Tim Johnson & Craig O’Neill
When you think about a large asteroid impact, you might imagine a moment of devastation: a violent collision, a blast of heat and debris, and then years of atmospheric disruption and damage left behind. But on the early Earth, the most important effect may not have been the crater and its aftermath. It may have been the heat from the impact, driven deep into our planet’s interior.
In our new study, we argue the long-lived effect of this impact heating has been greatly underappreciated in models of the Hadean aeon – the first half-billion years of Earth’s history. Rather than acting as brief interruptions to a planet cooling from within, repeated impacts kept the surface, or protocrust, of the young Earth hot, weak and geologically unstable for a very long time.
That matters because the Hadean is one of the biggest puzzles in Earth science. Tiny zircon crystals show that parts of Earth’s surface have survived for more than 4.3 billion years and that water was present very early. But almost no intact rocks survive from this time. The oldest known continental rocks are about 4.03 billion years old. So what happened during the missing interval?
There is a temptation to think of big asteroid impacts as short-lived surface events. Yet the Moon preserves a stark record of how common such collisions were in the early Solar System. These space rocks would have battered Earth even more intensely than they did the Moon, and the impacts carried enormous amounts of energy that were transferred deep into the planet itself.
In our modelling, we show that the impact heat was not merely a minor addition to Earth’s internal energy budget. Through most of the Hadean, impact heating appears to have vastly outweighed heat produced inside Earth itself. Just as importantly, the effect was not limited to warming the rocks at the surface. By adding heat and thinning or obliterating the crust, large impacts would have caused melting of the mantle beneath the impact site, generating large volumes of basaltic magma.
This finding has big consequences for how we picture the Hadean. Some researchers have argued that the early Earth may have been more similar to the modern Earth than we once thought, perhaps even capable of plate tectonics in a form not entirely unlike today’s. Our results point in a different direction. If impacts were adding this much heat to the system, the early crust was likely thin, weak and partly molten below shallow depths. That doesn’t look much like the modern Earth. It looks more like a planet whose outer shell was being repeatedly renewed.
At first glance, it might seem like impacts are bad news for the survival of continents – those buoyant landmasses we call home. But we argue the same forces were ultimately responsible for making continents. Large impacts would have fractured the young crust and helped water circulate through it for long periods, altering rocks near the surface. At the same time, melting of the mantle beneath impact sites would have supplied huge volumes of magma to and through the crust.
That may also help explain why the Hadean rock record is so sparse. If the crust were repeatedly heated, melted and recycled, much of Earth’s earliest crust may simply not have survived.
The Conversation