Someday, Mars’ moon Phobos will slip past a certain point in its degrading orbit and get ripped apart by tidal forces, forming a ring. This illustration depicts Phobos midway through that process, overlooking the Red Planet. Credit: Ron Miller for Astronomy
Phobos, it seems, is not long for this universe — at least on the large cosmic timescale.
Astronomers have long known that Phobos, the larger and nearer of the two martian moons, is slowly spiraling inward to eventual destruction. The end result won’t be pretty: Phobos will slip closer and closer toward Mars, then strike a gravitational line where the planet’s tidal forces will be strong enough to rip it apart. The rubble pile-like moon will break into smaller boulders, rocks, and dust, and will spread out in orbit around Mars.
Mars will join the gas giants in having a spectacular feature: a ring system.
It could be 25 million years from now. It could be up to 75 million years. Recent discoveries about the little moon’s composition and density, however, make it far more likely that its death dive will happen sooner. The pieces that don’t form a ring will fall to the surface, smashing with enough force to pockmark Mars with new craters.
“A lot of planetary science focuses on what happened in the past and what’s happening now,” says planetary scientist Benjamin A. Black. “It’s not often that we look at the future, at what will happen.” Black, a City University of New York professor, and graduate student Tushar Mittal from the University of California, Berkeley, have carried out a detailed examination of the eventual fate of Phobos.
Related: Japan launches sample return mission to Phobos
It appears the process of coming apart at the seams has already begun. Images of Phobos taken by the Viking orbiters and other spacecraft show a network of grooves in the tiny moon’s surface. At first they appeared to radiate from near Stickney Crater, and geologists assumed that the grooves were cracks caused by the ancient impact. Some certainly are just that, but not all. In 2015, Terry Hurford of NASA’s Goddard Space Flight Center and his colleagues reported a new analysis of the grooves. Most of them actually radiate from the side of Phobos that constantly faces Mars; tidal forces caused by Mars’ gravitational pull are deforming Phobos. Hurford believes the grooves are stretch marks, a visible sign of the inexorable grip of tidal forces on the moon.
The future martian ring will not be the only one in the solar system, of course. Nor will it be the only ring whose existence depends on a moon. There are rings across the giant planets: the four dusty rings of Jupiter; Uranus’ 13 dark, thin rings; and Neptune’s five faint rings and four enigmatic ring arcs are — like the future ring around Mars — all intimately linked to moons and moonlets. And of course, the most familiar ringed planet is Saturn, whose icy surrounding matter can be seen even through a small telescope.
Saturn’s magnificent rings
Galileo Galilei saw what turned out to be Saturn’s spectacular ring system in 1610. (He said the features looked like ears or handles.) But it wasn’t until 1655 that Christiaan Huygens identified them as an entire system of icy rings. In 1856, famed physicist James Clerk Maxwell showed that the rings must be composed of a huge number of tiny particles (he called them “brick-bats”), each independently orbiting Saturn.
Since then, debate has raged over the origin, age, and composition of Saturn’s rings. Are they leftovers from the formation of Saturn, or the remains of a shattered moon? As old as Saturn itself, or a relatively new addition? And why so much ice? Planetary scientist Robin M. Canup of the Southwest Research Institute in Boulder, Colorado, recently published a proposal that answers these questions. Canup suggests that Saturn’s rings are the very ancient remains of a Titan-sized moon.
“Saturn originally had multiple massive moons like Jupiter,” says Canup. These moons were large enough for their interiors to differentiate into layers of ices and a rocky core — less like a large comet and more like the four larger moons of Jupiter. When the large moons spiraled inward as Saturn finished its accretion, Canup says, the outer icy layers of at least one were stripped away. The core plunged into Saturn, and the icy remains eventually formed the planet’s main rings. This, she says, explains why the particles making up the rings are 99.9 percent pure water ice. It also explains the striking difference between Jupiter’s and Saturn’s rings and satellites.
“The existence of Saturn’s much more massive ring system is linked to Saturn having lost its large primordial inner moons,” Canup explains. “Jupiter retained its large inner moons, [while] its dusty ring system is vastly less massive than the ring system at Saturn.”
Related: How long will Saturn’s rings last before they disappear?
Rings of the giant planets
Jupiter’s four faint and dusty rings probably formed by a different mechanism than Saturn’s — and more recently — but its moons still play a vital role. Amalthea and Thebe are the likely sources of the material in Jupiter’s outer two “gossamer” rings. Thebe orbits within the outer gossamer ring, while Amalthea lies near the outer edge of the inner gossamer ring. Metis and Adrastea, two other small moons, orbit near the outer edge of Jupiter’s main ring, and are the sources of the dust grains making up the main and innermost halo rings.
Some forces work against the rings, keeping them relatively thin. Electromagnetic forces, a phenomenon called plasma drag, and even pressure from sunlight (called the Poynting-Robertson effect) continually remove the micron-sized particles from the rings. But particles sputtered off the moons by meteoroid impacts create dust and continually replenish the rings.
