THE red rocks of the Jack Hills in Western Australia have an incredible story to tell. Here, deep in the ancient heart of Australia, lie some of the oldest rocks ever found on Earth 鈥� up to 3.7 billion years old.
Beyond that time, however, the geological record in the Jack Hills peters out. And though older rocks have been found elsewhere, they don鈥檛 take us much further back. The period between the Earth鈥檚 creation 4.6 billion years ago and the oldest known rocks are the geological dark ages.
The accepted explanation for this gulf in information was that from 4.6 to 3.8 billion years ago, conditions on Earth were so brutal that even solid rock stood no chance of survival. Huge meteorites and comets pelted the planet 鈥� the moon still shows the scars. The barrage was particularly intense from 4 to 3.8 billion years ago, the so-called late heavy bombardment. No wonder geologists have dubbed the period from 4.6 to 3.8 billion years ago the 鈥淗adean鈥� after Hades, the ancient Greek god of the underworld.
Advertisement
Geologists have long believed that repeated impacts kept the Earth鈥檚 surface molten, which would explain why there are no rocks from this period. As for water, forget it. Any water touching the surface would boil off immediately. Most likely, our planet did not have an atmosphere until much later.
But recently, geologists have discovered fragments of the early Earth that cast doubt on the Hadean. These are zircons 鈥� tiny but tough crystals of zirconium silicate that are the oldest objects on Earth. Zircons can survive being baked up to 1600 掳C and can be washed down the course of an entire river without chipping. Most importantly for geologists, they can survive under tonnes of sediment without undergoing metamorphosis or melting, as other materials do. Nothing else, not even diamond 鈥� which is chemically less stable though harder 鈥� can survive such punishment.
The Jack Hills are full of zircons, and they suggest that the Hadean wasn鈥檛 quite as hellish as we once thought. And if they are telling the truth, there鈥檚 an even more stunning conclusion: Earth was ready to support life perhaps as long as a billion years before anyone thought possible.
Zircons are ubiquitous on Earth. They are found in almost all granites as hard, glinting rocks in which tiny crystals of quartz can be seen with the naked eye. Granite forms when molten material in the Earth鈥檚 crust rises and cools, perhaps in a volcanic eruption. As it solidifies, any zirconium in the melt snatches up silicate and crystallises out as zircons. Zircons are also a common component of sedimentary rocks once erosion has freed them from their original granites.
It was back in 1999 that doubts first surfaced about the notion of a hot, Hadean Earth. Two teams were searching for ancient zircons in the Jack Hills, one led by John Valley at Wisconsin State University in Madison, the other by Mark Harrison of the University of California, Los Angeles. As the Jack Hills contain some of the oldest and most geologically stable continental crust anywhere on Earth, it seemed a good place to look. Both groups shipped tonnes of 3-billion-year-old Jack Hills rocks back to their labs, knowing they could contain grains eroded away from even more ancient granites. Hidden in the rock was the researchers鈥� reward: a few dozen millimetre-sized zircons.
Time capsules
You can date zircons with an ion probe that compares the concentration of uranium to that of its radioactive decay product, lead. It works rather like carbon dating, which measures the ratio of carbon-14 to carbon-12 to indicate how old a once-living object is.
Sure enough, among the zircons the teams collected were a handful much older than the rocks they were buried in, time capsules from the Hadean. The oldest, one of Valley鈥檚, dated from 4.4 billion years ago, a mere 200 million years after the birth of the planet. 鈥淭his shows there was solid material on the surface of the Earth then that has been preserved,鈥� says Valley.
That was a pretty big discovery, but it was not all. The zircons also provided evidence that the early Earth was not hellish after all, but mild and wet. Both teams found 鈥渋nclusions鈥� 鈥� traces of quartz, mica and feldspar 鈥� trapped inside the zircons. These suggested that the zircons had formed from melted, metamorphosed sediments that might initially have been similar to wet mud or clay. Hard evidence for this idea came from measurements of the oxygen isotope ratios in the ancient zircons, revealing that they had a high concentration of the isotope oxygen-18. Rocks that form at a low temperature in wet conditions tend to absorb more oxygen-18 than other rocks. That meant there had to be water present on Earth 4.4 billion years ago.
