Mars, often called the Red Planet, is known for its two small, irregularly shaped moons, Phobos and Deimos. Unlike gas giants adorned with grand rings, Mars currently lacks any planetary rings.
Mars’ Two Natural Satellites
Mars’ two natural satellites, Phobos and Deimos, are small, non-spherical bodies. American astronomer Asaph Hall discovered them in August 1877, naming them after the Greek mythological attendants of Ares. Phobos translates to “fear,” and Deimos means “dread.”
Phobos, the larger, measures approximately 22 kilometers across, while Deimos is about 12 kilometers. Their irregular shapes indicate insufficient mass for gravity to pull them into a spherical form. Scientists believe these moons are captured asteroids from the main asteroid belt, though some propose they formed from debris ejected during a giant impact on Mars.
Phobos orbits Mars remarkably close, at about 6,000 kilometers from the surface, completing an orbit in just 7 hours and 39 minutes. This speed is faster than Mars’ rotation, causing Phobos to appear to rise in the west and set in the east, completing two full cycles each Martian day. Deimos orbits farther out, at approximately 23,460 kilometers, taking 30.3 hours to complete one revolution. From the Martian surface, Deimos appears much smaller, almost star-like, and rises in the east and sets in the west, similar to Earth’s moon.
The Absence of Rings
Unlike the prominent, icy ring systems of gas giants like Saturn, Jupiter, Uranus, and Neptune, Mars currently has no rings. This absence is due to a lack of sufficient material and the specific gravitational dynamics required for stable ring formation. Gas giants, being larger and more massive, possess stronger gravitational fields capable of holding vast amounts of material in stable orbits.
Planetary rings form when material, such as dust, ice particles, or fragments, orbits within a planet’s Roche limit. This limit is a distance where tidal forces overcome a smaller body’s internal gravity, causing it to disintegrate. Material inside this limit tends to disperse and form rings, while material outside it can coalesce into moons. Mars lacks sufficient material within its Roche limit to form a visible, stable ring system.
What Awaits Mars’ Moons
Phobos is slowly spiraling inward toward Mars. Tidal forces are gradually pulling it closer by about 1.8 meters per century. Scientists predict that within 30 to 50 million years, Phobos will reach Mars’ Roche limit.
When Phobos crosses this boundary, Mars’ gravitational pull will likely tear it apart. This disintegration would scatter the moon’s fragments, forming a temporary, dusty ring system around the Red Planet. This ring could persist for 1 million to 100 million years before its particles gradually fall onto the Martian surface. Deimos, in contrast, is in a more stable, distant orbit and is slowly moving away from Mars. It is expected to eventually escape Mars’ gravitational influence altogether.