Apollo missions impact looking at the Moon

The Apollo Moon landings, the first of which occurred 50 years ago tomorrow (Australian time), were not just “one giant step for mankind”. They set in motion a process that revolutionised our understanding of how the Moon was formed.

Prior to the first Apollo landing, on 20 July 1969, very little was known about the Moon, says Richard Carlson, a geochemist and geochronologist at the Carnegie Institution for Science in Washington, DC, US.

Based on its density, he says, scientists knew that it had to be mostly rock, without Earth’s large metallic core.

And they knew that its surface, however blinding it might look under the unfiltered sunlight of interplanetary space, is actually composed of relatively dark materials, a fact that led to speculation that it might be similar to a class of dark, high-carbon meteorites known as carbonaceous chondrites.

It was also presumed that it formed slowly and was cool enough that it only partially melted.

But the very first scoops of material brought back by Apollo 11 proved all of this to be wrong, Carlson says.

For one thing, the Moon wasn’t dark because it was loaded with carbon. Rather, we now know, it was dark because of a process known as space weathering, in which bombardment by energetic particles in the solar wind alter its surface in ways that turn normally bright substances dark.

But Apollo 11 also brought back sand grains and pebbles that were almost pure anorthosite, a mineral that is common enough on Earth, but is generally mixed with other minerals.

Figuring out how it could clump into entire pebbles took some head-scratching, but within a few months, a team led by John Wood of the Smithsonian Astrophysical Observatory, Cambridge, Massachusetts, realised that this meant that rather than being cool in its early stages, the Moon had to have been melted into a giant magma ocean.

Because anorthosite is less dense than the average material that would have comprised such an ocean, Wood’s team concluded, it would have risen to the surface as it crystalised, where it floated like rocky icebergs. {%recommended 9091%}

Prior to Apollo, there were three basic models for how the Moon formed. One, made famous 90 years before Apollo by British astronomer Sir George Darwin, was that the early Earth had somehow been spinning so fast that it fissioned, with the Moon thrown off like a giant discus hurled by an Olympic thrower. 

Under this theory, the Pacific Ocean basin was the divot left in the part of the Earth’s crust from which the Moon broke free. 

Another theory was that the Moon formed independently from the Earth and was somehow ensnared and captured by our planet’s gravity.

A third was that the Earth and the Moon formed together, and were always close companions.

Three theories: none of which held up in the face of Apollo.

To begin with, isotope dating of the Apollo Moon rocks revealed that the Moon formed 4.4 billion years ago, about 150 million years after the Earth. That doesn’t sound like a huge difference, but it kills the idea that the Moon and Earth formed together, simultaneously.

At the same time, Moon rocks and Earth rocks were incredibly similar in composition, especially in the signatures of rare isotopes we now know vary from planet to planet. “It’s really pushing the idea that the Moon is made of stuff that came out of the Earth,” Carlson says.

That, of course, led to the giant impact theory, under which the Earth, 150 million years after its formation, was clobbered by an object about the size of Mars, which blasted enough material off its surface to then coalesce into the Moon.

But however much this idea may have captured the popular imagination, evidence from the Apollo rocks is suggesting that even it might not be completely correct, Carlson says.

“It’s a nice idea, but [for it] the isotopic similarity [between Earth rocks and Moon rocks] is a problem,” he says.

That’s because there’s no reason to assume that the impactor should have had exactly the same isotope ratios as the Earth. So, for the giant impact theory to work, the collision must have been so violent that it mixed the materials of the impactor and the proto-Earth well enough that no trace of their original differences remains.

The current resolution to this conundrum, Carlson says, is a model that suggests that the young Earth was spinning very rapidly at the time it was struck, and that the impact may have been head-on, rather than a glancing blow.

That would have generated so much heat that the outer portions of the Earth would have reached temperatures of 10,000 °C, he says. That wouldn’t simply have melted its rock, but would have converted it (and much of the impactor) into a thoroughly mixed cloud of rock vapor that would have expanded into space like a “big, gaseous puffball” from which the Moon eventually condensed.

Whether this model is correct, Carlson doesn’t know.

But it’s interesting, and without the samples returned by the Apollo program, we might still be mired in 1960s planet-formation science.

David Kring, a lunar geologist and senior staff scientist at the Lunar and Planetary Science Institute in Houston, Texas, agrees. “The Apollo program demonstrated the extraordinary value of samples,” he says. “From a few gray rocks emerged big ideas.”

Carlson’s thoughts on the legacy of Apollo appear in a special issue of the journal Science dedicated to the fiftieth anniversary of the Apollo 11 landing.