Early Earth Was a Much More Pleasant Place Than Commonly Thought

New research suggests that Earth in its earliest states was a fairly reasonable place, and looked a lot like the planet we experience and thrive on today. It was neither a roiling sea of magma nor a harsh, hot grey landscape like that of Iceland, which is often thought to be an early Earth analog.

A description of the earliest era of planet Earth is about as unsettled as that of the earliest era of the universe. For many years, it was accepted that the planet was a hellish ball of molten rock, as habitable and life-harboring as Venus. That view changed about 30 years ago, when geologists first discovered zircon crystals dating back some 4 billion years. These crystals act as a time capsule of sorts, from which it's possible to extract information about the environment in which they formed.

The zircon crystals suggested that the early Earth (an era known as Hadean Earth) was cool, or at least much cooler than previously though. It was cool enough for some amount of water to be present on the surface, anyhow.

The effect of this discovery was to split geologists into two camps with two early-Earth interpretations. One of those holds that Hadean Earth looked a lot like Iceland: a rough, lifeless place where new crust is constantly being generated from crystallizing magma.The other interpretation is that Hadean Earth looked a lot like our current Earth.

A doctoral student at Vanderbilt University, Tamara Carley, began collecting samples of zircon crystals from all over the relatively young (geologically speaking) Iceland, arriving at a pool of material representing the 18-million-year history of the island. The hope was that comparisons between these crystals and the much older crystals representing early Earth would help settle the issue as to what this place actually used to look like.

Carley's efforts led to a study this week in the journal Earth and Planetary Science Letters that debunks Iceland-like descriptions of Hadean Earth. The investigation, led by Vanderbilt professor Calvin Miller, found that the crystals formed within Iceland were created by temperatures much higher than those of the very early Earth. This suggests that Hadean Earth was a much cooler, more hospitable place than Iceland is now. It was possibly even quite wet.

"This statistically robust zircon-based comparison between Iceland and the early Earth reveals distinctions in chemistry that suggest fundamental differences in magmatic environments," Miller et al write in the current study. "Many Hadean zircons and almost all Icelandic zircons grew from magmas with substantial contributions from materials that had interacted with surface waters,"

"In the Hadean case, the interaction occurred at low temperatures, while in Iceland, it was a high-temperature interaction," the paper continues.

"Our conclusion is counterintuitive," said Miller in a statement. "Hadean zircons grew from magmas rather similar to those formed in modern subduction zones, but apparently even 'cooler' and 'wetter' than those being produced today."

Nonetheless, it remains likely that Earth existed for some time with a massive, heavy atmosphere laden with CO2. Life would still have to wait long enough for oceans to soak some of that up, leaving more habitable atmospheric conditions. But perhaps it wouldn't have to wait quite as long as the hot, magma-strewn, Icelandic model might suggest.