Scientists fire tiny laser beams at zircon crystals to reveal Earth's galactic companion

Earth's oldest mineral, zircon, is a treasure for Earth and space scientists alike

In a new study published in GeoScienceWorld, an international team of Earth and space scientists looked at rocks from two of Earth's oldest continents, which preserve the history of our planet's earliest continental formation—the North American Craton in Greenland, and the Pilbara Craton, in Western Australia.

"We crushed these samples into powders, then used various techniques to separate tiny grains of a mineral called zircon," Dr. Tim Johnson, associate lecturer in Earth and Planetary Sciences at Curtin University and co-author of the study, explains to IE.

"Zircon is cool for many reasons," adds Johnson. The mineral occurs in tiny volumes in the rocks that make up the continents and is incredibly robust to almost all the chemical and physical processes that destroy weaker minerals.

Dr. Chris Kirkland, leader of the Timescales of Mineral Systems Group at Curtin University and co-author of the study, reveals to IE, "we dated minerals (zircon crystals) using the decay of U [Uranium] to Pb [lead], which acts much like a stopwatch."

"We can use various machines that fire tiny laser beams at the zircon grains to measure the U and Pb, which tells us when they grew," describes Dr. Johnson.

Better yet, the decay of uranium in zircon crystals found in these ancient regions creates a geological timeline spanning 1 billion years-from 3.8 billion to 2.8 billion years ago.

This corresponds with a time on Earth that geologists described as the Archean eon when the Earth's crust had cooled enough for continents to form and for the earliest known life to start. For geologists, the Archean rocks are a goldmine for pondering how Earth became the only planet known to have continents.

A 200-million-year rhythm to early crust production

In an article published in The Conversation, Dr. Phil Sutton, senior lecturer in astrophysics at the Univesity of Lincoln and lead author of the study, explains that with these two pieces of information – age and composition – the team could then reconstruct a timeline of crust formation.

Next was the study's distinguishing element; the scientists applied a mathematical analysis called the Fourier Transform. This enabled them to identify whether a reoccurring event with a common frequency was present in the data they had—much like unscrambling the ingredients used to bake a cake.

In an interview with IE, Sutton clarifies, "what this means is that the scientists looked for patterns in crust production by transforming the data from the time domain into the frequency domain."

The results from this approach suggest an approximate 200-million-year rhythm to crust production on the early Earth. Right, so what's the connection with the cosmos?

Earth's big cosmic journey also takes about 200 million years

"Interestingly, statistical analysis of the age data of zircon grains shows a periodicity in crust production that matches periods of time during which the solar system entered and exited the spiral arms of the Milky Way galaxy," Dr. Tim Johnson explains to IE.

The spiral arms of the galaxy are the main production centers of young stars. Our galaxy, the Milky Way, has two main spiral arms, the Perseus arm, and the Scutum-Centaurus arm. There are also two smaller arms, sometimes referred to as spurs, called the Sagittarius and the Orion, Orion-Cygnus, or Local Arm (which is where our solar system is located and parts of which are also sometimes referred to as the Norma or Inner arm and the Outer Arm). There is also a more recently discovered arm called the "Far-3 kiloparsec arm," which is shorter than the two major arms and lies along the bar of the galaxy.

Our Solar System and the spiral arms of the Milky Way are all spinning around the supermassive black hole at the galaxy’s center. "We can think of this as cars traveling down a multi-lane road. As stars all orbit in a common direction, there are differences in their relative velocities," explains Sutton.

The spiral arms orbit at about 210 kilometers per second, while the Sun is speeding along at 240km per second, meaning our Solar System is surfing into and out of the galaxy’s arms. It is believed that each of these transits takes around 200 million years.

Dr. Phil Sutton explains this model in the video below:

So, the timing of crust production on Earth and the length of time it takes for our solar system to orbit the galactic spiral arms are both around 200 million years– but how is this connected?

The Oort cloud effect- an increased bombardment of comets toward Earth

The scientists highlight that when stars are close to one another, as they are in the spiral arms of the galaxy, there are stronger gravitational interactions between them. These interactions cause a gravitational tug which can perturb or alter the orbits of planets, asteroids, and comets that orbit the star.

"For us, the tugs are small but enough to dislodge comets at the edge of the solar system and send them inwards," highlights Dr. Phillip Sutton.

The scientists believe that while the solar system is in a spiral arm, gravitational forces from the more densely packed stars interact with the Oort cloud- a distant cloud of rocky debris orbiting our sun- and cause the dislodgement of large amounts of material from the cloud, which then hurtles towards the inner Solar System.

Some of this material could strike Earth at speeds as fast as 52 kilometers per second (Earth typically experiences asteroid impacts with an average velocity of around 15 kilometers per second).

Giant meteorite impacts built Earth’s continents billions of years ago

The researchers believe that these high-energy impacts are being recorded by the presence of Hafnium isotopes in the Zircon minerals found in the ancient crusts they studied.

This is because Hafnium isotopes suggest an influx of juvenile magma — magma that has never reached the surface before. Such an influx makes sense in a scenario where a blast from a high-energy meteorite has caused a perturbation in Earth's surface, leading to decompression melting of the mantle. The researchers describe this as "not too dissimilar from popping a cork on a bottle of fizz," in an article in The Conservation.

Still, evidence from this team of scientists for how meteorite impacts produced continental crust on Earth is not completely new. In fact, it was only earlier this year when IE covered a previous study by the team which provided evidence for the role of meteorite impacts on continent formation during the first billion years or so of our planet's 4.5 billion-year history.

The fundamental difference with this new study is that the scientists have pinpointed that the rhythm of crust production on the early Earth and an enhanced rate of Earth bombardment by comets occurs around every 200 million years.

Therefore, it is the team's theory that Earth's journey through the galaxy helped shape the planet's geology.

Could this help us in our search for life on exoplanets?

"When looking for exoplanets, it might want us to check out their local neighborhood," reveals Dr. Phillip Sutton. " It can also give us a sense," adds Dr. Tim Johnson.

For instance, if the exoplanets are in a different part of the galaxy, they might alter the orbits of the planets more or less. This might be useful for looking at the very long-term habitability of the planet.

Still, we know that just because a planet is in a habitable zone does not mean it is habitable. As in the case of potentially habitable planets found orbiting red dwarf stars, their atmospheres can be stripped away as a result of being too close to stars.

Or perhaps to figure out the next mass extinction event...

While what the scientists report is related to deep time, i.e., Earth’s early history, they also highlight that there have been similar cycles involving impacts that have caused mass extinction events. Again, these may be linked to solar systems' movement through the Milky Way.

With this thought in mind, could deciphering these rhythms of bombardment and continent formation further help us to predict future extinction events on Earth, or perhaps on exoplanets? Of course, we do not know- yet.

This work also makes the most fundamental question of "how did we come to be on this planet?" a lot more interesting.

"I think this study, and our almost synchronous Nature paper, put some solid footings to some vague ideas that have been around at least for many decades, but probably much longer. We will annoy lots of people in the scientific community and beyond," Dr. Tim Johnson shares with IE.

Still, whether it turns out that these ideas eventually turn out to be right or wrong, they will lead us to a deeper understanding. That's how science should work, right?