The planet Mars had rings

The more massive planets nearby have large rings and moons. Mars has only two small, misshapen moons: Phobos and Deimos. However, one of them provides an indication of the Red Planet’s past. Deimos has a slightly inclined orbit. According to scientists from the SETI Institute and Purdue University, this is only explained for one reason: The planet Mars had rings.

Moons, not asteroids

The two Martian moons discovered in 1877 were thought to be asteroids. But their orbits are almost at the same level as Mars’ equator. Therefore, they must have formed at the same time as Mars. However, the orbit of the smallest moon, Deimos, is inclined by two degrees.

“This fact was not considered important,” says lead author Matija Cuk, a research assistant at the SETI Institute. “But Deimos’ orbit tendency revealed a big secret.”

When they studied it, they noticed something. The inner moon of Mars, Phobos, is losing height as its small gravity interacts with the Martian globe. From an astronomical point of view, the orbit of Phobos will soon be too low. The gravity of Mars will break it into pieces to form a ring around the planet. For billions of years, it has been believed that generations of Martian moons in rings have been destroyed. Each time the ring created a new, smaller moon to repeat the cycle.

Orbital resonances

This cyclical theory of the Martian moon has a crucial element that enables the tilt of the Deimos. A newborn moon would drift away from the ring and Mars. A moon that moves outside the rings can encounter an orbital resonance. In this case, the orbital period of Deimos is three times that of the other moon.

These orbital resonances are demanding, but predictable in the direction in which they intersect. Only a moon that moves outwards could have greatly influenced Deimos. That means that Mars must have had a ring that pushes out the inner moon. Cuk and his team conclude that this moon could be 20 times more massive than Phobos. It existed perhaps 3 billion years ago, followed by two more ring-moon cycles, with the last moon being Phobos.

This modest inclination of the lunar orbit has major consequences. The discovery of orbital resonance in the past almost confirms the cyclic ring-moon theory for Mars. This implies that the planet Mars had rings for much of its history. While Deimos is billions of years old, Cuk and his team believe that Phobos is young. It may have been founded just 200 million years ago, just in time for the dinosaurs.

The Japanese space agency JAXA plans to send a spaceship to Phobos in 2024. It is said to collect samples from the lunar surface and bring them back to Earth. Cuk hopes that this will give us firm answers to the dark past of the Martian moons, as stated in a statement.

This research is published in the Astrophysical Journal Letters.