A figment of our imagination? Maybe the early solar system was a pleasant place
Simulations suggest that the purported spike in asteroid and comet strikes on the inner solar system about 3.9 billion years ago might be a fiction.
The 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 impacts such as that between the early Earth and a Mars-sized object that may have spawned the moon.
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. It has been suggested that this bombardment could have brought the first water and even prebiotic molecules to the infant planet.
One of the strongest pieces of evidence for the Late Heavy Bombardment is that rocks brought back from the moon by Apollo astronauts all seem to have ages that cluster around this date. This suggests that the lunar surface experienced a cataclysmic battering at this time. The dating methods looked at a potassium isotope with a half-life of approximately 1.25 billion years, which decays to argon gas. If a nearby impact heats 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, of the University of California, Los Angeles. 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.
Boehnke and his colleague Mark Harrison created a model that divided 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 spike 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.
The Late Heavy Bombardment theory has been a fundamental assumption in planetary science for decades. However, more recent findings have led to a reassessment. Harrison points out that the theory 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 at around 3.9 billion years, but rather have a wide range of ages. Geologic samples from the Earth also suggest that it was a comparatively pleasant place at this time, with liquid water, plate tectonics, and perhaps even early life.