The crescent Earth rises above the lunar horizon in this undated NASA handout photograph taken from the Apollo 17 spacecraft in lunar orbit during the final lunar landing mission in the Apollo program in 1972.
| Photo Credit: Reuters

During the Apollo 17 mission in 1972 – the last time people walked on the moon – U.S. astronauts Harrison Schmitt and Eugene Cernan collected about 243 pounds (110.4 kg) of soil and rock samples that were returned to Earth for further study.

A half century later, crystals of the mineral zircon inside a coarse-grained igneous rock fragment collected by Schmitt are giving scientists a deeper understanding about the moon’s formation and the precise age of Earth’s celestial partner.

The moon is about 40 million years older than previously thought – forming more than 4.46 billion years ago, within 110 million years after the solar system’s birth, scientists said on Monday, based on analyses of the crystals.

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The leading hypothesis for lunar formation is that during the solar system’s chaotic early history a Mars-sized object called Theia slammed into primordial Earth. This blasted magma – molten rock – into space, forming a debris disk that orbited Earth and coalesced into the moon. But the exact timing of the moon’s formation has been hard to nail down.

Mineral crystals were able to form after the magma cooled and solidified. The researchers used a method called atom probe tomography to confirm the age of the oldest-known solids that formed after the giant impact, the zircon crystals inside the fragment of a type of rock called norite collected by Schmitt.

“I love the fact that this study was done on a sample that was collected and brought to Earth 51 years ago. At that time, atom probe tomography wasn’t developed yet and scientists wouldn’t have imagined the types of analyses we do today,” said cosmochemist Philipp Heck, senior director of research at the Field Museum in Chicago, a University of Chicago professor and senior author of the study published in the journal Geochemical Perspectives Letters.

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“Interestingly, all the oldest minerals found on Earth, Mars and the moon are zircon crystals. Zircon, not diamond, lasts forever,” UCLA planetary scientist and study co-author Bidong Zhang added.

The rock containing the zircon was collected in the Taurus-Littrow valley at the southeastern edge of the lunar Mare Serenitatis (Sea of Serenity) and stored at NASA’s Johnson Space Center in Houston.

“Zircons are very hard and tough and survive the breakdown of rocks during weathering,” Heck said.

A study led by Zhang published in 2021 used a technique called ion microprobe analysis to measure how many atoms of uranium and lead were in the crystals, calculating the age of the zircon based on the decay of radioactive uranium to lead over time. That age needed to be confirmed through another method because of a potential complication involving lead atoms if defects existed in the zircon crystal structure.

The new study used atom probe tomography to determine there were no complications involving the lead atoms, confirming the age of the crystals.

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“I see this as a great example of what the nanoscale, or even atomic scale, can tell us about big-picture questions,” said study lead author Jennika Greer, a cosmochemist at the University of Glasgow in Scotland.

The moon, which orbits Earth at an average distance of about 239,000 miles (385,000 km), has a diameter of about 2,160 miles (3,475 km), a bit more than a quarter of our planet’s diameter.

“The giant impact that formed the moon was a cataclysmic event for Earth and changed Earth’s rotational speed. After that, the moon had an effect on stabilizing Earth’s rotational axis and slowing down Earth’s rotational speed,” Heck said. “The formation date of the moon is important as only after that Earth became a habitable planet.”

“The moon helps stabilize Earth’s axis for a stable climate,” Zhang added. “The moon’s gravitational pulls help shape the ocean’s ecosystem. The moon is inspirational to human cultures and explorations. And NASA and other space agencies see the moon as a steppingstone for future deep-space explorations.”

In May 2020, some unusual rocks containing distinctive greenish crystals were found in the Erg Chech sand sea, a dune-filled region of the Sahara Desert in southern Algeria. On close inspection, the rocks turned out to be from outer space: lumps of rubble billions of years old, left over from the dawn of the Solar System. Image for Representation.
| Photo Credit: Getty Images/iStockphoto

In May 2020, some unusual rocks containing distinctive greenish crystals were found in the Erg Chech sand sea, a dune-filled region of the Sahara Desert in southern Algeria.

