Did Mercury and Uranus Have a Rough Youth After All?

NANTES, FRANCE—How violent was the early solar system? Over the past decades, astronomers have invoked giant cosmic collisions to explain a wide variety of weird planetary properties. For instance, a catastrophic impact could have stripped away most of Mercury's rocky mantle, leaving the planet with its relatively huge iron core. Farther out, a similar blast would have tipped over Uranus, which lies on its side while orbiting the sun.

But when NASA's MESSENGER spacecraft detected lots of sulfur and potassium in the crust of Mercury earlier this year, many planetary scientists believed this ruled out the giant-impact scenario for the innermost planet. An energetic collision, they concluded, would have melted the planet and released such volatile constituents.

As for Uranus, no one could explain why the paths of its satellites are also tilted. They still orbit in the equatorial plane of the fallen giant, a path you wouldn't expect if a cosmic interloper had struck only the planet. What's more, says physicist Stanton Peale of the University of California, Santa Barbara, a single impact can't explain the orbits of Pluto's moons, as some scientists have proposed. So should we abandon the idea of protoplanets smashing into each other during the solar system's youth?

Not so fast, say some researchers here at a joint meeting of the European Planetary Science Congress and the Division for Planetary Sciences of the American Astronomical Society. Planetary scientist Sarah Stewart of Harvard University is confident that Mercury could have been stripped while keeping hold of its volatiles. And today, astronomer Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice, France, presented a new impact model that could explain the Uranus system.

"You only lose volatiles if they somehow can get separated from the heavier, more refractory elements," Stewart explains. "For instance, our own moon is low in volatile elements because it slowly coalesced from a churning, hot disk of material surrounding the Earth." This disk, she says, was boiling hot for long enough to lose most of its light elements. "But in the case of Mercury, the impact debris cooled pretty quickly," Stewart says, "so most of the potassium and sulfur was preserved."

Astrochemist Richard Starr of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and physicist Patrick Peplowski of the Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland, both gave talks at the conference yesterday announcing the death of the giant impact theory for Mercury. But they acknowledge that no one has ever carried out detailed calculations of elemental loss during giant collisions. "Sarah brings up a very interesting point," Peplowski says.

Meanwhile, Morbidelli thinks Uranus's giant impact can also be saved—but it must have occurred much earlier than researchers had assumed, and in multiple stages. "In the early days of the solar system, there may have been many hundreds of protoplanets drifting around in the system's outer regions," he says. "Giant collisions were, of course, more likely to occur very early on."

In Morbidelli's revised model, Uranus was hit before its satellites formed from a disk of gas and dust surrounding the planet. The disk would have been heavily disturbed by the impact and then would have settled back into the planet's tilted equatorial plane, as a result of friction. Only later would the material in the disk clump into satellites.

However, Morbidelli needs two or more subsequent collisions to explain why the satellites orbit in the same direction as the planet's rotation. The axial tilt of Uranus is 98°, so effectively the planet is now spinning in a direction opposite to its orbital motion around the sun. But the particles in the disk would have kept rotating in the original direction, even though they would have ended up in the new equatorial plane. "So if the huge tilt of the planet was produced by just one impact," Morbidelli says, "the disk, and the satellites, would end up orbiting opposite to the planet's rotation."

This problem would disappear if one impact tilted the planet to, say, 50° or so, and a second blow delivered an additional tilt of 48°. A larger series of smaller impacts is also possible. In each case, the planet and the disk material would remain orbiting in the same direction. "Two or more impacts may look improbable, but it all depends on the number of objects out there," Morbidelli says.

So what about the satellite system of Pluto? Maybe multiple collisions occurred there, too, producing both the large moon, Charon, and the three smaller ones, known as Nix, Hydra and P4. "Our simulations indicate that there are some additional stable regions around the dwarf planet where small satellites could end up," Peale said at the conference last Tuesday. If NASA's New Horizon spacecraft should discover more moons during its July 2015 flyby of Pluto, they would certainly help in reconstructing the violent youth of the system.