As Carl Sagan once said, “If you wish to make an apple pie from scratch, you must first invent the universe.” And if you wish to make a moon from scratch, according to new research, you must first create planets with rings (after inventing the universe, of course).
Earth's moon may have emerged from a long-vanished ring system, much like the rings still encircling Saturn—and the same goes for many of the satellites orbiting the other planets. The bulk of the solar system's regular satellites—those moons that stick close to their planets in roughly equatorial orbits—formed this way, rather than taking shape simultaneously with the planets as a direct result of planet formation, French astrophysicists have concluded. The researchers reported their findings in a November 2012 issue of Science.
Through theoretical modeling, Aurélien Crida of the University of Nice Sophia-Antipolis and Sébastien Charnoz of the University of Paris Diderot found that the moon-formation action begins at the edge of a planetary ring, where a satellite can take shape without being shredded by the gravitational pull of the planet. There moonlets coagulate from the ring material before migrating outward. As the ring system spits out moonlet after moonlet, the small objects merge to form larger moons, which may merge in turn as they spiral outward from the planet.
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The new hypothesis seems to explain a key commonality among the regular satellites of Saturn, Uranus and Neptune—namely, that moons farther from their respective planet tend to have larger masses than their closer-in neighbors. Like a snowball rolling downhill, the coalescing moons grow larger and larger as they drift farther from the planet and its rings and undergo progressively more mergers along the way. The end result is a neatly ordered satellite system, with small moons on the inside built from few moonlets and large moons farther out built from numerous moonlets.
Planetary scientists generally accept that a giant impact into the newly formed Earth ejected a huge cloud of material that became our moon. In Crida and Charnoz's conception, that ejecta first flattened into a ring around the planet, which then spread out and coagulated into the moon.
The new hypothesis is not without its problems. For example, if extensive, Saturn-like ring systems once adorned Neptune and Uranus, where are they now? “We have a few ideas, but nothing too convincing,” Crida says. “But I think we can find good reasons for the disappearance of the rings, and the satellites remain as the smoking gun.”