Mars is a captivating world that inspires curiosity about its features and history. Questions about its appearance, especially when compared to the magnificent ring systems of the gas giants, frequently arise. Mars currently does not have a visible ring system. It is orbited by two small natural satellites, which are not rings.
Observing Mars Today
Mars currently lacks the broad, flat disk of orbiting material that defines a planetary ring system. Scientists have a definitive answer for the common question of whether Mars has rings: it does not. Instead, it is orbited by two small, irregularly shaped moons named Phobos and Deimos, which are thought to be captured asteroids.
Phobos is the larger of the two, with a diameter of about 22 kilometers, and orbits extremely close to the planet, circling Mars three times per Martian day. Deimos is about half the size of Phobos and orbits much farther away. Neither of these small, lumpy bodies resembles the vast, icy structures seen around Saturn.
Phobos orbits at an altitude of only about 6,000 kilometers above the Martian surface, closer than any other known moon orbits its host planet. This close proximity allows Phobos to complete an orbit in just over seven and a half hours.
The Future Formation of Martian Rings
While Mars does not have rings now, scientific models predict it will acquire a ring system in the future. The fate of Phobos is tied to this prediction because its orbit is slowly decaying, drawing it closer to the planet by about 1.8 meters each century. This inward spiral results from tidal forces exerted by Mars on its close-orbiting satellite.
Phobos is expected to reach the Roche Limit within the next 30 to 70 million years. The Roche Limit is the boundary where the planet’s gravitational tidal forces overcome the moon’s internal gravity. Since Phobos is essentially a loosely consolidated rubble pile, these tidal stresses will tear it apart once it crosses this boundary.
The resulting debris—a mix of rock, dust, and boulders from the shattered moon—will spread out into an orbiting disk. This process will create a temporary ring system around Mars, likely composed of dark, carbonaceous material from Phobos. This ring is not expected to be permanent, with estimates suggesting it will only last between one and 100 million years before the debris falls onto the Martian surface. This cycle may have occurred multiple times in Mars’s history.
The Physics of Planetary Ring Systems
The existence and stability of planetary rings are governed by the same principles of gravity that will eventually destroy Phobos. Ring systems are generally composed of countless small particles, ranging from fine dust to meter-sized boulders, all orbiting a host planet in a thin disk. A planet’s gravitational field plays the primary role in both the creation and maintenance of these structures.
The classic, massive rings of the gas giants, like Saturn, are distinct from the predicted Martian ring. Saturn’s rings are primarily composed of bright water ice, giving them their high visibility, and they may have formed from the breakup of a large, icy moon or from primordial material. In contrast, the future Martian ring will be made of much darker, carbon-rich rock, limiting its brightness and visibility.
The stability of large ring systems is managed by smaller satellites called shepherd moons. These moons orbit near the edges of the rings and use their gravitational influence to confine the ring particles, preventing them from spreading out or dissipating quickly. This mechanism helps maintain the complex structure and longevity of ring systems like Saturn’s. The formation of any new Martian ring is expected to be a transient event because it will lack the stable configuration and potential shepherd moons that characterize the long-lasting ring systems of the outer solar system.