This Article is From Nov 28, 2015

Scientists Link Moon's Tilt and Earth's Gold

Scientists Link Moon's Tilt and Earth's Gold

A dearth of lunar metals argues for a few large metal-rich objects hitting the Earth rather than many small ones.

The moon's orbit is askew, and two planetary scientists believe that they have come up with a good reason.

Intriguingly, their idea also explains why gold and platinum are found in the Earth's crust, well within diggable reach.

The moon is believed to have formed out of a giant cataclysmic collision early in the history of the solar system when an interplanetary interloper the size of Mars slammed into Earth and lofted a ring of debris circling over the Equator. The debris coalesced into the moon.

At its birth, the moon was quite close to the Earth, probably within 20,000 miles. Because of the tidal pulls between the Earth and moon, the moon's orbit has slowly been spiraling outward ever since, and as it does, Earth's pull diminishes, and the pull of the sun becomes more dominant.

By now, with the moon a quarter million miles from Earth, the sun's gravity should have tipped the moon's orbit to lie in the same plane as the orbits of the planets.

But it has not. The moon's orbit is about 5 degrees askew.

"That the lunar inclination is as small as it is gives us some confidence that the basic idea of lunar formation from an equatorial disk of debris orbiting the proto-Earth is a good one," said Kaveh Pahlevan, a planetary scientist at the Observatory of the Cote d'Azur in Nice, France. "But the story must have a twist."

Writing in this week's issue of the journal Nature, Pahlevan and his observatory colleague Alessandro Morbidelli propose the twist.

The moon did indeed form in the Earth's equatorial plane, the scientists said, but then a few large objects, perhaps as large as the moon, zipping through the inner solar system repeatedly passed nearby over a few tens of millions of years and tipped the moon's orbit.

A series of computer simulations show that the idea is plausible.

"This mechanism works for a broad range of physical conditions," Pahlevan said.

Eventually the crisscrossing mini-planets would have been tossed out of the solar system, swallowed by the sun, or slammed into the Earth or the other planets.

Robin M. Canup, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who wrote an accompanying commentary in Nature, said the thousands of close passes that typically occur before an impact were a "really new realization" by Pahlevan and Morbidelli.

"While a single scattering event will only change the moon's tilt slightly," Canup said, "it's the cumulative effect of these many passes that can produce this tilt."

The scars of one or more moon-size objects hitting Earth would have long been erased by the tectonics of the shifting surface, but those impacts would explain the gold, platinum and other precious metals in the Earth's crust but not on the moon.

Metals on the early Earth should have sunk to the interior. Thus, planetary scientists think that after the moon was created, later collisions that provided the last 1 percent or so of the Earth's mass added a veneer of precious metals.

A dearth of lunar metals argues for a few large metal-rich objects hitting the Earth rather than many small ones.

The computer simulations show that the chances of the moon's getting hit are low. In the simulations, if there was one object buzzing by, the moon was hit 9 percent of the time. With four objects, the chances of a lunar impact rose to 25 percent.

"Not an overly likely outcome, which is good," Canup said.
Scientists including Canup had proposed other explanations for the tilt.

"I would say those relied on certainly more complex processes and required rather narrow sets of conditions for success," Canup said. "I think where this has really stepped in is it's a very simple mechanism."
© 2015, The New York Times News Service
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