The rain that fell on Bruce Watson and Mark Harrison as they collected rocks in Western Australia’s arid shrublands in June 2005 was a fitting epilogue to the two scientists’ previous year’s research, published in the journal Science
the month before. The area around Western Australia’s Jack Hills had a reputation for being dry, barren and hot. It’s also home to the oldest Earth material scientists have ever found, and Watson and Harrison were there to replenish their supply. “Ironically,” said Watson, “it rained on us nearly every day.”
The scientists’ encounter with cool, wet weather where they expected a searing, parched landscape echoed the story that mineral crystals from the Jack Hills have begun to tell about the history of the very early Earth. The crystals appear to contradict the conventional notion that the first 500 million years of Earth’s history—the Hadean Eon—were a continuously violent and chaotic time, when endless volcanism and continual meteor bombardment kept a global magma ocean simmering across the surface of the newly formed planet.
Instead, the chemical make up of the Jack Hills crystals suggests that they formed in the presence of liquid water, likely even an ocean. These crystals provide evidence that even the very early Earth was cooler and wetter than scientists used to think. A gentler Hadean could have permitted life to evolve far earlier in the planet’s history than scientists originally supposed.
Evidence of the Earliest Earth
It’s hard for scientists to know exactly what went on during the Hadean because no one has found any rocks that survived Earth’s infancy. Scientists use the chemical and physical structure of rocks to infer not only how old they are, but also something about the conditions in which they formed.
In the absence of Hadean rocks, scientists have relied on indirect evidence of what went on for 500 million years after the formation of the Earth about 4.57 billion years ago. Lunar observations and astronomical models of solar system formation paint a picture of a violent Earth, bombarded with meteors, covered with an ocean of magma, and boiling with volcanic activity. This fiery, Hades-like image gave rise to the period’s name.
For many years, there was no direct evidence to challenge that picture. But in the early 1980s came the discovery in Western Australia of single grains of an extremely durable mineral crystal known as zircon. Even after they are recycled through countless generations of rock, zircon crystals retain hints about the physical and chemical conditions in which they formed.
At the time, scientists didn’t immediately appreciate the full worth of the ancient zircons from Australia. “They were found out of context,” explained Watson, a geochemist from Rensselaer Polytechnic Institute, “weathered out of the rocks they had been embedded in and washed out into river sediment. Geologists didn’t think much verifiable ‘environmental’ information could be gleaned from them. And although the zircons were known to be old—the first really old zircons from the Jack Hills were dated at maybe 4.1 billion years old—we had rocks that were almost that old anyway.”
Since then, however, scientists have found zircons that date to almost 4.4 billion years ago. “That might not seem like a big difference—4.1 versus 4.4—but it is another 300 million years closer to the origin of the Earth,” said Watson. As the age of the zircons kept getting pushed back, interest among scientists grew, and they began to develop techniques for teasing information from the zircons.
Among the first important discoveries, says Watson, came out in 2001. Analysis of the relative amounts of different isotopes of oxygen indicated that the ratio was skewed toward “heavy” oxygen-18, as opposed to the more common “light” oxygen-16. “When a geologist sees a heavy oxygen signature in rocks,” said Watson, “it’s commonly understood to be a sign that the rocks formed in cool, wet, sedimentary processes at the Earth’s surface.” Thus, the magma that eventually gave rise to the zircons might have been formed from what had once been sediments deposited on the floor of an ancient ocean.
A Titanium “Thermometer”
Watson’s own study of the Australian zircons supports the idea of a cooler, wetter Hadean. Several years ago, Watson and the students working in his lab began work on a titanium “thermometer”—an equation that describes how the titanium concentration in zircons changes as their crystallization temperature changes. Once they had the equation, geologists could estimate the temperature at which the Australian zircons crystallized more than 4 billion years ago by measuring the amount of titanium they contain.
To develop the “thermometer” equation, Watson and his team grew synthetic zircons in the presence of titanium at controlled temperatures in his lab. Then they measured how much titanium appeared in crystals grown at different temperatures. They also analyzed titanium concentrations in naturally occurring zircons found at several locations across the globe, whose crystallization temperature had been determined through independent tests by other scientists.
“I knew all along that by far the most compelling application of the thermometer would be to the [Jack Hills] zircons, but people don’t part with them too willingly,” explained Watson. To gain access to the zircons, Watson teamed up with an old friend, Mark Harrison of the Australian National University, whose department had made studying the zircons a top priority. “He had access, I had the tool, and so we pooled our resources,” explained Watson.
