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 that the long-lived effect of this impact heating has been greatly underappreciated in models of the Hadean eon — 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?
A deeper look at impacts
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 was transferred deep into the planet itself.
In our modeling, we show that the impact of 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.
Heat transferred into the mantle from the biggest impacts likely influenced volcanism and tectonic behavior for tens to hundreds of millions of years.
A very different early Earth
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.
How continents could emerge from a hot beginning
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 that 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.
As these processes repeated, the surface ended up being composed of more silica, which is what we see in the pale-colored rocks that characterize continental crust.
That may also help explain why the Hadean rock record is so sparse. If the crust was repeatedly heated, melted, and recycled, much of Earth’s earliest crust may simply not have survived.
What changed 3.9 billion years ago
It’s apparent from studies that have analyzed the impact history of the Moon that, by around 3.9 billion years ago, the global effect of impact heating in the inner Solar System had become much less important. That is also around the time Earth began to preserve large tracts of continental crust.
That timing seems unlikely to be a coincidence.
Once the bombardment eased, the crust would have had a chance to cool, solidify, and thicken. Only then may long-lived continents have become possible.
All this means repeated impacts may have limited when Earth could begin to preserve a more lasting crustal record.
Our study does not end the debate about the Hadean. But it does suggest that any realistic picture of the early Earth has to take impacts, and impact heating, seriously.
If that is right, then the young Earth was not just scarred by impacts. It was reshaped by them, and the first continents may only have endured once the violence began to fade.
Tim Johnson, Professor, Geology, Curtin University, and Craig O’Neill, Associate Professor in Geophysics and Remote Sensing, Queensland University of Technology
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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