Imagine you’re digging around your back yard—perhaps making a nice herb garden or digging posts for a deck—and you come across a rock along the way. That’s not too hard to imagine, since either of those activities are bound to turn up a large number of rocks, but perhaps you’re more prone to couch surfing than home improvement.
In any event, you find a rock.
Ever wonder how old that rock might be? Where it might have traveled? Perhaps, for example, it had been stepped on by a dinosaur once upon a time?
These musings become even more interesting if you happen to live in western Australia, particularly in the Jack Hills region. On the outcrops of that sparsely inhabited region of land, you might just come across tiny bits of rock that date back to a few hundred million years after the Earth was first formed. We’re talking rocks that are 4.4 billion years old.
That’s right, billion. With a B.
How do geologists come up with this number? Well, rather than relying on made-up texts, researchers look to the ratios of certain isotopes contained within the sample.
Remember if you will that an isotope is an atom of an element—defined by that atom’s number of protons—that has a varying number of neutrons. For example, the most abundant form of carbon on Earth has eight protons and eight neutrons, but there are also forms of carbon found on earth that have eight protons and seven or six neutrons.
When you get to really disparate numbers–like eight protons and two neutrons, for example–the atom becomes unstable. A proton might spontaneously turn into a neutron to even out the ratio, giving off bits of energy in the process.
That energy is also known as radiation.
Thanks to accelerators around the world that can create these unstable isotopes, scientists know which isotopes eventually turn into what stable atoms. They also know the likely paths they take to get there and how long it takes for this process to occur. So if you know how many original unstable isotopes there are, and you also know how many final stable atoms that have decayed there are, you can figure out how long its been since the sample had only the unstable isotopes—it’s first formation.
Researchers have done this with a few rare zircon rocks from the Jack Hills region, and discovered that the ratio of rare lead ions indicates that the rocks in question are at least 4.4 billion years old. But some people require more proof. What if the ratios got thrown off by atoms being added later in the rock’s life, for example?
John Valley, a geochemist from the University of Wisconsin, has given them this proof. He lead a study that used a new technique called atom-probe tomography that actually maps and weighs the individual atoms in a microscopic sample of zircon. By checking out again the ratio of isotopes and determining their location within the crystalline structure of the zircon, Valley was able to corroborate the earlier findings that the rock comes from 4.4 billion years ago.
But that’s not the end of the story.
Rather than being randomly distributed throughout the sample, the lead isotopes were clustered together like raisins in a pudding. The clusters of lead atoms formed 1 billion years after crystallization of the zircon, by which time the radioactive decay of uranium had formed the lead atoms that then diffused into clusters during reheating.
“The zircon formed 4.4 billion years ago, and at 3.4 billion years, all the lead that existed at that time was concentrated in these hotspots,” Valley says. “This allows us to read a new page of the thermal history recorded by these tiny zircon time capsules.”
The study, according to Valley, strengthens the theory of a “cool early Earth,” where temperatures were low enough for liquid water, oceans and a hydrosphere not long after the planet’s crust congealed from a sea of molten rock.
“The study reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago,” and possibly life not long after, says Valley.
“The Earth was assembled from a lot of heterogeneous material from the solar system,” Valley explains, noting that the early Earth experienced intense bombardment by meteors, including a collision with a Mars-sized object about 4.5 billion years ago “that formed our moon, and melted and homogenized the Earth. Our samples formed after the magma oceans cooled and prove that these events were very early.”
The study, “Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography,” was published by Valley and a whole host of associates from the University of Wisconsin; the University of Puerto Rico; CAMECA in Madison, Wisconsin; Curtin University of Western Australia; and the University of Western Ontario.