Arguing over the Moon

If Earth was hit by a huge object that created the Moon, where did evidence of that object go?

By Keith A. Spencer

Senior Editor

Published January 7, 2018 7:30PM (EST)

 (Getty/Giuseppe Cacace)
(Getty/Giuseppe Cacace)

To be human is to have a relationship with our nearest celestial neighbor, the Moon. Every culture has lore and legends, rituals and beliefs associated with it. Some worshipped it as a god — which is sensible given how uncommon and unusual Earth's moon is, comparatively speaking. Not only are we the only terrestrial planet in the solar system with such a sizable moon — Mercury and Venus are moonless, while Mars possesses asteroid-size pipsqueaks — but life on Earth owes a great debt to ours. The moon's stabilizing gravitational power stopped the Earth from tilting too much on its axis, thus stabilizing our seasonal calendar. Hence, it was the Moon that afforded Earth a modicum of climate stability, and the evolution of organisms that depended on climatic routine.

A couple of new scientific papers in Nature reflect recent research into how and why our planet-moon system is so unique among our friends in the solar system. And it turns out that while the main hypothesis for formation is widely accepted, the devil is in the details. And what a devil it is.

The short, and most widely accepted answer for what created the moon is that Earth was hit with a glancing blow from a planetesimal — a big planet-size sphere that wasn't in a stable orbit around the sun — that sloughed off a huge amount of mass, briefly creating a ring around Earth, which ultimately accreted into the moon we know today. I might emend the word "Earth" in that last sentence to read "proto-Earth" — for the Earth then was, as a chef might say, al dente, and became true Earth only when much of its mass merged with the mass of this planetesimal. Some call the proto-Earth "Gaia" and the planetesimal Theia; meaning, Gaia and Theia struck, chaos ensued, and the Earth and Moon system were born in the aftermath.

This theory, known as the "giant impact hypothesis," is convincing inasmuch as it explains much of what we see today. That the moon's composition is similar to the crust of Earth could be explained by the idea that it was merely sloughed-off crustal material. And the intriguing gravitational connections between the two systems, a situation called tidal lock, hints at a shared origin. The only problem is that it doesn't quite explain what happened to Theia. If Theia hit Gaia to create Earth and Moon, shouldn't we see some leftover evidence of Theia?

The key to the mystery lies in understanding the composition of the Earth and the Moon. All of the planets in the solar system have specific ratios in the composition of the elements within them; this is how we know, for instance, when a rock on Earth is actually a meteorite from elsewhere — the ratio of different elements, specifically different isotopes of individual elements, tends to vary. (In case you've forgotten your high school chemistry, an isotope is merely a specific atomic configuration of an element — meaning, a variant on how many neutrons it has. For instance, oxygen has multiple isotopes of different masses, all of which behave the same chemically. With any element, some isotopes are radioactive and may ultimately decay to other elements.)

Pick up a rock anywhere on Earth, crush it and put it in a spectrometer. The ratio of oxygen isotopes to other oxygen isotopes, or between potassium isotopes, will be the same — though different on Mars, Venus or Titan. A different ratio of isotopes in a solar system body hints at different evolutionary origins. In short, isotopes do not lie — and they appear, decay, and exist in such specific quantities as to betray their precise origins.

You may recall the hubbub in the mid-1990s over the unsexily-named ALH84001, a meteorite discovered in Antarctica that was thought to have originated on Mars (and which was in the news because in one analysis it appeared, maybe, to have what looked like cellular life). The 1976 Viking lander on Mars took measurements of the planet's chemistry, which made it possible for scientists to say definitively that a meteorite discovered in Antarctica was actually from the Martian surface: it matched the previously observed unique Martian chemistry.

So, to return to the question of the Earth and the Moon: how do we know that they formed from Gaia and Theia's collision? The key lies in finding differences in the composition of the Earth and Moon. If Theia hit Earth, and a bunch of rocks and dust vaporized, floated around in space, and eventually coalesced into the Moon, there should be some leftover evidence of Theia; likely, it would have a different composition than Earth (or the Moon), and would have left some evidence.

Kaveh Pahlevan explains in Nature that in simulations of the collision, the "debris disk" created post-impact "is sourced mainly from the impacting body" — in other words, the disk would be mostly made of Theia stuff. Earth's mantle, on the other hand, "is sourced mainly from the proto-Earth," Pahlevan continues. In other words: if Theia hit proto-Earth, the debris disk it would form around Earth would be mainly comprised of Theia-material; while Earth's mantle would be mostly made of Gaia stuff.

But that's not what we see. We have detailed measurements of the composition of the Moon because of all the rocks brought back by Apollo astronauts over the years — hundreds of pounds of them. In all cases, the "isotopic similarity" is just about the same as Earth's. In other words, Earth and Moon look almost identical in terms of their compositions.

"The isotopic similarity of Earth and Moon is consistent with the giant-impact hypothesis," Pahlevan explains. We came from the same stuff. Yet "isotopic equivalence" poses problems, too. If the Moon formed via impact of two planetary bodies, where is the isotopic evidence for the second body?

Desperate to solve the mystery, scientists have turned to increasingly precise measurements of different isotopes. Elements are like teenagers in the high school courtyard — they like to hang with very specific groups, the composition of which doesn't change. Oxygen, titanium and silicon isotope measurements did not yield significant insight as to where Theia went. Now, scientists have turned to Tungsten-182 — which, due to the unique crowds that it runs around with, has "special significance in unravelling the lunar origin story," as Pahlevan explains.

Another recent paper in Nature Geoscience studied whether slight differences in Tungsten-182 composition in the Earth and the Moon could be explained by the way in which Earth was bombarded with rocks from the Theia-Gaia collision — in other words, smearing around the mass with a slight degree of randomness. It's akin to looking at a fingerpaint painting and trying to suss out what the original colors were that comprised the mixed paints. Still, "where did Theia go?" is still an open question. The more the isotopes stay the same, the more the science changes.

By Keith A. Spencer

Keith A. Spencer is a social critic and author. Previously a senior editor at Salon, he writes about capitalism, science, labor and culture, and published a book on how Silicon Valley is destroying the world. Keep up with his writing on TwitterFacebook, or Substack.

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Related Topics ------------------------------------------

Astrophysics Earth Gaia Moon Nature Nature Geoscience Physics Planetary Science Science Solar System Theia