A figment of our imagination? Maybe the early solar system was a pleasant place Stocktrek/getty

A proposed period of intense asteroid and comet strikes on the inner solar system might be a fiction. Simulations suggest that the purported spike in impacts about 3.9 billion years ago could merely be an artefact of limited samples.

“It’s been an overly simplistic interpretation of complicated data that leads to this illusion,” says Mark Harrison at the University of California, Los Angeles.

Our solar system coalesced from a giant cloud of gas and dust around 4.6 billion years ago. Colossal planetesimals and other fragments were constantly colliding during this era, leading to dramatic wallops such as the impact between the early Earth and a Mars-sized object that may have spawned the moon.


Lucky break for Earth

After a few hundred million years, these first enormous chunks cleared out and impacts became rarer. But approximately 3.9 billion years ago, during a period known as the Late Heavy Bombardment, a second deluge of asteroids and comets seemed to rain down on the inner solar system. Some suggest this was a lucky break for Earth: that bombardment could have brought the first water and even prebiotic molecules to the infant planet.

One of the strongest pieces of evidence for this upheaval is that rocks Apollo astronauts brought back from the moon all seem to cluster around this particular date, suggesting the lunar surface experienced a cataclysmic battering. The dating methods looked at a potassium isotope with a half-life of approximately 1.25 billion years, which decays to argon gas. Should a nearby impact heat up the lunar rocks, they will release some argon gas. The leftover argon then theoretically gives an estimate of the last time the rock was hit.

“The problem is that your rock might not just be getting hit once, but multiple times,” says Patrick Boehnke, also at UCLA.

Each subsequent outgassing would make the rocks appear younger and younger. Because all of the moon’s surface formed at roughly the same time, perhaps 4.3 or 4.4 billion years ago, multiple impacts could drive down the surface rocks’ ages to the same value, providing an imaginary spike.

Hellish and horrible or quite nice?

Harrison and Boehnke created a model divvying up the moon’s surface into 1000 regions. They simulated what it would look like if a decreasing number of asteroids and comets battered this area over time, and found they could produce an illusory uptick at 3.9 billion years. Rather than a sudden deluge of meteorites, the moon might have experienced a slow, steady rain of rocks until the solar system cleared out.

Though it’s been a bedrock assumption in planetary science for decades, other indications seem to be moving away from the Late Heavy Bombardment theory. Harrison points out that it originated with the Apollo samples, which come from only 4 per cent of the moon’s surface. Subsequent lunar meteorites found on Earth – blown off from random parts of the moon – don’t show a spike around 3.9 billion years, but rather have a wide range of ages.

Geologic samples from the Earth suggest it was a comparatively pleasant place too, with liquid water, plate tectonics, and perhaps even early life. “How can one line of evidence say this was a hellish horrible time period and this other that says maybe it was quite nice?” asks Boehnke.

“I’m not shocked at this result,” says Meenakshi Wadhwa at Arizona State University in Tempe, who also studies isotopic dates, mostly in meteorites from the asteroid Vesta. “We’re not seeing any big evidence for a cataclysm.”

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1611535113