“This is really a smoking gun,” Nesvorny says. “It rules out the Nice model as a source of the Late Heavy Bombardment.”
An early instability makes sense. According to Raymond, the most natural trigger for the planetary movement was the loss of the gas disk in which they were born. Within roughly 10 million years, most of the gas had either been swept up by the planets or dispersed by the Sun. The gas would have had a dampening effect. Once it was gone, the planets could more easily tug at one another gravitationally, changing their orbits. Nesvorny’s 100 million years was an upper limit, with the circumstances that he modeled among the most optimistic.
That doesn’t necessarily mean there wasn’t an LHB — only that if there was one, the rearrangement of the ice giants didn’t set it off. However, the Nice model was the best argument for causing an LHB, so the demise of this connection makes yet another argument against the hypothesis.
Not ready to give up
The chatter among the scientific community makes it seem as though time has run out for the LHB. Perhaps there wasn’t a sudden increase of material slamming into the terrestrial planets after all, only the last of the debris that had built the planets being slowly swept up by collisions. Case closed, right?
Bottke isn’t completely sold. “There’s things that still need to be worried about,” he says. He wants to know why Mare Imbrium and Mare Orientale, both huge basins, formed at the end of the line. Statistically, the largest rocks should have collided the earliest, leaving the more numerous smaller objects to make the final scars. More large impacts at the end can come from a spike in collisions, a change in the characteristics of the colliding population, or just bad luck. “What are the odds of two of the biggest [collisions] at the end of the line?” he asks. “I think that’s going to be a really low probability number.”
The South Pole-Aitken Basin, the largest and oldest impact feature on the Moon, is another location with a mysterious age. According to Bottke, estimates range from 4.1 billion to 4.5 billion years. Nailing down its age would be “a big deal,” he says. If it’s only 4.1 billion years old, why don’t we see signs of the craters that should have formed earlier in lunar history? Although several researchers have suggested that some process erased the most ancient craters, he remains unconvinced. “It’s not enough to just get rid of basin rims,” he says. “You have to get rid of gravitational signals and any compositional differences you have.” He’s not certain those changes are showing up on the Moon.
Finally, he points to concerns with meteorites. Meteorites from the Moon and the asteroid belt show signs that their parent bodies lost gas as a result of impact shock waves in two episodes — one around 4.5 billion years ago and the other between 3.5 billion and 4 billion years ago — but remain strangely quiet between 4 billion and 4.5 billion years ago. Bottke says this points to two components of inner solar system bombardment.
“I’m not quite ready to give up,” he says.
One of the best ways to settle the debate over whether Earth and the Moon suffered from increased collisions early in their lifetimes would be to return to the Moon for more samples. NASA didn’t select the Apollo landing sites for the purpose of nailing down crater ages, but perhaps that could be a key consideration on the next trip. Right now, NASA considers the South Pole-Aitken Basin a top choice for future landing sites. Confirming the basin’s age could help to unravel some of the mystery surrounding the history of lunar bombardment.
“If we can be more selective and careful about landing sites that give us samples from the impact basin, they can help us to fill in some of the questions we still have,” Zellner says.
With those ages in hand, perhaps scientists will be able to nail down what happened in the early solar system, revealing the end game of planet formation and perhaps unveiling when life could have first arisen on Earth and, perhaps, Mars.
“It’s an exciting time of confusion,” Morbidelli says. “Stay tuned.”