‘Moon rocks’ made here: tranquillityite discovered in Western Australia

The Apollo missions yielded more than just great views of our home planet. NASA

By the time the Apollo Program ended in 1972 it had cost NASA roughly US$170 billion dollars (in today’s terms). It was seen as a waste of money by some, but almost 40 years since the launch of Apollo 17, we are still seeing significant returns on the investment.

Among the most significant of those returns is the valuable information the lunar landings provided about our moon and, in turn, the planet we call home.

Have you seen my rock collection?

Prior to the Apollo missions, knowledge about the moon was limited to remote sensing, modelling and speculation. It was unclear what the moon was composed of, whether it was young or old, whether it harboured life, and whether it contained water. There were many theories but few facts.

On July 20, 1969, the Apollo 11 Lunar Module touched down in the Sea of Tranquillity. Soon after landing, Neil Armstrong and “Buzz” Aldrin became the first humans to walk on the moon. During their two-and-a-half hours outside the spacecraft, they collected 58 samples, weighing a total of 21.6 kilograms.

Five days later, the first box of samples was opened before a global audience. Although the first dusty samples looked like overcooked potatoes, they soon started answering some fundamental questions, and raising new ones.

A microscope image of a thin slice of coarse-grained basalt collected from the Sea of Tranquillity by the Apollo 11 astronauts. Birger Rasmussen

Among the samples were coarse-grained basalts that were found to contain minerals not known on Earth: pyroxferroite, armalcolite and tranquillityite. Within a decade the first two of these were identified on Earth but tranquillityite was not, and it came to be regarded as the “moon’s own mineral”.

Tranquillityite is a silicate mineral containing zirconium, titanium and iron that crystallises from a magma with other late-forming minerals.

Australian-made

Rocks of broadly similar composition to the coarse-grained “lunar basalts” also occur here on Earth. These rocks are referred to as dolerite, and contain many of the same minerals.

My colleagues and I recently collected samples of Western Australian dolerite while searching for minerals to use for dating. Our research involved detailed examination of thin slices of dolerite using optical and scanning electron microscopes.

In a sample from the Pilbara region, small, foxy red crystals were identified that had the same X-ray element spectra – a kind-of compositional fingerprint – as tranquillityite from the moon.

Examination of thin slices of the mineral using a high-resolution transmission electron microscope revealed it had the same structure as annealed (slowly cooled) lunar tranquillityite.

Since the first discovery, we’ve now identified tranquillityite in dolerite from six new localities in Western Australia. This suggests tranquillityite may be a widely distributed trace mineral in these types of rocks.

A sample of dolerite from the Pilbara region of Western Australia containing tranquillityite. Birger Rasmussen

Dating rocks

Tranquillityite contains trace amounts of uranium and so attempts were made to date it by ion microprobe – an instrument that fires a focused beam of charged particles at the sample. We now know that tranquillityite is particularly good for dating and we can deduce the age of the mineral quite accurately.

By analysing basalt collected during the Apollo 11 mission we can deduce that the Sea of Tranquillity was a vast lava field 3.7 billion years ago. Rocks this old are rare on Earth and are typically highly deformed and metamorphosed, having experienced a complex geological history.

On the moon, by contrast, the absence of tectonic processes, water and microbial life has left most lunar rocks virtually unaltered since they formed. Indeed, the 3.7 billion-year-old lunar basalts appear “fresher” than lavas that erupt today in Hawaii or Iceland.

Because of its ideal properties for dating, tranquillityite could also be used to date the dolerite in Western Australia. It turns out that the dolerite is more than a billion years old – much older than expected – necessitating a re-evaluation of the geological history of this region of Australia.

In our own backyard …

Two questions immediately arise from our discovery of tranquillityite on Earth: why was it first found on the moon, and why did it then take 40 years to identify on Earth?

It was probably first identified on the moon for a couple of reasons.

Firstly, scientific interest in lunar material was intense and within five years of the first returned lunar samples, nearly 1,000 scientists from 200 research groups across the world had closely scrutinised the samples.

In 1970, six independent groups had already identified a mysterious new mineral in Apollo 11 basalts variously referred to as “phase A”, “an iron, titanium, zirconium silicate” and “an unnamed yttrium-zirconium silicate”.

In 1971, Australian geologist John Lovering and co-authors jointly published a paper describing the new mineral and naming it “tranquillityite” after the Sea of Tranquillity.

An optical microscope image showing a crystal of tranquillityite (red-brown) in a sample of dolerite from Western Australia. Birger Rasmussen

Secondly, at that time, new technology was emerging in the form of scanning electron microscopes and electron microprobes, which allowed scientists to extract every bit of information possible from these priceless samples.

Although tranquillityite also formed on Earth, it was probably not discovered earlier because these rocks were not as intensely studied. Tranquillityite is small and rare, and can be mistaken for other minerals. Also, dolerite is mostly examined today using techniques that require crushing of entire samples to determine chemical composition or to extract minerals for dating. These approaches do not favour the discovery of small, rare minerals.

The identification of tranquillityite on Earth 40 years after its discovery on the moon is a reminder of the many achievements of the Apollo mission, including countless scientific discoveries, revolutionary advances in technology and, perhaps most importantly, the inspiration of a generation of scientists and engineers.

From the vantage point of 2012, the estimated US$170 billion cost (in today’s terms) of the Apollo mission seems increasingly modest, and is surpassed many times over by the long-term economic, societal and scientific returns.