Oct 24 2023

Age of the Moon Revised

There are a few interesting stories lurking in this news item, but lets start with the top level – a new study revises the minimum age of the Moon to 4.46 billion years, 40 million years older than the previous estimate. That in itself is interesting, but not game-changing. It’s really a tweak, an incremental increase in precision. How scientists made this calculation, however, is more interesting.

The researchers studied zircon crystals brought back from Apollo 17. Zircon is a crystal silicate that often contains some uranium. These crystals would have formed when the magma surface of the Moon cooled. The current dominant theory is that a Mars-sized planet slammed into the proto-Earth about four and a half billion years ago, creating the Earth as we know it. The collision also threw up a tremendous amount of material, with the bulk of it coalescing into our Moon. The surface of both worlds would have been molten from the heat of the collision, but it is easier to date the Moon because the surface is better preserved. The surface of the Earth undergoes constant turnover of one type or another, while the lunar surface is ancient. So dating the Moon tells us something about the age of the Earth also.

The method of dating employed in this latest study is called atom probe tomography. First they use an ion beam microscope to carve the tip of a crystal to a sharp point. Then they use UV lasers to evaporate atoms off the tip of the crystal. These atoms pass through a mass spectrometer, which uses the time it takes to pass through as a measure of mass, which identifies the element. The researchers are interested in the proportion of uranium to lead. Uranium is a common element found in zircon, and it also undergoes radioactive decay into lead at a known rate. In any sample you can therefore use the ratio of uranium to lead to calculate the age of that sample. Doing so yielded an age of 4.46 billion years old – the new minimum age of the Moon. It’s possible the Moon could be older than this, but it can’t be any younger.

This study reminded me of what a NASA scientist said about the recent return of samples from the comet Bennu (the OSIRIS-REx mission) – in 50 years scientists will be studying these samples using instruments that don’t exist yet to answer questions we haven’t asked yet. Apollo 17 brought back these lunar sample in 1972, 51 years ago, and here is a new study using a technique that did not exist in 1972. This definitely goes in the win column for NASA.

As fascinating as all this is, I think there is an even bigger story here. This latest study adds one more small piece to the overall mountain of evidence as to the age of the Earth-Moon system. It’s a great example of how science works at the large scale. Scientists use evidence to test hypothesis about the nature of the universe. Slowly they build a picture of what exists and how it works. We never, philosophically speaking, get to any ultimate Truth with a capital “T”, but we can develop models that can make such reliable and precise predictions that, for practical purposes, we can treat them as true. We sent a probe to Pluto, on a 3 billion mile journey, and were rewarded with incredible pictures of the distant world. If our models of basic physics did not reflect reality to a significant degree, this would not have worked.

One of the approaches that scientists use to model reality is to compare independent lines of evidence and see if they all line up at the same answer. There is only one reality, so the findings of science must all be internally consistent. They also try really hard to prove themselves wrong, so they can say that a model is working, at least so far.

When it comes to dating stuff, the science has progressed tremendously, and is yielding increasing precise and internally consistent results. It still takes time, because no single method is perfect, but using various methods, depending on the context, dating methods generally provide a consistent picture of history. My recent post about the age of the oldest humans in the Americas is a great example. First pollen from sea plants were used with a carbon-14 dating method to age the substrate of the human footprints, yielding an age of 21-23 thousand years old. But plants growing in water may not have the same starting ratio of carbon-14 and carbon-12, so there was still some doubt as to the accuracy of this result. Further study was done using pine pollen from land plants, eliminating this uncertainty, and verifying the results. But then another method was used, the optically stimulated luminescence of quartz, which relies on a completely different physical process, and also resulting in the 21-23 thousand year old result. Multiple independent lines of evidence triangulating to the same result.

I also wrote recently about tree ring data (dendrochronology), which can actually use tree rings to line up with each other creating a calendar year record going back about 15,000 years. Tree rings can also be dated by using their place in the calendar year sequence, but also by carbon-14 dating, which provides a way to independently validate the carbo-14 method, and also to calibrate it by creating a record of the starting composition of the atmosphere to then date other organic samples.

An independent line of evidence comes from ice cores. Ice cores also have layers, because of winter snowfall and summer melt, and the ice preserves a record of the atmosphere in trapped gas, and also isotopes in trapped minerals. Volcanic eruptions are preserved as layers of ash in the ice, and those can be dated by various radioactive decay methods. Organic material can be dated by carbon-14 dating. Using these various methods we have a continuous ice-core record from Greenland going back 150,000 years and in Antarctica going back 800,000 years. The oldest individual ice core sample is possibly 2.7 million years old.

All of this is extremely difficult to square with a 10,000 year old Earth. But of course, that does not deter young-Earth creationists. If you are sufficiently motivated, you can find reasons to cast doubt on any individual piece of evidence. To some extent, this is a healthy part of science – as I said, try to shoot down any hypothesis as much as possible. But this has to be done is a fair and consistent manner, looking at all the evidence. That is not what YEC deniers do. For example regarding ice cores, this site raises two points. Many refer to crashed WWII airplanes buried in the ice. They calculate the depth and argue that at that rate the ice sheet can be only 1,000 years old.  But their analysis is nonsense – the plane in question was buried by glacial flow, not annual layered deposits. Their analysis is simply wrong.

They also just deny ice core dating by arguing that perhaps precipitating rates were different in the past. But this is just a made-up handwaving argument, and is not supported by evidence. Remember – we have layer counts, which are not dependent on precipitating rates (that would make the layers thinner, but not change the number of layers). Also, this would not affect dating volcanic ash layers in the ice.

The deniers are not addressing the multiple independent lines of evidence for the age of the Earth. They are just throwing out flawed or made-up special pleading arguments to deny pieces of the evidence, in order to create seemingly plausible deniability for the believers.

Meanwhile, real science keeps chugging along, with each new piece to the puzzle fitting into the larger picture. Of course there are still lots of unknowns and apparent contradictions – that is h0w science slowly progresses. But for some questions the evidence becomes overwhelming. The evidence for an ancient Earth and Moon are one of those situations.


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