Scientists still have many questions about the earliest years of our planet.
Earth formed about 4.55 billion years ago, but almost no rocks from that time have survived.
Without those ancient rocks, it is difficult to know what Earth looked like in its earliest days or when important processes such as plate tectonics began.
To uncover clues, researchers study extremely durable minerals called zircons.
These tiny crystals can survive for billions of years without being significantly altered.
Because they form inside molten rock, they record chemical details about the environment at the time they formed. In a way, they act like microscopic time capsules from Earth’s distant past.
Some of the oldest known zircons are about 4.4 billion years old. Many of them come from the Jack Hills region of Western Australia, one of the few places where minerals from the earliest Earth can still be found.
A new study from California Institute of Technology, commonly known as Caltech, examined these ancient crystals and uncovered surprising information about the early planet.
The research was led by geochemist Shane Houchin, working in the laboratory of Francois Tissot.
Their findings were published in the journal Proceedings of the National Academy of Sciences.
The scientists analyzed both the cores and the outer rims of zircon grains. Some zircons originally formed deep in magma billions of years ago, but later geological events caused new layers of zircon to grow around them.
These layers can contain valuable chemical information about different periods of Earth’s history.
By studying tiny amounts of elements such as uranium and titanium trapped inside the crystals, the team was able to learn about the conditions present when the zircons formed.
One of their most surprising discoveries involved the early chemical environment of Earth. For decades, scientists imagined the earliest Earth as a harsh, “hellish” world filled with volcanic gases and almost no oxygen or water. This period is known as the Hadean Eon.
However, the new research suggests that Earth’s surface environment may have changed faster than previously believed. The researchers found that zircon rims formed about 4.1 billion years ago contained uranium in a highly oxidized state. Oxidation is often linked to the presence of oxygen or water in the environment.
This result suggests that Earth’s crust may have become oxidized only about 350 million years after the planet formed. In other words, the early Earth may not have been as dry and hostile as scientists once thought. Instead, significant amounts of water could already have been present.
The zircons also provided clues about another major question: when plate tectonics began. Plate tectonics is the process in which large pieces of Earth’s crust move and interact, forming mountains, earthquakes, and volcanoes.
The team discovered that the zircon crystals had once been exposed to very high pressure but relatively low temperatures. These conditions resemble those found in subduction zones, where one tectonic plate slides beneath another and sinks deep into the planet.
This evidence suggests that plate tectonics may have already been operating at least 3.35 billion years ago. Scientists have long debated when this process began, and the new study provides an important data point in that discussion.
To make these discoveries, the researchers used a specialized technique called U XANES oxybarometry, which allowed them to measure the oxidation state of uranium in the crystals. They carried out these measurements using powerful synchrotron facilities at the Advanced Photon Source at Argonne National Laboratory.
The team hopes to analyze hundreds more zircon crystals in the future. By doing so, they may continue to uncover new details about how Earth transformed from a young planet into the world we know today.
