Late Heavy Bombardment Slows to a Trickle

Late Heavy Bombardment Slows to a Trickle

Evidence for an early period of intense meteoritic impacts on Earth continues to grow. From 4.1 to 3.8 billion years ago, these events would have regularly sterilized Earth’s surface.

Recent research indicates that, although the rate of the sterilizing events dropped significantly, they continued at a slow pace until 550 million years ago. This research provides a partial answer to the question of why God waited until Earth was 4.5 billion years old before creating humanity.

According to the scientific data, modern humans appeared on Earth no earlier than 200,000 years ago (RTB argues for a date around 100,000 years ago). However, Earth formed over 4.5 billion years ago. If God created humanity, why would he wait so long?

Different groups of people answer the question differently. Young-earth creationists often point to the gap as evidence for a much younger Earth. Skeptics usually reject the idea that God was involved at all. Reasons To Believe affirms that God created humanity and accepts the scientific data for Earth’s antiquity. So, we must offer some explanation for the long gap between Earth’s formation and humanity’s appearance. Recent research into the history of astronomical impact events on Earth helps provide part of the explanation.

A growing body of evidence points to a period of intense meteoritic activity on Earth between 3.8 and 4.1 billion years ago. Known as the late heavy bombardment (LHB), this era included hundreds of impacts rivaling the one that wiped out the dinosaurs 65 million years ago (the Chicxulub crater). As dramatic as that sounds, some of the impacts liquefied the top mile or so of Earth’s crust, sterilizing the surface of all life. Although scientists find evidence for life appearing immediately after the LHB ended, new data indicates that these large impact events continued for another two billion years—though at a much slower pace.

When a large comet or asteroid collides with Earth, it ejects a large amount of sand-sized droplets of melted crust into the atmosphere. As these droplets cool, they fall back to Earth and form global layers up to a few centimeters thick. The active life/water/erosion cycle usually alters these layers beyond recognition, but certain conditions will preserve the layers. Scientists searched for preserved layers and found a number of them in rock formations dating back 3.5 billion years ago1. Their research indicated that the frequency of Chicxulub-sized (or larger) impacts was significantly higher prior to 2.5 billion years ago. Thus, the LHB did not end abruptly but diminished gradually over the next couple of billion years.

Another team of scientists found similar evidence for a diminishing impact rate after the LHB. Specifically, they cited evidence of at least seven Chicxulub-sized events between 3.47–3.23 Gya2, four from 2.63–2.49 Gya, and one between 2.1–1.7 Gya3. With this data, the scientists used solar system formation models to determine the source of the bodies causing the impacts. These models include the migration of Jupiter and Saturn from their formation locations to their current orbits. These migrations destabilized the orbits of comets and asteroids, likely causing the LHB and subsequent impacts. The team found that if the asteroid belt originally extended closer to Mars, it would have contained the right distribution of bodies with orbital velocities to explain the large-sized impacts found in the geological record.

Given the processes by which planets form, habitable planets like Earth will experience numerous collisions large enough to cause mass extinctions and even sterilizations. The migration of Jupiter and Saturn ensured that the bulk of these impacts occurred early in Earth’s history where they would have little lasting effect on our planet’s biosphere. The fact that the collisions persisted for another 2–3 billion years helps explain why God waited to introduce multicellular life—and, ultimately, humanity—onto the scene.

  1. B. C. Johnson and H. J. Melosh, “Impact Spherules as a Record of an Ancient Heavy Bombardment of Earth,” Nature 485 (May 3, 2012): 75–77.
  2. “Gya” stands for “giga years ago”; one Gya is equivalent to 109 years. 
  3. William F. Bottke et al., “An Archaean Heavy Bombardment from a Destabilized Extension of the Asteroid Belt,” Nature 485 (May 3, 2012): 78–81.