On close inspection, the rocks turned out to be from outer space: lumps of rubble billions of years old, left over from the dawn of the Solar System.

They were all pieces of a meteorite known as Erg Chech 002, which is the oldest volcanic rock ever found, having melted long ago in the fires of some now-vanished ancient protoplanet.

In new research published in Nature Communications, we analysed lead and uranium isotopes in Erg Chech 002 and calculated it is some 4.56556 billion years old, give or take 120,000 years. This is one of the most precise ages ever calculated for an object from space – and our results also cast doubt on some common assumptions about the early Solar System.

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The secret life of aluminium

Around 4.567 billion years ago, our Solar System formed from a vast cloud of gas and dust. Among the many elements in this cloud was aluminium, which came in two forms.

First is the stable form, aluminium-27. Second is aluminium-26, a radioactive isotope mainly produced by exploding stars, which decays over time into magnesium-26.

Aluminium-26 is very useful stuff for scientists who want to understand how the Solar System formed and developed. Because it decays over time, we can use it to date events – particularly within the first four or five million years of the Solar System’s life.

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The decay of aluminium-26 is also important for another reason: we think it was the main source of heat in the early Solar System. This decay influenced the melting of the small, primitive rocks that later clumped together to form the planets.

Uranium, lead and age

However, to use aluminium-26 to understand the past, we need to know whether it was spread around evenly or clumped together more densely in some places than in others.

To figure that out, we will need to calculate the absolute ages of some ancient space rocks more precisely.

Looking at aluminium-26 alone won’t let us do that, because it decays relatively quickly (after around 705,000 years, half of a sample of aluminium-26 will have decayed into magnesium-26). It’s useful for determining the relative ages of different objects, but not their absolute age in years.

But if we combine aluminium-26 data with data about uranium and lead, we can make some headway.

There are two important isotopes of uranium (uranium-235 and uranium-238), which decay into different isotopes of lead (lead-207 and lead-206, respectively).

The uranium isotopes have much longer half-lives (710 million years and 4.47 billion years, respectively), which means we can use them to directly figure out how long ago an event happened.

Meteorite groups

Erg Chech 002 is what is known as an “ungrouped achondrite”.

Achondrites are rocks formed from melted planetesimals, which is what we call solid lumps in the cloud of gas and debris that formed the Solar System. The sources of many achondrites found on Earth have been identified.

Most belong to the so-called Howardite-Eucrite-Diogenite clan, which are believed to have originated from Vesta 4, one of the largest asteroids in the Solar System. Another group of achondrites is called angrites, which all share an unidentified parent body.

Still other achondrites, including Erg Chech 002, are “ungrouped”: their parent bodies and family relationships are unknown.

A clumpy spread of aluminium

In our study of Erg Chech 002, we found it contains a high abundance of lead-206 and lead-207, as well as relatively large amounts of undecayed uranium-238 and uranium-235.

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Measuring the ratios of all the lead and uranium isotopes was what helped us to estimate the age of the rock with such unprecedented accuracy.

We also compared our calculated age with previously published aluminium-26 data for Erg Chech 002, as well as data for various other achondrites.

The comparison with a group of achondrites called volcanic angrites was particularly interesting. We found that the parent body of Erg Chech 002 must have formed from material containing three or four times as much aluminium-26 as the source of the angrites’ parent body.

This shows aluminium-26 was indeed distributed quite unevenly throughout the cloud of dust and gas which formed the solar system.

Our results contribute to a better understanding of the Solar System’s earliest developmental stages, and the geological history of burgeoning planets. Further studies of diverse achondrite groups will undoubtedly continue to refine our understanding and enhance our ability to reconstruct the early history of our Solar System.