So it seemed that, far from being a 鈥渕agma ocean鈥� with no atmosphere, the Earth 4.4 billion years ago was solid, cool and wet. And if there was liquid water then there had to be a thick atmosphere: otherwise the water would have boiled off. Hell on Earth was suddenly looking rather balmy.
Perhaps this should not have come as too much of a surprise. For some time climate scientists have had models suggesting that the bombardment of early Earth was not as devastating as once thought. In the late 1980s, for example, Jim Kasting and colleagues at NASA鈥檚 Ames Research Center in Moffett Field, California, showed that the early Earth cooled fast, with a surface temperature probably a long way below 400 掳C by 4.4 billion years ago.
The models also showed that, because the Earth formed from material that contained a lot of water, the young Earth would have had an atmosphere rich in water vapour. And as soon as the planet had cooled sufficiently, this would have condensed as an ocean, perhaps as early as 200 million years after the planet formed. 鈥淭he ocean has been here for a long time,鈥� says Kasting, now at Pennsylvania State University in University Park. 鈥淚t may have been vaporised by large impacts after that time, but the resulting steam atmospheres would rain out in a few thousand years.鈥�
As far as geologists were concerned, however, the models were not hard evidence. And so the idea of the Hadean Earth survived.
Valley and Harrison鈥檚 zircon results also clashed with the received wisdom. One sceptic at the time was Bruce Watson of Rensselaer Polytechnic Institute in Troy, New York. Watson had worked on artificial zircons for decades, and the new discoveries catapulted his expertise to centre stage. 鈥淚鈥檓 sceptical,鈥� he said back in 2003. 鈥淥f course I would like it to be true that our ideas were wrong about the hot, violent beginnings of our planet. But I鈥檓 not ready to say I believe that.鈥�
One problem was that the groups had only tested a handful of zircons each. It was also difficult to rule out the possibility that during their long, long lifetimes, the crystals had undergone some kind of alteration that had affected their oxygen isotope concentrations.
鈥淔ar from being a magma ocean, the Earth of 4.4 billion years ago was solid, cool and wet鈥�
Watson鈥檚 main quibble was with those oxygen isotope results. His studies on artificial zircons had shown that oxygen isotopes can leak in and out of the crystals, suggesting that the values in natural zircons didn鈥檛 necessarily reflect the conditions under which they had formed. Though Valley鈥檚 group had published some work showing that 2.7 billion-year-old zircons retained their oxygen isotope ratios, it wasn鈥檛 clear whether this applied to Hadean zircons, which have probably been buried and redeposited many times since they formed. Watson thought it was possible that the oxygen isotopes, and the inclusions too, had got into the Hadean zircons later in geological history. In which case, they had nothing to say about the early Earth.
It wasn鈥檛 immediately obvious how to counter this criticism. But help soon arrived from an unlikely source: Watson himself. He remembered reading a PhD thesis which reported that the concentration of titanium varied enormously in natural zircons. He wondered whether this might be related to the temperature at which the zircon formed. So he did some experiments on artificial zircons, and by 2002 he had confirmed that the higher the temperature at which a zircon forms, the less titanium it absorbs. So measuring the titanium concentration in the Jack Hills zircons could reveal the temperature at which they formed, providing an independent test of the new ideas about the early Earth.
Last week, Watson and Harrison reported the results of titanium measurements on around 50 Jack Hills zircons (Science, vol 308, p 841). The temperature they get is around 700 掳C, which in zircon terms is positively frigid and points to something in the crust lowering the crystallisation temperature. 鈥淭hat screams water,鈥� says Harrison. The pair calculate that when the ancient zircons formed, the crust must have been at least 5 per cent water by weight, compared with less than 1 per cent at the depth at which zircons form today. And this much water in the crust points inevitably to water at the surface. 鈥淚 find myself being a convert,鈥� says Watson. What is more, Valley鈥檚 group has now published the results of oxygen isotope data on 50 zircons, supporting their previous report. The idea of a mild, wet early Earth is suddenly on firm ground.
And that has important implications for another highly charged scientific debate: when did life begin? Depending on who you believe, the earliest fossil evidence dates from anywhere between 1.9 and 3.85 billion years ago (快猫短视频, 22 February 2003, p 28). Fossils, of course, are not definitive. Earlier evidence might simply have been wiped out, Valley points out. We鈥檒l never know for sure. But thanks to the ancient zircons of the Jack Hills, we now know that, close to the dawn of its creation, the planet was ready for life.
