"What we call it is ‘proto continental crust,’" said zircon researcher John Valley of the University of Wisconsin at Madison, who is regarded as a member of the Cool Early Earth camp. The rocks in which the zircons formed may not have been quite the same as continental crust today, says Valley, but they were heading in that direction.
On the other hand, Valley agrees with Coogan that there is still a lot of work to be done to settle the matter and that "It’s a mistake to think that any of the answers are final yet."
The Hadean zircon question is important and getting a lot of attention from scientists for several reasons, explained geoscientist Paul Hoskin of Central Washington University in Ellensburg, Wash.
"This is extremely interesting because it may mean that life was able to evolve in the early oceans much sooner — by about 1 billion years — than previously thought," said Hoskin. Right now the earliest evidence for life is at about 3.4 billion years ago.
There are also implications for other planets, like Mars, says Martin Whitehouse, a zircon researcher at the Swedish Museum of Natural History.
If, for instance, the ancient zircons are from an early Earth that did not have plate tectonics or oceans, but still had a proto-crust of some sort, it might have been a lot like the four billion-year-old surface of Mars seen today.
If that’s the case, it could suggest that water is the key difference between the planets, says Whitehouse. If Mars had had more of it to help flux hot rocks into magma and lubricate crustal plates, perhaps it might have evolved into a planet a lot more like Earth, he explained.
"The more we understand the early crust on Venus and Mars the more we are going to understand Earth," said Whitehouse.
In other words, the planetary science goes both ways. And since scientists have found no zircons in Martian meteorites, he said, the Hadean zircon research could help select where to land future missions to the Red Planet.
For now, however, about the only thing getting clearer about Earth’s early crust is how much we don’t know.
"The paper of Coogan & Hinton highlights the fact that there is much work to be done yet to understand Earth’s earliest history and it reminds us that alternative, more mainstream explanations, may turn out to be more satisfactory," said Hoskin.
