Mars does not currently possess a ring system, unlike the massive bands that encircle the gas giants of the outer solar system. Planetary rings are essentially a vast collection of debris, ranging from microscopic dust to house-sized boulders, all orbiting a central planet. While Mars is ringless now, the unstable dynamics of its innermost moon suggest this situation is temporary. The Red Planet is on a path to acquire a spectacular, though short-lived, ring system in the deep future, driven by gravity.
Mars’s Current Orbital Companions: Phobos and Deimos
Mars currently hosts two small, irregularly shaped moons, Phobos and Deimos. These bodies are believed to be captured asteroids due to their lumpy forms and composition, which resembles carbonaceous chondrites found in the main asteroid belt. Phobos is the larger of the two, measuring about 14 miles (22 kilometers) across, while Deimos is smaller.
Phobos circles Mars at an altitude of just 3,700 miles (6,000 kilometers), making it the closest-orbiting moon in the solar system. It completes a revolution in less than eight hours, much shorter than a Martian day. This tight, fast orbit is the source of the moon’s instability and the key to Mars’s ring-filled future. Deimos orbits much farther out and is slowly drifting away from the planet.
The Mechanics of Ring Formation and Mars’s Deficit
The formation of stable, visible planetary rings requires specific conditions that Mars currently does not meet, starting with the planet’s size and gravitational strength. Terrestrial planets like Mars have a much weaker gravitational influence than the giant planets. This means their gravitational threshold for tearing an object apart is much closer to the surface. This barrier is defined by the Roche Limit, a distance within which a planet’s tidal forces exceed the gravitational force holding an orbiting body together.
If an object crosses inside this limit, tidal forces cause it to fragment into pieces, which then spread out to form a ring. The gas giants have immense mass, allowing their Roche Limits to extend far into space, where they can capture and fragment material. Mars’s weaker gravity means its Roche Limit is so close that most incoming debris would simply crash into the planet rather than being torn into a ring system. Furthermore, the lack of icy material, which makes up a large part of Saturn’s rings, contributes to Mars’s current deficit.
Phobos’s Doomed Orbit: The Creation of a Future Ring System
The closest moon, Phobos, is an exception because its orbit is already decaying due to the tidal forces exerted by Mars. Because Phobos orbits faster than Mars rotates, the moon is constantly being pulled inward, spiraling about six feet (1.8 meters) closer to the planet every century. This process is accelerating the moon’s descent toward the Roche Limit, setting the stage for disruption.
Scientists predict that in 30 to 50 million years, Phobos will finally cross this gravitational boundary. The moon is thought to be a “rubble pile,” a loose collection of material held together by a thin crust, making it highly susceptible to tidal stresses. Once tidal forces overcome the moon’s weak self-gravity, Phobos will be ripped into millions of pieces, which will rapidly disperse into a dense, temporary ring system around Mars.
The resulting ring could initially be comparable in mass density to some of Saturn’s rings, stretching across thousands of miles in a relatively narrow band. This new ring will not be permanent, as the debris will be subject to atmospheric drag from Mars’s thin atmosphere, causing the particles to slowly spiral downward. The material will rain down onto the Martian surface over a period estimated to last between one million and 100 million years, eventually clearing the orbit.
How Mars’s Potential Ring Compares to the Gas Giants
The predicted Martian ring will be fundamentally different from the iconic systems of the gas giants, particularly Saturn, in both composition and longevity. Saturn’s rings are massive, broad, and stable, composed primarily of bright water ice particles, and are estimated to be billions of years old. This stability is a function of Saturn’s size and the volume of icy material available in the outer solar system.
In contrast, the future Martian ring will be dark, composed of the carbonaceous, rocky material from Phobos, reflecting very little sunlight. It will be a narrow and relatively thin structure compared to the scale of the gas giants’ systems. Most importantly, the Martian ring will be a fleeting geological event, lasting a maximum of 100 million years before the debris falls to the planet, a cosmic blink.