A New Look at the ‘Big Whack’

Somewhere a decade or so back in these pages a Centauri Dreams commenter described the event that formed our Moon as ‘the big whack.’ Although I hadn’t run across it before, the phrase turns out to have been common parlance for what is now thought to be a massive collision between the Earth and an early planetesimal. But whatever the case, we know a bit more about the cataclysm thanks to new work out of UCLA, as reported in the journal Science.

The impactor, which struck about 4.5 billion years ago, is commonly called Theia. So how do we analyze such a remote event? The key, as discussed in this UCLA news release, is oxygen, which makes up 90 percent of the volume of lunar rocks the team of geochemists studied, and 50 percent of their weight. Usefully, oxygen can manifest itself in various isotopes, the most common on Earth being O-16, meaning each atom holds eight protons and eight neutrons.

Image: Light image of a lunar rock from the Apollo 17 mission. Credit: NASA.

Heavier isotopes like O-17 (with one extra neutron) and O-18 (with two) occur, though on Earth 99.9 percent of the oxygen is O-16. But each of the planetary bodies in our Solar System has a unique ratio of O-17 to O-18, making for a characteristic signature.

To analyze these ratios, the team studied seven moon rocks from Apollo missions 12, 15 and 17, while also working with five volcanic rocks from Hawaii and one from Arizona. Led by UCLA’s Edward Young, the team found no difference between the Earth’s oxygen isotopes and those of the Moon. This contradicts a 2014 study from Germany (also published in Science) that argued for a distinct ratio of oxygen isotopes on the Moon as opposed to the Earth.

Theia was probably a growing object that would have become a planet in its own right. It may have been as large as the Earth, though some believe it was closer in size to Mars. If the UCLA work is correct, the oxygen isotope data tell us that the collision would have been head-on, for if it had been a glancing blow, the great bulk of the Moon would have been made up of Theia, and would thus have shown different oxygen isotope ratios than the Earth’s. A head-on collision should have produced a similar chemical composition on both Earth and Moon.

“Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them,” Young said. “This explains why we don’t see a different signature of Theia in the moon versus the Earth.”

Image: Artist’s depiction of a collision between two planetary bodies. Such an impact between the Earth and a Mars-sized object likely formed the Moon. Credit: NASA/JPL-Caltech.

Thus a head-on collision, as theorized in 2012 by Matija Ćuk (SETI Institute) and Sarah Stewart (UC-Davis), and also by Robin Canup (SwRI) in the same year, now has more weight. The catastrophic impact’s possible role in removing any water found in the early Earth is not known, though water from small asteroid and comet impacts would eventually be plentifully available.

The paper is Young et al., “Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact,” Science Vol. 351, Issue 6272 (2016), pp. 493-496 (abstract). For the German work in 2014, see D. Herwartz, A. Pack, B. Friedrichs, A. Bischoff, Science 344, 1146–1150 (2014).