By David Tenenbaum
The oldest rocks so far identified on Earth are one-half billion years younger than the planet itself, so geologists have relied on certain crystals as micro-messengers from ancient times. Called zircons (for their major constituent, zirconium) these crystals "are the kind of mineral that a geologist loves," says Stephen Mojzsis, an associate professor of geological sciences at the University of Colorado at Boulder. "They capture chemical information about the melt from which they crystallize, and they preserve that information very, very well," even under extreme heat and pressure.
The most ancient zircons yet recovered date back 4.38 billion years. They provide the first direct data on the young Earth soon after the solar system coalesced from a disk of gas and dust 4.57 billion years ago. These zircons tend to refute the conventional picture of a hot, volcanic planet under constant assault by asteroids and comets.
One modern use for the ancient zircons, Mojzsis says, is to explore the late heavy bombardment, a cataclysmic, 30- to 100-million-year period of impacts that many scientists think could have extinguished any life that may have been around 4 billion years ago.
With support from a NASA Exobiology grant, Mojzsis has begun examining the effect of impacts on a new batch of zircons found in areas that have been hit by more recent impacts. Some will come from the Sudbury, Ontario impact zone, which was formed 1.8 billion years ago.
"We know the size, velocity and temperature distribution, so we will be looking at the outer shell of the zircons," which can form during the intense heat and pressure of an impact, he says. A second set of zircons was chosen to span the Cretaceous-Tertiary (KT) impact of 65 million years ago, which exterminated the dinosaurs.
"The point is to demonstrate that the Hadean zircons show the same type of impact features as these younger ones," Mojzsis says. The Hadean Era, named for the hellish conditions that supposedly prevailed on Earth, ended about 3.8 billion years ago.
The oldest zircons indicate that Earth already had oceans and arcs of islands 4.45 to 4.5 billion years ago, just 50 million years after the gigantic collision that formed the moon. At that time, Mojzsis says, "Earth had more similarities than differences with today. It was completely contrary to the old assumption, based on no data, that Earth's surface was a blasted, lunar-like landscape."
Zircons are natural timekeepers because, during crystallization, they incorporate radioactive uranium and thorium, but exclude lead. As the uranium and thorium decay, they produce lead isotopes that get trapped within the zircons. By knowing the half-lives of the decay of uranium and thorium to lead, and the amount of these elements and their isotopes in the mineral, it's possible to calculate how much time has elapsed since the zircon crystallized.
Zircons carry other information as well. Those that contain a high concentration of the heavier oxygen isotope O-18, compared to the more common O-16, crystallized in magma containing material that had interacted with liquid water. A new "titanium thermometer," developed by Bruce Watson of Rensselaer Polytechnic Institute and Mark Harrison of the University of California at Los Angeles, can determine the temperature of crystallization based on the titanium concentration.
Both these analyses showed that zircons from as far back as 4.38 billion years ago crystallized in relatively cool conditions, such as at subduction zones where water and magma interact at the intersection of tectonic plates.
To Mojzsis, the message from the most ancient zircons is this: just 50 million years after a mammoth impact formed the moon, Earth had conditions we might recognize today, not the hellish conditions long favored by the conventional viewpoint.
For reasons related to the orbital dynamics of the solar system, that bucolic era was brutally interrupted about 3.96 billion years ago by the "late heavy bombardment," a period of intense asteroid impacts that churned the planet's surface.
The zircons record this period in the form of a narrow, 2-micron-thick zone that most likely formed during a brief exposure to very high temperature. Careful radioactive dating shows that these zones formed essentially simultaneously, even in Hadean zircons of different ages, Mojzsis says. "We found the most amazing thing. These zircons, even if the core ages are different, all share a common 3.96 billion year age for this overgrowth."
The zones also record "massive loss of lead, which happens when the system is heated quite catastrophically and then quenched," Mojzsis adds. "So it looks like these zircons were sort of cauterized by some process" that both built up the zone and allowed the lead to escape. The cause, he says, was likely "some extremely energetic event" at 3.96 billion years ago, a date that "correlates very nicely to other estimates of the beginning of the late heavy bombardment."
The intense impacts of this period would seem to have exterminated any life that had formed previously. And yet Mojzsis says this conclusion may be overturned by the zircon data. "From the Hadean zircons we can understand further what the thermal consequences for the crust were, and test our models for habitability during the late heavy bombardment. Most people think it sterilized Earth's surface, but our analysis says that is not the case at all. For a microbial biosphere at some depth in crustal rocks and sediments, impact at the surface zone did not matter," he says.
Indeed, University of Colorado post-doctoral student Oleg Abramov has calculated that the habitable volume of Earth's crust actually increased by a factor 10 for heat-loving thermophiles and hyperthermophiles during the impacts, Mojzsis says.
This raises the possibility that life survived the period of heavy impacts. "The bombing, however locally devastating, creates quite an ample supply of hydrothermal altered rock and hydrothermal systems, worldwide," says Mojzsis.
Although that's bad for organisms that require cool conditions, "thermophiles do not even notice," he says. "This goes back to an old idea, maybe the late heavy bombardment pruned the tree of life, and selected for thermophiles. Whatever the diversity of life was like before the late heavy bombardment, afterwards it was diminished, and all life henceforth is derived from these survivors."
