Earlier Water On Earth? Oldest Rock Suggests Hospitable Young Planet

"This appears to be evidence of the earliest existence of liquid water on our planet," says Margaret Leinen, assistant director of NSF for geosciences. "If water occurred this early in the evolution of earth, it is possible that primitive life, too, occurred at this time."

By probing a tiny grain of zircon, a mineral commonly used to determine the geological age of rocks, scientists from the University of Wisconsin-Madison, Colgate University, Curtin University in Australia and the University of Edinburgh in Scotland have found evidence that 4.4 billion years ago, temperatures had cooled to the 100-degree Centigrade range, a discovery that suggests an early Earth far different from the one previously imagined.

"This is an astounding thing to find for 4.4 billion years ago," says John Valley, a geologist at UW-Madison. "At that time, the Earth's surface should have been a magma ocean. Conventional wisdom would not have predicted a low-temperature environment. These results may indicate that the Earth cooled faster than anyone thought." Previously, the oldest evidence for liquid water on Earth, a precondition and catalyst for life, was from a rock estimated to be 3.8 billion years old.

The accepted view on an infant Earth is that shortly after it first formed 4.5 to 4.6 billion years ago, the planet became little more than a swirling ball of molten metal and rock. Scientists believed it took a long time, perhaps 700 million years, for the Earth to cool to the point that oceans could condense from a thick, Venus-like atmosphere. For 500 million to 600 million years after the Earth was formed, the young planet was pummeled by intense meteorite bombardment. About 4.45 billion years ago, a Mars-size object is believed to have slammed into the Earth, creating the moon by blasting pieces of the infant planet into space.

The new picture of the earliest Earth is based on a single, tiny grain of zircon from western Australia found and dated by Simon Wilde, of the School of Applied Geology at Curtin University of Technology in Perth, Western Australia. Valley worked with William Peck, a geologist at Colgate University, to analyze oxygen isotope ratios, measure rare earth elements, and determine element composition in a grain of zircon that measured little more than the diameter of two human hairs. Colin Graham's laboratory analyzed the zircon to obtain the oxygen isotope ratios. Graham is a contributor to the paper and geochemist at the University of Edinburgh.

"What the oxygen isotopes and rare earth analysis show us is a high oxygen isotope ratio that is not common in other such minerals from the first half of the Earth's history," Peck says. In other words, the chemistry of the mineral and the rock in which it developed could only have formed from material in a low-temperature environment at Earth's surface.

"This is the first evidence of crust as old as 4.4 billion years, and indicates the development of continental-type crust during intense meteorite bombardment of the early Earth," Valley says. "It is possible that the water-rock interaction (as represented in the ancient zircon sample) could have occurred during this bombardment, but between cataclysmic events."

Scientists have been searching diligently to find samples of the Earth's oldest rocks. Valley and Peck say such ancient samples are extremely rare because rock is constantly recycled or sinks to the hot mantle of the Earth. Over the great spans of geologic time, there is little surface material that has not been recycled and reprocessed in this way.

The tiny grain of zirconium silicate or zircon found by Wilde in western Australia was embedded in a larger sample containing fragments of material from many different rocks, Valley says. Zircons dated at 4.3 billion years were reported from the same site a decade ago, but the new-found zircon grain is more than 100 million years older than any other known sample, giving scientists a rare window to the earliest period of the Earth.

"This early age restricts theories for the formation of the moon," Valley says. "Perhaps the moon formed earlier than we thought, or by a different process." Another intriguing question is whether or not life may have arisen at that early time. Low temperatures and water are preconditions for life. The earliest known biochemical evidence for life and for a hydrosphere is estimated at 3.85 billion years ago, and the oldest microfossils are 3.5 billion years old.

"It may have been that life evolved and was completely extinguished several times" in catastrophic, meteorite-triggered extinction events well before that, Valley says. The research conducted by Valley, Peck, Graham and Wilde was also supported by the U.S. Department of Energy, the U.K. Natural Environment Research Council and a Dean Morgridge Wisconsin Distinguished Graduate Fellowship.