The 13 known rings of Uranus are unlike those of Jupiter or Saturn. All but the innermost and two outermost of the rings are quite narrow, ranging from just 0.6 to 59.6 miles (1 to 96 kilometers) wide. Their particles are larger than those in Jupiter’s rings, but there’s little dust. Instead, they’re largely made up of ice with organic chemicals mixed in to give the dark appearances, unique among the outer solar system rings.
But there are similarities: Like Jupiter’s and Saturn’s rings, Uranus’ rings are intimately associated with moons and moonlets. Primordial moons the size of Puck, 100 miles (162km) in diameter, or larger would have had a good chance of surviving for several billion years. But not all would escape a devastating fate. Computer simulations show that the 11 inner moons of Uranus are likely the remains of original larger moons broken up by cometary impacts. What’s more, the current rate of meteoroid impacts among moons and other icy bodies at Uranus’ distance is enough to create all the observed rings and dust bands circling that planet.
Because the rings appear to be young, probably not more than 600 million years old, the material in the rings must be continually renewed. Particles blasted off the tiny moons and still-unseen moonlets by collisions and meteoroid strikes continually add material to the rings, while the dust continues to dissipate.
Neptune’s five rings and various dust bands are probably even younger than those of Uranus, and the same processes are likely responsible for them and the moons that orbit in or near them. Naiad and Thalassa orbit in the gap between the innermost Galle and Le Verrier rings, and Despina orbits just inside the Le Verrier ring. Galatea lies slightly inside the outermost Adams ring. These tiny moons are likely rubble-pile objects, agglomerations of fragments from earlier neptunian moons, weakly held together by gravity. The ring particles are material continually blasted off the moons by meteoroid impacts. Unlike the uranian rings, the rings of Neptune are quite dusty, thanks to the destruction of a satellite; at least 20 percent of the material is the size of smoke particles, and in some of the rings, that rises to 70 percent.
The Adams ring also has five distinct clumps or arcs of dust spanning about 40° in longitude. How they can exist for any length of time is a mystery, as these tenuous rings should have faded away. One explanation, by planetary ring expert Carolyn Porco, leader of the Cassini orbiter’s imaging team, is that a resonance effect caused by Galatea’s eccentric orbit may act to keep the particles in the arcs from spreading out and dissipating.
Creating a martian ring
Phobos isn’t nearly the size of a planet, but many of the same mechanisms will drive its destruction. Its companion moon Deimos is about 7.8 miles (12.6 kilometers) in diameter and orbits Mars at an average distance of 14,580 miles (23,460km), far enough away to avoid Phobos’ fate. Phobos is 13.8 miles (22.2km) in diameter. It circles the planet at an average distance of 5,827 miles (9,377km). With an orbital period of just 7 hours, 39.2 minutes, Phobos is one of only 18 of the 181 known moons in the solar system whose orbital period is less than its planet’s rotation period.
That’s one of a few reasons why Phobos is doomed. “There are four factors in action,” explains Mittal. “The tidal force of Mars; the centrifugal forces on Phobos, which is rotating; the gravity of Phobos; and the strength of Phobos. There’s a balance among these. Tidal and rotational stresses act to pull Phobos apart; self-gravity and [tensile] strength act to hold Phobos together.”
Just as our own Moon’s gravity raises tides in both Earth’s oceans and landmasses, Phobos creates a tiny tidal bulge in Mars. The bulge moves as the moon circles Mars, just as the Moon-caused tidal bulge moves around Earth. Because Phobos has an orbital period faster than the martian day, the tidal bulge lags behind Phobos and acts to gradually slow it in its orbit, sending Phobos slowly spiraling in toward the martian surface. The rate is about 0.79 inch (2 centimeters) per year. It’s not much, but there is nothing to stop it.
“As Phobos gets closer to Mars,” says Mittal, “the tidal stresses increase.” The moon’s self-gravity and tensile strength oppose the tidal forces, but eventually Phobos will drop to a distance — called the Roche limit — where the martian gravity will win the tug-of-war. Several studies confirm that Phobos’ inward spiral will lead to its destruction in 25 million to 75 million years.
Phobos could stave off obliteration if it were a dense body, more able to resist the pull of gravity past the Roche limit. Unfortunately, data from the European Space Agency’s Mars Express orbiter show that the moon has a density of just 1.9 grams per cubic centimeter. By comparison, our Moon has a density of 3.3g per cubic centimeter. Even tiny Themisto, Jupiter’s smallest regular moon at just 4.9 miles (8km) in diameter, has an estimated density of about 2.6g per cubic centimeter. Mars Express also revealed that Phobos is porous, likely containing large voids, so it is probably a rubble pile, like many of Uranus’ and Neptune’s tiny moons.
