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.