mars at an incredibly close distance of about 6,000 kilometers above the surface. This makes Phobos the moon with the closest orbit to its planet in the entire solar system. Phobos is heavily pockmarked with craters, the most prominent being the Stickney Crater, which spans roughly 9 kilometers and takes up a large proportion of the moon’s surface area. Deimos is noticeably smaller, with an average diameter of about 12 kilometers, and it orbits much farther out, at a distance of about 23,460 kilometers from Mars.
The difference in size and orbital distance leads to distinct surface appearances. Phobos’s surface is rough and heavily cratered, with a composition similar to carbonaceous asteroids, making it one of the darker objects in the solar system. Deimos, conversely, has a much smoother surface, a result of loose, dusty material called regolith that has accumulated and partially filled in many of its craters. Both moons are tidally locked, meaning they always present the same face toward Mars. Phobos completes its orbit in just 7 hours and 39 minutes, while Deimos takes 30 hours to circle the planet.
Theories on Their Formation
The exact origin of Phobos and Deimos remains a long-standing scientific puzzle, with two primary hypotheses. One leading theory suggests they are captured asteroids, pulled into Mars’ orbit by the planet’s gravity early in the solar system’s history. This idea is supported by their composition, which resembles that of C- or D-type asteroids found in the outer asteroid belt. However, this capture theory is complicated by the moons’ nearly circular orbits and their low inclination, which lies almost perfectly in Mars’ equatorial plane.
An alternative theory proposes that the moons formed from the debris ejected after a massive object collided with Mars, similar to the prevailing model for the formation of Earth’s Moon. This giant impact hypothesis better explains the near-circular, equatorial orbits of the satellites, as the material would have settled into a disk around the planet before coalescing. Recent models have explored a hybrid approach, suggesting a large impact created a temporary ring that later broke apart to form the two moons. The upcoming Japanese Martian Moons Exploration mission, which plans to return a sample from Phobos, aims to provide the definitive compositional evidence needed to settle this debate.
Phobos’s Unique Orbital Fate
Phobos is subject to a unique orbital fate due to its extremely close proximity to Mars. Gravitational interactions create tidal forces that are slowly causing Phobos to spiral inward toward the Martian surface, a process known as orbital decay. This process is drawing the moon approximately 1.8 meters closer to Mars every century. Scientists predict that this inward spiral will lead to the moon’s inevitable destruction within the next 30 to 50 million years.
When Phobos drops below the Roche limit, the tidal forces will exceed the moon’s gravitational pull, tearing it apart. Phobos will disintegrate into a vast ring of debris around Mars, which may persist for millions of years before eventually crashing onto the planet’s surface. In contrast, Deimos is located far enough away that tidal forces cause it to slowly drift away from the planet, similar to Earth’s Moon. The shallow grooves and striations seen on Phobos’s surface may be early signs of stress fractures caused by these relentless tidal forces.