Mars is the only inner solar system planet, besides Earth, that hosts moons. Named Phobos (fear) and Deimos (dread) after the mythical attendants of the Greek god of war, these two tiny satellites present a profound mystery to planetary scientists. Unlike Earth’s single, large, and spherical Moon, Mars’ companions are small, irregularly shaped, and orbit unusually close to their host planet. Understanding the origin of this peculiar pair points to a turbulent past for the Red Planet.
The Distinctive Nature of Phobos and Deimos
Mars’ two moons are small; Phobos measures about 22 kilometers across, and Deimos spans only about 13 kilometers. Their appearance is highly irregular, often described as potato-shaped, contrasting sharply with the near-perfect sphere of Earth’s Moon. This non-spherical shape results from their small mass, meaning their gravitational pull is too weak to form a round shape.
Both moons have dark, heavily cratered surfaces, making them among the darkest objects in the solar system. Their low reflectivity suggests they are made of carbon-rich material, similar to C- or D-type asteroids found in the outer asteroid belt. Density measurements indicate that Phobos is highly porous, likely a “rubble pile” of rocky fragments loosely held together by gravity. This low density and dark composition provide important clues for their formation story.
Competing Theories on Martian Moon Formation
The unusual characteristics of Phobos and Deimos have led to two primary hypotheses explaining their existence. The historical view, known as the Capture Hypothesis, suggests the moons were once asteroids that strayed too close to Mars and were snagged by its gravity. The dark, asteroid-like composition of their surfaces initially supported this idea.
However, significant problems exist with the capture scenario, primarily concerning the moons’ orbits. Captured objects typically settle into highly elliptical orbits with a high inclination, meaning they are tilted relative to the planet’s equator. Phobos and Deimos, by contrast, have nearly circular orbits that lie almost exactly in Mars’ equatorial plane. Scientists struggle to find a plausible mechanism that could have circularized and aligned the orbits of two captured asteroids so precisely over time.
The leading modern explanation is the Giant Impact Hypothesis, which posits that a massive object, perhaps a protoplanet one-third the size of Mars, struck the Red Planet early in its history. This collision would have ejected a vast amount of material, forming a debris disk around the planet. This scenario is similar to the prevailing theory for the formation of Earth’s Moon, but with a different outcome.
Instead of coalescing into a single large moon, the debris formed numerous small moonlets. Phobos and Deimos are thought to be the final two survivors that formed in the outer region of this debris disk. This theory neatly explains their circular, equatorial orbits and accounts for their low density and porous internal structure. Recent spectral analysis of Deimos also suggests its composition is more similar to Mars’ rocky material than to a pristine carbonaceous asteroid, strengthening the impact theory.
The Unique Orbital Dynamics and Impending Fate
Phobos has one of the closest orbits of any moon in the solar system, circling Mars at an average distance of only about 6,000 kilometers above the surface. Its speed is so great that it completes an orbit in just 7 hours and 39 minutes, meaning it circles the planet three times for every Martian day.
This extremely close orbit places Phobos deep within the gravitational influence of Mars, causing strong tidal forces to act upon it. The friction generated by these forces is gradually slowing Phobos down, resulting in orbital decay, where the moon is spiraling inward at a rate of approximately 1.8 centimeters per year. Current predictions estimate that within 30 to 50 million years, Phobos will descend below the Roche limit, the point at which Mars’ tidal forces will exceed the moon’s gravitational self-attraction.
At this point, Phobos will likely be torn apart into millions of fragments, forming a temporary planetary ring system around Mars. This ring may persist for millions of years before the debris eventually rains down onto the Martian surface. In contrast, Deimos orbits much farther out, at about 23,460 kilometers, and follows the more typical pattern seen in other moon systems. It is slowly receding from Mars, moving slightly farther away each year, ensuring its long-term survival as the sole Martian satellite.