Three New Findings About the Moon

Researchers have announced interesting news concerning the Moon, especially about how and when it formed, and why the "Man in the Moon" constantly stares at us whenever the lunar disk is fully lit.

Last Friday, the 13th, a full Moon came and went at 12:11 a.m. Eastern Daylight Time. Though not a cause for worry, this coincidence of ominous date and lunar phase was rather unusual — as reckoned in Universal Time, it won't occur again until for another 35 years!

The occasion coincided with a week that featured a trio of interesting research findings about the Moon.

When Did the Moon Form?

Cosmic thinkers have wondered where the Moon came from for a very long time. A century ago, in the absence of any real proof one way or the other, researchers had proposed three possibilities: the Moon formed alongside Earth as a pair (an idea termed "co-accretion"), it became trapped by Earth while passing close by (the "capture scenario"), or it was spit out when Earth spun so rapidly that it became unstable and split in two ("fission").

All are interesting, but once geochemists got their hands on actual lunar samples, they quickly concluded that none of these are likely correct. Instead, computer simulations in the 1970s showed that the Moon most likely formed when a Mars-size object struck the young Earth. This ejected a huge jet of debris that formed a ring around our young planet and rapidly coalesced into the Moon. (The putative impactor is often called Theia, named for the Titan in Greek mythology who gave birth to Selene, the goddess of the Moon.)

This "giant-impact hypothesis" neatly explains many unique aspects of lunar composition. For example, Moon rocks contain isotopes of oxygen, tungsten, and other elements in ratios that are very close matches to Earth's (more on this point in a moment). Yet lunar rocks contain very little iron and (virtually) no water or other volatile elements. Also, the angular momentum of the Earth-Moon system is very high.

So when did this big splat occur? It's hard to pin down exactly, because geochemists have to rely on difficult measurements of trace radioactive isotopes and their daughter products in lunar rocks. Until recently, the best answer was "about 100 million years after the solar system's formation," which would put the age at about 4.47 billion years.

However, new results, presented this week at the Goldschmidt Geochemistry Conference, suggest it happened some 60 million years earlier. Guillaume Avice and Bernard Marty (University of Lorraine, France) analyzed xenon gas trapped inside quartz-bearing rocks from South Africa and Australia. Their model takes into account how much xenon was present when the solar-system formed (as determined from meteorites); how much was created by the fission of radioactive iodine, plutonium, and uranium; and how much has lost to outer space (during, say, a major impact).

"The xenon gas signals allow us to calculate when the atmosphere was being formed," Avice notes in a press release, "which was probably at the time the Earth collided with a planet-sized body, leading to the formation of the Moon. Our results mean that both the Earth and the Moon are older than we had thought.”

So, based on this result, we should all reset our Earth-Moon clocks to 4.53 billion years and counting.

How Did the Moon Form?

The giant-impact hypothesis fits the observational data well but still has some holes in it (no pun intended).

For example, the Moon should be a compositional mélange that's part proto-Earth and part Theia. Computer simulations have consistently suggested that a glancing blow would have been required in order to squirt enough material into orbit. Yet previous analyses have shown that the ratios of oxygen's three isotopes match exactly in terrestrial and lunar rocks. This could only occur if Theia and Earth started out with identical blends of oxygen isotopes (virtually impossible) or if somehow Earth and its newly formed Moon swapped white-hot vapor with each other (far-fetched at best) so that the isotopes could "equilibrate". (Or, conceivably, no giant impact occurred at all.)

The good news is that the oxygen ratios aren't exact after all. In Science for June 6th, a team led by Daniel Herwartz (George August University, Göttingen, Germany) used improved techniques to derive the isotopic ratios of oxygen-16, -17, and -18. First, they tried analyzing lunar meteorites that have fallen onto Earth, but their technique was sensitive enough to detect terrestrial contamination. So instead they tested lunar samples returned by the Apollo 11, 12, and 16 missions.

Using these, the researchers found a difference of 12 ± 3 parts per million in the ¹⁷O:¹⁶O ratios of terrestrial and lunar rocks. "The differences are small and difficult to detect, but they are there," says Herwartz. The upshot is that the giant collision really happened. It also puts some strict limits on Theia's composition, though one lingering problem is that we don't know how massive it was. Some recent modeling suggests that, if Earth were already spinning rapidly when it got smacked, then a relatively small impactor could have triggered the Moon's formation. In that case, less than about 10% of the eventual lunar mass must have come from Theia.

Why is the Moon Two-Faced?

This week's third lunar result takes a stab at resolving a 55-year-old mystery. Since the Moon's rotation is locked in such a way that we see only one hemisphere, we didn't know what the unseen half looked like. Then, in 1959, the Soviet spacecraft Luna 3 radioed the historic first-ever views of the lunar farside that showed it to be very different: the familiar dark maria so common on the nearside are almost completely absent.

Various researchers have tried to explain this dichotomy in the decades since, but with no real consensus emerging. Now Penn State researchers Arpita Roy, Jason Wright, and Steinn Sigurdsson think they have the answer.

Writing in the June 9th issue of Astrophysical Journal Letters, they explain that when the Moon first formed it was much closer, just 5% to 10% of its present distance, and that its rotation quickly became tidally locked. Both bodies were still hot from the collision, with Earth's surface a red-hot 4,500°F (2500°C). This meant the lunar nearside was continually seared by the looming, glowing Earth that dominated its sky, too hot for any minerals to condense into solid.

But the farside, out of sight from Earth, cooled much quicker. The first solids to form were rich in aluminum and calcium, minerals that have relatively low densities. These early minerals became the building blocks for the lunar crust, which accumulated faster and grew substantially thicker on the generally cooler farside.

Later on, titanic collisions battered all of the young Moon's surface. The nearside's relatively thin crust, fractured deeply by the largest impacts, provided an easy conduit for dense, metal-enriched magmas to rise from the deep interior to the surface. These formed the pattern of dark maria — the familiar "Man in the Moon" — seen today. But similar eruptions were almost nonexistent on the farside, due to its thicker crust.

And that, claim Roy and her colleagues, is how the Moon got its two-faced appearance.

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Want to compare the Moon's nearside and farside for yourself? Check out Sky & Telescope's terrific new lunar globes. Choose either the natural-hued Moon you see by eye or the color-coded topographic version.