Privileged Planet: very early water (Introduction)

by David Turell , Tuesday, May 14, 2024, 14:43 (194 days ago) @ David Turell

Just 600 million years after Earth formed:

https://www.livescience.com/planet-earth/geology/earth-may-have-had-freshwater-and-cont...

'Earth's first continents may have emerged from the planet's primordial oceans much earlier than we thought, just six hundred million years after the planet formed, new research suggests.

"The researchers found that ancient zircon crystals from the Jack Hills in Western Australia contain evidence of fresh water, which indicates that patches of land must have been present as fresh water can only form if there's land for it to pool on following precipitation.

"The composition of early Earth has long puzzled scientists. When our planet first formed 4.6 billion years ago, it was a roiling sphere of magma. The eon after that, called the Hadean (4.6 billion to 4 billion years ago), is poorly understood. While we know that this magma eventually solidified and formed a crust, we do not know precisely what happened after that.

"Some scientists have suggested that Earth may have been mostly covered by water as early as 4.4 billion years ago — aligning with the oldest zircons ever discovered. However, it is unclear how water arrived. It may have been part of the planet's original composition or may have been the result of bombardment by water-bearing asteroids soon after its formation.

"Fresh water would only have been present if a hydrological cycle — evaporation and precipitation — had already begun by that point of Earth's life, and that water devoid of the minerals present in salt water could collect on emerged portions of continental crust according to the presentation abstract.

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"The scientists found that zircons extracted from rocks in the Jack Hills contained higher levels of light oxygen isotopes than zircons formed in the presence of seawater, indicating that they formed as magma rose to the surface and interacted with fresh water. They dated the crystals by measuring ratios of different uranium isotopes in the samples. Of the 1,400 zircons analyzed, the presentation abstract claimed, a few dated to 3.4 billion years ago and another few dated to 4 billion years ago. Most were much younger, with the most recent crystals dating to 1.85 billion years ago.

"Zircons are extraordinarily resilient. As a result they linger in rocks that are much younger than they are, and young and old zircons end up jumbled together. The rock in which the zircons from the Jack Hills was found was 3 billion years old according to the presentation.

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"If the researchers are correct, lonely outposts of terra firma may have been jutting from the primordial waves earlier than we thought."

Comment: it happened to our planet, and we ended up with two-thirds of the surface covered. Astronomers are not seeing this elsewhere. Why just us?