When Watson compared the titanium concentration of the Jack Hills zircons to his thermometer, he discovered something interesting. All the zircons appeared to have formed within a very restricted temperature range: about 680 plus or minus 20 degrees Celsius.
To a geologist, that is a very special temperature range, explains Watson. “Any rock heated in the presence of water—any rock, at any time, in any circumstance—will begin to melt at between 650 and 700 degrees. This is the only terrestrial process that occurs so predictably,” said Watson.
Watson thinks the most likely scenario is that the Jack Hills zircons formed in a continental crust environment similar to what we know today, in which water-saturated rocks melt as they are subducted along plate boundaries or are buried in deep sediment. “Some critics have said the water could have been coming up into the zircon-forming area from down in the mantle [the layer of Earth beneath the crust], and our results don’t directly contradict that explanation,” acknowledged Watson. “But taken with previously published zircon evidence such as the oxygen isotope data, we feel our results point more strongly toward the idea of surface water.”
Tracking down a mistake
How do we reconcile the idea of abundant liquid water with the Hell-on-Earth scenario traditionally theorized for the Hadean? According to David Morrison, senior scientist at NASA’s Astrobiology Institute, some of the things we first assumed about the Hadean may simply be wrong. Earth’s bombardment by asteroids and comets, for example, probably did not happen the way it is often illustrated, with multiple, simultaneous impacts streaking across a red-tinged sky. Based on the number and size of craters on the Moon, a hypothetical observer perched on a hilltop on Earth would probably not have seen more than one impact over a human lifetime.
“The idea of a global magma ocean is also probably overblown,” says Morrison. “We know from experience on Earth that lava cools very rapidly on the surface. I have walked across a lava field in Hawaii within 10 hours of its emplacement with no more harm than slightly singed soles on my boots, and within a month, a lava flow is cool to the touch.” A planet with intermittent volcanism—even at much higher rates than we see on Earth today—is likely to have a relatively cool, solid crust most of the time, not a seething magma ocean. It is even possible for much of such a planet to be ice-covered.
Although the Australian zircons indicate average conditions less Hell-like that the name “Hadean” might suggest, there were still intervals of global catastrophe far more violent than anything we can experience today. The craters left on the Moon tell us that asteroids large enough to boil away the ocean probably hit Earth several times during the Hadean.
Such impacts would be planet-sterilizing, at least for any life in the oceans. Thus life might have formed and been snuffed out several times in the Hadean. Eventually these sterilizing impacts ceased, and even the largest subsequent impacts left a few survivors in protected locations, such as ocean-floor hot springs. Evidence of this history appears in the organisms that sit at the root of the universal family tree of life: the most ancient common ancestors of life on Earth are heat-loving microorganisms that don’t need oxygen to survive. Such heat-loving microbes are the sort of creatures we would expect to survive the final, near-sterilizing impacts of the Hadean.
Scientists are beginning to imagine a Hadean world that experienced far more dramatic swings in surface conditions than anything revealed in subsequent geologic history. Most of the time there appears to have been a stable, liquid water ocean, perhaps ice covered. Intermittently, however, massive volcanic eruptions or asteroid impacts destabilized conditions, creating something like the “Hell” scientists once thought prevailed then. It was in such conditions that life arose on our planet, and it is out of this maelstrom that our microbial ancestors emerged.
Links
- NASA Astrobiology Institute. Earth’s Oldest Mineral Grains Suggest an Early Start for Life. Accessed February 27, 2006.
- Valley, J.W., Peck, W.H., and King, E.M. (1999). Zircons are Forever. The Outcrop for 1999: University of Wisconsin-Madison Geology Alumni Newsletter, 34-35. Accessed February 27, 2006.
- University of California, Los Angeles. Scientists from UCLA and Australia Find Evidence of Water on Earth 4.3 Billion Years Ago. Accessed February 27, 2006.
References
- Morrison, D. (2006) Habitable Conditions on the Early Earth. Accessed February 27, 2006.
- United States Geological Survey. (1999) Age of the Earth. Accessed February 27, 2006.
- Watson, E.B., and Harrison, T.M. (2005) Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth. Science, 308 (5723), 841-844.
- Watson, E.B., and Harrison, T.M. (2006) Response to Comments on “Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth”. Science, 311 (5762), 779.
NASA Earth Observatory story by Rebecca Lindsey with contributions from David Morrison; design by Robert Simmon
