Moons of Mars: New theory for their past and future
As soon as the moons of Mars were discovered by the American astronomer Asaph Hall in 1877, scientific curiosity went into overdrive to answer the age-old question that drives children and scientists alike: Why? Researchers have long asked: Why are they there, and how did they form? We’re exploring those angles plus another aspect: Where are they heading?
My research group has developed a new hypothesis for the origin and evolution of the moons of Mars, based on analysis of their orbits and computer simulations of their origin. The moons were named for Greek mythological twins, Phobos (fear) and Deimos (dread). Some context: In Greek mythology, the twins went into battle with their father, Ares, the God of war. Ares later was known to the Romans as Mars — hence, the moons Phobos and Deimos are planetary “sons,” orbiting around their father, Mars.
Phobos and Deimos have long been a puzzle, as their orbits are low-eccentricity (nearly circular), low-inclination (low-tilt), and prograde (orbiting in the same direction as the spin of the planet), suggesting they formed in a disk around Mars. But it has been difficult to come up with a consistent scenario for their formation. A big sticking point is that Phobos and Deimos orbit on either side of the Mars synchronous orbit distance, which is the distance at which the moons’ orbital period equals the rotation period of the planet.
Phobos orbits inside the synchronous distance. Therefore, the “tidal” forces (the gravitational effects that cause bulging, or stretching) that Phobos exerts on Mars cause Phobos to spiral inward, while Deimos orbits outside the synchronous orbit and its tides cause it to spiral outward, much like Earth’s moon does. When the solar system originated 4.5 billion years ago, Phobos and Deimos would have been right next to each other, but today Phobos is much closer to Mars and has roughly 70 million years left before it gets close enough to Mars to be ripped apart by strong tides or slam into the planet.
Our hypothesis is grounded on the idea that when Phobos becomes close enough to Mars, it will be torn apart and form a ring. But what if this isn’t the first time that has happened?
The cool thing about rings is that they behave a bit differently than satellites. The collisions between ring particles cause an effect known as “viscous spreading,” which extends the ring both inward and outward. When the outermost edge of the ring starts to cross a special place known as the Roche limit — the distance at which the tidal forces of a larger celestial body overcome the gravitational forces in an orbiting body that hold it together — ring material can start to coalesce into new satellites. The Roche limit is named for French astronomer Édouard Albert Roche, who theorized that the planetary rings of Saturn were formed when a moon got too close to our solar system’s second-largest planet and was done in by gravitational forces.
New satellites formed among ring particles induce orbital resonances in the ring material known as Lindblad resonances, which take their name from the observations and deductions of Swedish astronomer Bertil Lindblad. The disturbances of ring material by resonances can exert a gravitational effect on the satellites that is very similar to tides but always causes the satellite to spiral away from the rings. All of these phenomena are well studied and observed at Saturn and other ring/satellite systems.
The combination of tides, ring repulsion, and the locations of the Roche limit and synchronous orbits set up the conditions for a ring/satellite cycle at Mars. We hypothesize that a giant impact — perhaps the one that formed the massive, 10,000-km-diameter Borealis Basin of the northern hemisphere of Mars, which covers 40 percent of the planet — created a debris disk somewhat like that from which Earth’s moon was formed.
Our NASA-funded research indicates that Deimos formed from the material that spread beyond the synchronous orbit, but also that inside the synchronous orbit, a much more massive moon (perhaps more than 100 times larger than present-day Phobos) coalesced. We believe this proto-Phobos rapidly drifted toward Mars due to tides, until it got close enough to be disrupted into a ring.
Next in the cycle, the ring would viscously spread, dumping much of its mass onto Mars at the inner edge — but also distributing material outward, until it crossed the Roche limit and formed a new satellite. Lindblad torques would repel the new satellite from the ring, preventing tides from driving it back inward. As the ring lost mass, the Lindblad torques would become less effective, and the satellite ultimately would be driven toward the planet.
Once this second-generation satellite got close enough, it would disrupt into a new ring, less massive than the first. Our modeling suggests this process has repeated multiple times, and each time, the next generation of ring and satellite has been lower in mass and taken longer to cycle than the previous generation.
The result? The cycles that have occurred over approximately 4 billion years could have produced a satellite roughly the mass of Phobos, on its way to forming a new ring in about 70 million years.
David Minton
Associate Professor
Department of Earth, Atmospheric, and Planetary Sciences
College of Science
Purdue University
