Moon theory packs a real wallop
HOW did the moon form? It’s a question that has been on astronomers’ minds for a very long time, and there have been several theories.
It was once thought the moon could be a piece that separated from the still-forming and rapidly spinning Earth. Other possibilities that have been considered are that it was captured into orbit after getting too close to Earth, or, more simply, that the two bodies formed at the same time, as a pair of adjacent objects.
However, there are strong scientific reasons for rejecting these ideas.
The leading theory today is that the moon formed as a result of a dramatic collision between Earth and another planet — a Mars-sized one that has been named Theia.
I have written on this subject before. However, in recent months, this idea has been strengthened.
Researchers based at Arizona State University in the US, led by Qian Yuan, think that pieces of Theia may lie buried deep in Earth’s mantle.
They base their thoughts on two very large and unusually dense parts of the mantle, called large low shear velocity provinces, abbreviated to LLSVPs.
We can’t examine them directly, but we know they are there because of measurements of seismic waves moving through Earth’s interior.
Earthquakes can be quite devastating, but the passage of waves from an earthquake as they are affected by the internal structure of our planet and then measured elsewhere — often a very long way away — can tell us much about Earth’s interior.
The two LLSVPs are centred on opposite sides of Earth, one below the Pacific Ocean and the other below Africa.
However, they are quite irregularly shaped in terms of depth and lateral extent.
The researchers found that volcanic material from Samoa and Iceland, assumed to have come from deep in the mantle, showed evidence for the LLSVPs being at least 4.45 billion years old.
This is quite consistent with the time that the moonforming collision is thought to have taken place, and suggests that these features are actually parts of Theia itself. They also think that Theia’s mantle was denser than that of Earth, which is why it sank to a lower level.
This latest research comes not long after another piece of supporting evidence came to light. In 2020, Justin Simon
and Tony Gargano, from NASA’s Astromaterials Research and Exploration Science Division at Johnson Space Center in Houston, examined the amount of chlorine, and the proportion of heavy chlorine, in samples from Earth and the moon.
Heavy chlorine, also called chlorine-37, is one of two stable isotopes of chlorine. (Isotopes of an element have nuclei containing the same
number of protons but different numbers of neutrons.) The nucleus of a chlorine atom always contains 17 protons, but the two stable types of that element contain either 18 or 20 neutrons. Chlorine-37 has 20, so is the “heavier’’ of the two types.
In the lunar material brought back by the Apollo astronauts, there is a much lower proportion of chlorine and other “volatile” elements than is found on Earth, but of the chlorine that is present, there is a higher proportion of heavy chlorine.
By examining elements in the same family as chlorine, called the halogens, the researchers were also able to deduce that the loss of chlorine took place in a high-energy event — which is likely to have been the “giant impact” — and that there were no events later in the moon’s history that gave rise to the chlorine proportions we measure today.
It is, therefore, looking good for the giant impact theory, which explains so many observations and measurements that have been made — including those of the interior of our planet. Results like these also remind us of the importance of the lunar samples we can study. The vast majority were brought back to Earth between 1969 and 1972 by the astronauts of the Apollo missions, during which 12 men walked on the moon, set up experiments and collected samples.
It’s something to think about as we gaze at the moon, which will return to our evening skies as a lovely crescent during the coming week.