By Kelly Beatty
Researchers have announced interesting new lunar research results, especially about how and when the Moon formed, and why the “Man in the Moon” looks down on us whenever the lunar disk is fully lit.
When the Moon is full, its most dominant features are the dark maria (lava seas) that cover much of the hemisphere facing Earth.
It’s Friday, the 13th. My go-to almanac, the Observer’s Handbook, tells me that full Moon came and went at 12:11 a.m. Eastern Daylight Time. Soothsayers might have you believe that this is a time to worry. Not so! But this coincidence of ominous date and lunar phase is rather unusual — as reckoned in Universal Time, it won’t occur again until for another 35 years!
So rather than raise some eyebrows, let’s use the occasion to raise our collective appreciation for Earth’s closest neighbor in space. This week features a trio of interesting research findings.
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: it formed alongside Earth as a pair (termed “co-accretion”), became trapped by Earth while passing close by (the “capture scenario”), or was spit out by an Earth spinning so rapidly that it became unstable and split in two (“fission”).
Artist’s depiction of a collision between two planetary bodies. Such an impact between the Earth and “Theia,” a Mars-sized object, likely formed the Moon.
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 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. And 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 came together,” 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 start ticking 4.53 billion years ago.
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 melange that’s part proto-Earth and part Theia. Computer simulations have consistently suggested that a glancing blow would have required in order to squirt enough material into orbit. Yet past analyses have shown that the ratios of oxygen’s three isotopes are an exact match 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 in the 17O:16O ratios of 12