If geochronology is “the science of determining the ages of earth materials” (according to the center’s website), then Renne must know his gnat’s eyebrow. For those of us lay-folk, it’s about 5,000 years.

Renne and his colleagues have a new paper in Science pinpointing the dates of both the impact and the dinosaur extinction, placing them within the same time of each other—providing evidence, once again, for an asteroid or comet impact being the cause of extinction.

The 110 mile-wide Chicxulub (cheek’-she-loob) crater, off the Yucatan coast of Mexico, is likely the result of a six-mile in diameter asteroid or comet. Using and refining a technique called argon-argon dating, the scientists determined that the impact occurred 66,038,000 years ago, plus or minus 11,000 years.

The same argon-argon dating put the dinosaur extinction at 66,043,000 years ago, with the same margin of error.

The first link between the impact event and dinosaur extinction was published in 1980 by UC Berkeley’s Luis and Walter Alvarez. Since then, many other scientists have supported or refuted the theory, sometimes putting the extinction several hundred thousand years before the impact.

“When I got started in the field, the error bars on these events were plus or minus a million years,” says UC Berkeley paleontologist William Clemens. “It’s an exciting time right now, a lot of which we can attribute to the work that Paul and his colleagues are doing in refining the precision of the time scale with which we work.”

Despite the synchronous impact and extinction, Renne cautions that the impact was not the sole cause of extinction. Dramatic climate variation over the previous million years, including long cold snaps amidst a general Cretaceous hothouse environment, probably brought many creatures to the brink of extinction, and the impact kicked them over the edge.

“These precursory phenomena made the global ecosystem much more sensitive to even relatively small triggers, so that what otherwise might have been a fairly minor effect shifted the ecosystem into a new state,” Renne says. “The impact was the coup de grace.”

So imagine the surprise, forty years ago, when the Apollo astronauts brought back moon rocks with magnetic properties. How is that possible?

This week, two teams of scientists attempt to solve the mystery with separate papers in Nature.

Christina Dwyer, of UC Santa Cruz, and her team offer one theory. Early in its history, the Moon orbited Earth at a much closer distance than it does now, and it continues to gradually recede from Earth—even today! At close distances, tidal interactions between Earth and the Moon caused the Moon’s mantle to rotate slightly differently than the core. This differential motion of the mantle relative to the core stirred the liquid core, creating fluid motions that could give rise to a magnetic field.

Michael Le Bars, of Non-Equilibrium Phenomena Research Institute in Marseille, France, and his team have another theory. Large impact events like asteroids a few billion years ago could have caused sloshing within the lunar core for up to 10,000 years at a time.

So is it the asteroids’ fault or Earth’s? New Scientist doesn’t take sides:

Both models offer “a way out of a pretty major conundrum,” says Ben Weiss at the Massachusetts Institute of Technology.

Both theories produce a magnetic field of the right strength—about one fiftieth of what we experience here on Earth’s surface—but how do we decide which one is correct?  Sky & Telescope explains:

Distinguishing between these theories will depend in part on figuring out which rocks were magnetized when. Big bull’s-eyes happened pretty rarely in lunar history. If an impact created a dynamo, any molten surface rock around the time of the crash—such as lava created by the hit itself—would record the magnetic field created. But lava that erupted on the surface between these infrequent events wouldn’t. If most lunar rocks everywhere were magnetized during a particular time period, including rocks not made by impacts, that would sway the balance toward the precession argument, Weiss says. If impact melts are always associated with a magnetic field, the balance swings the other way.

Or maybe a combination of both? Wired makes the point that the two ideas aren’t mutually exclusive:

Dwyer herself has suggested that both models could have some parts correct, with tidal forces pushing the mantle steadily for a time and giant impacts speeding up the motion occasionally.

Image: Luc Viatour / www.Lucnix.be

… Or went, anyway, until last week. An international team of scientists led by Alessandro Morbidelli, presenting at a planetary sciences meeting in France, knocked the large impact theory on its side.

There has always been a significant flaw in the notion of a body a few times more massive than Earth colliding with Uranus. The bright blue planet’s 27 known moons should have continued to orbit the planet at their original angles, but they too lie at almost exactly 98 degrees.

Using computer simulations, Morbidelli and his team examined the large impact theory. The researchers soon discovered that the collision must have happened early in Uranus’ history. The scientists found that if Uranus had been hit when still surrounded by a protoplanetary disk—the material from which the moons would form—then the disk would have reformed into a fat doughnut shape around the new, highly-tilted equatorial plane. Collisions within the disk would have flattened the doughnut, which would then go on to form the moons in their current positions.

From National Geographic News:

When Uranus was hit, this disk was disrupted but then reformed around the planet’s tilted equator, eventually giving rise to the moons in the positions we see today.

However, the simulation also threw up an unexpected result: in the above scenario, the moons displayed retrograde motion—meaning they orbited in the opposite direction to that which we observe. Morbidelli’s group tweaked their parameters in order to explain this.

The surprising discovery was that Uranus was not tilted in one go, as is commonly thought, but rather that two or more smaller collisions transformed the system, tilting the planet and giving the moons the orbits we observe today.

This research is at odds with current theories of how all planets form—not just Uranus, says Morbidelli.

The standard planet formation theory assumes that Uranus, Neptune and the cores of Jupiter and Saturn formed by accreting only small objects in the protoplanetary disk. They should have suffered no giant collisions. The fact that Uranus was hit at least twice suggests that significant impacts were typical in the formation of giant planets. So, the standard theory has to be revised.

Image: Lawrence Sromovsky, University of Wisconsin-Madison, Keck Observatory