The oldest known zircons date back to 4.4 billion years ago, about 200 million years after Earth was formed. They lie in Australian rocks formed approximately 3 billion years ago from the erosional debris of prior rocks.
Geologists favoring the cold Earth theory argue that the chemical composition of these crystals shows that they formed from water-rich magma—similar to that which forms today's continents.
In other words, early Earth must have had climate conditions much like today's—and have been cool enough to allow continents to form.
But Coogan was suspicious. "To my mind, this is a large interpretation to make on the basis of a few crystals that aren't in their original rock," he said.
He and a Scottish colleague collected from seabeds samples of modern rocks formed by an entirely different process.
"We analyzed the composition of the zircons and found that it is basically the same as the Hadean ones," he said.
This shows that zircons don't have to form on continents.
But Coogan says more evidence is needed to conclusively support a theory of a hot early Earth.
"I certainly wouldn't say that we've closed the case [for a hot, nasty Hadean]," Coogan said.
Instead, the study shows that zircons don't provide good evidence for a cool Earth theory, he says.
Alternate Explanation
Norm Sleep, a geophysicist at Stanford University, disagrees.
The problem, he says, is that the zircons stuck around for more than a billion years. Rocks located in places other than continents disappear very quickly.
"Something sat around for 1.2 billion years or so before it got eroded. That's the key point," he said.
He offers an alternate explanation for the formation of Coogan's zircons. In today's oceans, he says, magma rises beneath mid-ocean ridges. Occasionally the roof of the magma chamber collapses, and water mixes with the magma.
That process can form a rock with zircons similar to those in continental crust, he says.
"[Coogan and his colleagues] are saying that the zircons from that kind of rock and the ones from the Hadean are about the same," he said. "That may be well and true," but it doesn't mean that Hadean zircons didn't come from continents.
For those reasons, Sleep doubts that early Earth was all that hellish. Asteroid impacts occurred, he says, but not frequently enough to turn the planet into a steamy slag heap.
"Unless you were very unlucky, you would not get killed by an asteroid," he said. "You would not have to sweep meteor dust off your driveway every night. Except right after an asteroid impact, it doesn't really correspond to what a good Baptist would think hell should be."
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