When did the Earth actually form? There are a number of ways to approach that question. We can use radioactive dating to look at material that has fallen to Earth after remaining largely undisturbed since the formation of our Solar System, or we can obtain a date for the oldest materials we've found on Earth. But those methods simply provide an upper and lower limit; the Earth formed some time after the smaller material in the Solar System, while the earliest materials on Earth would have been produced some time after its formation.
Now, researchers have provided a new date for the formation of the Earth, based on the last time the planet was entirely molten—an event that was triggered by a collision with a body that ultimately created the Moon. The data is calculated using what we know about the early Solar System combined with the debris that fell to Earth after the big collision.
The early Solar System was a violent place, as small particles aggregated into bodies that then grew larger by collisions. These collisions eventually produced planetesimals the size of large asteroids, which merged to form the current collection of planets. So there was no clear start to what would ultimately become the Earth, but there was a clear end to the primary process of its formation.
That end came when the proto-Earth was smacked by a Mars-sized body. The resulting collision would have left the Earth a magma ocean, blown away its atmosphere and any volatile liquids on its surface, and put enough debris in orbit to form the Moon. In effect, it acted like a reset button for the timing of the Earth's formation, remixing all its components so that the raw material for radioactive dating—the stable maintenance of isotope differences caused by radioactive decay—was eliminated. Everything started afresh.
Figuring out the timing of that collision is important if we want to understand the conditions on the early Earth and in the early Solar System in general. The new study uses a clever way of providing an estimate, based on the fact that the last major collision in the Earth's history didn't mean that the Earth escaped further bombardment from space.
The logic is pretty clever. In the wake of the Moon-forming collision, the entire Earth was molten, which allowed the iron to sink to the core. A number of heavy elements that have an affinity for iron sunk to the core with it. This would have left the surface of the Earth completely stripped of these metals. (These elements, which include gold and platinum, are generically referred to as siderophiles.) But it's possible to mine this material from the crust.
The elements are there because the Earth's supply was partly refreshed by the arrival of new material in collisions with other planetesimals and smaller asteroids. (The most famous asteroid, the one that killed the dinosaurs, was first identified through the presence of a layer of another siderophile, iridium, which it carried to Earth.) If we total how much of these elements are in the crust and compare that number to the composition of asteroids in our Solar System, we can get an estimate of how much mass must have struck the Earth after the Moon-forming collision.
How do you go from that mass to a date? The authors recognized that over time, these bodies were lost through collisions with Earth and the other planets. Thus, early in the Solar System's history, there was an ever-shrinking stock of planetesimals that could have delivered material to Earth. To find out the timing of when the stock was depleted, the researchers relied on models of Solar System formation.
In fact, they relied on two different types of models: one that keeps the giant outer planets in their current locations and a second class in which Jupiter and Saturn engage in what's called a "Grand Tack," moving inward early in the Solar System's history before receding back out to their current locations. The Grand Tack simulations were more likely to produce a set of rocky inner planets that look like our Solar System, but in both cases, the planetesimals got depleted pretty rapidly.
Their vanishing act sets limits on when the Moon-forming collision could have happened and, thus, when the Earth could have formed. Too early, and the Earth would have seen lots of additional collisions and had its crust loaded with the elements that are now rare. Too late, and there wouldn't be enough around to give us any significant quantities. Consequently, the authors calculate that there's only a 0.1 percent chance that the Moon-forming collision took place prior to 40 million years after material started condensing in our Solar System. Instead, they place the likely date at 95 million years, with a margin of error of 30 million years on either side.
The authors suggest that we might want to revisit some of the results that were generated using isotope data—a few of these put the collision at 30 million years, and understanding why they got the number wrong may provide some details of the mechanics of the collision itself.
They also think that the reloading of the Earth with rare metals tells us something about the distribution of mass during the formation of the Solar System, which can constrain our models of planet formation. Right now, those models don't do a very good job of creating many of the tightly packed systems that we're currently discovering, so any improvements to them would be a positive step.