Scientists used to think that when Earth first formed, it was an uninhabitable wasteland, covered with molten lava and without a solid crust or ocean. They thought it was, literally, Hell on Earth. The Hadean period, covering the first 500 million years on Earth, is even named after Hades, the Greek word for Hell.
Now, new research turns this hypothesis on its head. The first 500 million years of Earth's life may actually have looked an awful lot like it does today. Our newborn planet had solid land masses, tectonic plates and oceans fully formed 500 million years before scientists originally thought.
The research compares zircon that formed 4 billion years ago to zircon that formed more recently in Iceland is the premise behind this new hypothesis. Iceland was previously thought to be geologically analogous with ancient Earth. However, the study, titled, "Iceland is not a magmatic analog for the Hadean: Evidence from the zircon record," says this isn't the case. The article was released this weekend in the November 1 issue of the journal Earth and Planetary Science Letters.
Comparison of ancient zircon from Jack Hills in Western Australia to zircon that formed naturally in Iceland shows that conditions in Hadean Earth were surprisingly cool and wet.
Scientists thought for over a hundred years that, because no major stone formations from the Hadean period still existed, the climate must have been so hot it melted all stone. However, this new theory that the planet was cool and wet backs evidence that solid land and some surface water definitely existed.
"We reasoned that the only concrete evidence for what the Hadean was like came from the only known survivors: zircon crystals - and yet no one had investigated Icelandic zircon to compare their telltale compositions to those that are more than 4 billion years old, or with zircon from other modern environments," said Calvin Miller, one of the researchers.
The team of researchers who authored the new study came up with a dataset mapping elements found in the Icelandic zircon and compared that to the elemental makeup of the four-billion-year-old zircon from the Hadean period.
The team found that there was evidence of the presence of magma in both the Icelandic zircon and the Australian zircon. However, the Icelandic zircon likely formed at a high temperature, whereas the zircon from the Hadean period formed at a lower temperature. This sheds new doubts on the theory that ancient Earth was incredibly hot. The team also found evidence that the zircon from the Hadean period was formed in the presence of surface water, like an ocean.
The researchers saw big differences in the elemental makeup of the magma from the Hadean period and the Icelandic magma that these zircon stones formed in. For example, the Hadean zircon had much more Ytterbium, which is rare on Earth today.