Soon after formation the Earth would have had fresh water and continents
Ancient zircon crystals contain chemical clues that fresh water may have existed on Earth soon after its formation.
Earth's first continents may have emerged from the planet's primordial oceans much earlier than we thought, as little as six hundred million years after the planet formed, new research suggests.
Ancient zircon crystals from the Jack Hills in Western Australia contain evidence of fresh water, indicating that areas of land must have been present as fresh water can only form if there is land to later collect to precipitation.
The team described the zircons at a European Geosciences Union conference in April 2024. The composition of early Earth has long puzzled scientists. When our planet first formed, 4.6 billion years ago, it was a bubbling ball of magma. The next eon, called the Hadean (4.6 to 4 billion years ago), is little known. While we know that this magma eventually solidified and formed a crust, we don't know exactly what happened next. Some scientists have suggested that Earth may have been mostly covered in water as early as 4.4 billion years ago, aligning with the oldest zircons ever discovered. However, it is unclear how the water got there. It may have been part of the original composition of the planet or it may have been the result of bombardment by water-containing asteroids soon after its formation.
Fresh water would have been present only if a hydrological cycle - evaporation and precipitation - had already begun at that moment in the Earth's life, and if that water devoid of the minerals present in salt water could collect on emerged portions of the continental crust according to the presentation abstract.
Rainwater contains lighter isotopes, or versions of oxygen, because heavier isotopes are more resistant to evaporation.
Salt water contains more heavy isotopes of oxygen, which evaporate less easily.
Scientists found that zircons extracted from Jack Hills rocks contained higher levels of light oxygen isotopes than zircons formed in the presence of seawater, indicating that they formed when magma rose to the surface and interacted with fresh water. They dated the crystals by measuring the ratios of different uranium isotopes in the samples. Of the 1,400 zircons analyzed, some date back to 3.4 billion years ago and others to 4 billion years ago, according to the presentation abstract. Most were much younger, with the most recent crystals dating back 1.85 billion years.
Zircons are extraordinarily durable. As a result they linger in rocks much younger than themselves and the young and old zircons end up mixing. According to the presentation, the rock in which the Jack Hills zircons were found was 3 billion years old. Due to their resilience, zircons are extremely useful for understanding when exactly the continental plates formed by the crust began to break through the surface of the global ocean.
The oldest rock ever found has been dated to 4.03 billion years ago, but because the earliest zircons are hundreds of millions of years older, they provide rare insights into the planet's early history. If researchers are right, solitary land outposts may have emerged from primordial waves sooner than we thought.