In addition, Phobos’ makeup is similar to carbonaceous chondrite meteorites like the ones that fell on Canada’s Tagish Lake in 2000. These primitive meteorites are made of a bevy of loose, easily broken materials, including magnetite, olivine crystals, phyllosilicates, and complex organic molecules such as amino acids, and are about 10 percent water, formed in oxygen-rich regions of the early solar system. Phobos is not only a rubble pile with a weak gravitational field, but the rubble itself is somewhat frangible.
Then there’s Stickney Crater, the giant impact basin on Phobos. Sometime in its past, Phobos was hit by an object that left a crater 5.6 miles (9km) wide — nearly half the moon’s diameter. The impact surely fractured much of Phobos’ interior, leaving it even weaker. From there, billions of years of meteoroid impacts have churned the surface of Phobos into a layer of fine dust about a hundred meters deep. Given the extremely weak surface gravity (escape velocity for Phobos is a mere 4.56 feet per second), the rock making up the rubble pile just barely stays put.
Black and Mittal used a numerical geotechnical model designed for large underground construction projects to estimate what they call the moon’s “rock mass strength.” The results place the finale for Phobos between 20 million and 40 million years from now, and give more details on its violent end.
As Phobos reaches its Roche limit, about 3,400 miles (5,470km) from the martian surface, tidal forces will start pulling Phobos apart. Just as Saturn’s tidal forces may have peeled away the icy mantle of a Titan-sized moon billions of years ago, the hundred meters or so of regolith will be stripped off Phobos. It will happen quickly: In as little as a week, the dust will spread into a ring circling Mars, the moon no more.
Depending on how much material is stripped off Phobos, the ring could initially have a mass density similar to that of today’s rings of Saturn. The martian ring will be very dark, unlike Saturn’s bright icy rings, more like the dusty rings of Jupiter that are darker, thinner, and more diffuse. It will practically hug Mars, closer in relative terms to the planet than the other planetary rings in the solar system. And the ring, say Black and Mittal, will not last long; they estimate its lifetime at between 1 million and 100 million years.
Eventually, the rocky rubble left behind will meet a quicker but equally dramatic end. It will plummet down along slanting paths to impact the surface of Mars and leave a string of elongated craters along the planet’s equator.
Other once and future rings
Is it possible that Mars possessed a ring system in its distant past? “We’re not certain,” says Black, “but it would be worth investigating whether past inwardly migrating moons may have existed. Some fraction of moons might be expected to have an orbital configuration similar to that of Phobos and Mars.”
Some planetary scientists think Mars once did have more than two moons. Researchers have identified 258 elliptical craters on Mars formed by objects hitting the surface at grazing angles. At least some of them could well have been made by oblique impacts of ancient martian moons. If so, others may have broken up before hitting the atmosphere, leaving short-lived rings of rocks and dust around Mars.
There’s no evidence that Mercury or Venus ever possessed ring systems. Earth did, though, for an extremely brief period during the formation of the Moon 4.5 billion years ago when our planet was struck by a Mars-sized body dubbed Theia. Much of Theia merged with Earth, but the “Big Splash” would have blown the remaining material into space. Computer simulations indicate about 20 percent of Theia’s mass would have gone into orbit around Earth as a ring. About 10 percent of the ring’s material then quickly coalesced into the Moon, with the rest eventually falling back to Earth. The ring would not have lasted long, perhaps as little as a month but probably no more than 100 years.
Curiously, our Moon also could end as it began, according to astronomer Lee Anne Willson, university professor emerita at Iowa State University. As part of her research on the fate of Earth as the Sun expands into a red giant, she found that the Moon stood a chance of becoming a ring around Earth.
The Moon is receding from Earth at a rate of about 1.6 inches (4cm) per year. Left unchecked, the Moon will eventually migrate out to a distance where it will take 47 days to orbit Earth. By then, Earth’s rotation also will have slowed to 47 days. The two will then keep the same face to each other, as Pluto and its moon Charon do today.
Before this happens, though, some 5 billion years from now, the Sun will enter its red giant phase. It will start expanding in size, and swallow up Mercury and Venus. As the Earth-Moon system orbits through the Sun’s expanded outer atmosphere, drag forces will cause the Moon’s orbit to begin decaying. The Sun probably will continue to expand, and destroy both Earth and the Moon. On the other hand, if the Sun should blow off about 20 percent of its mass first, the Moon will continue to spiral down to its Roche limit. Tidal forces will tear it apart, just as they will destroy Phobos.
And then, 9 billion years after the Moon’s birth from a ring of molten impact ejecta, and nearly 5 billion years after the birth and death of the ring around Mars, Earth will once again have a ring.
This story was originally published online August 29, 2019.
Joel Davis is a freelance science writer whose articles have also appeared in Analog, Final Frontier, New Scientist, and Science Digest. His books include Flyby: The Interplanetary Odyssey of Voyager 2 and Journey to the Center of Our Galaxy.