What ‘time of day’ did the heavy bombardment period begin?

Cosmic Pinball: When Did the Solar System’s Wild Ride Begin?

Picture the early solar system: not the neat, orderly arrangement we see today, but a cosmic demolition derby. One of the most intense phases of this chaotic period is what scientists call the Late Heavy Bombardment (LHB). Basically, it was a time when asteroids and comets were slamming into the inner planets left and right, like a cosmic pinball machine gone wild. This pummeling had a huge impact on the surfaces – and maybe even the potential for life – on Earth, Mars, Venus, and Mercury. But when did this crazy ride actually start? That’s a question scientists are still trying to nail down.

The Late Heavy Bombardment: What Was It, Anyway?

The Late Heavy Bombardment, sometimes called the lunar cataclysm, was essentially a period when the inner solar system got bombarded by a lot of space rocks. The leading idea is that this happened somewhere between 4.1 and 3.8 billion years ago – that’s way back in the Neohadean and Eoarchean eras, for those keeping score. This was after the solar system initially formed and things had (supposedly) calmed down a bit. But then, BAM! All hell broke loose.

Lunar Rocks Tell a Tale… Or Do They?

So, how do we know this happened? Well, the main evidence comes from moon rocks brought back by the Apollo missions. When these rocks get hit by asteroids, they can melt and then re-solidify. By dating these “impact melt rocks,” scientists found that a bunch of them seemed to be from around the same time, suggesting a major surge in impacts around 3.9 billion years ago.

But here’s the thing: it might not be that simple. Some scientists argue that this “surge” could be misleading. The Apollo samples only come from a small part of the moon. It’s like trying to understand the history of the entire Earth by only looking at rocks from, say, Texas. You might get some interesting information, but it might not be the whole story.

Rewriting History: Maybe the Bombardment Started Sooner?

Now, things get even more interesting. Some recent research is shaking up the traditional timeline. A 2019 study, for example, looked at a bunch of meteorites and didn’t find any sign of a big impact spike at 3.9 billion years ago. Instead, they found that many of these meteorites had been melted by impacts way earlier, around 4.48 billion years ago – practically right after the solar system formed!

This suggests that the LHB wasn’t a sudden burst of chaos, but more like a gradual winding down of the intense bombardment that was already happening in the early solar system. This also fits better with evidence that Earth might have had liquid water as early as 4 billion years ago, and that life might have popped up between 3.8 and 3.9 billion years ago. If the bombardment ended earlier, Earth would have had more time to cool down and become habitable.

Planetary Shuffle: Did the Giant Planets Cause the Chaos?

One popular theory to explain the LHB is that the giant planets – Jupiter, Saturn, Uranus, and Neptune – went on a bit of a road trip. The “Nice model” (yes, that’s really what it’s called) suggests that these planets shifted their orbits, stirring up the asteroid and Kuiper belts and sending a hailstorm of space debris towards the inner planets. While this planetary migration idea is still a leading contender, the timing is still up for debate. That 2019 study I mentioned earlier? It suggests that this planetary shuffle might have happened much earlier, contributing to that proposed 4.48 billion-year-old bombardment.

So, When Did the Wild Ride Really Begin?

The truth is, we still don’t have a definitive answer. The timing and nature of the Late Heavy Bombardment are still a cosmic puzzle. While the old view put the start date around 3.9 billion years ago, new evidence hints at an earlier start, maybe around 4.48 billion years ago. Future missions to the Moon and more in-depth studies of meteorites and lunar samples will be key to finally piecing together this chaotic chapter in our solar system’s history. It’s a bit like being a detective trying to solve a very, very old cold case – and the clues are written in the rocks.