A PROPOSED period of asteroid and comet strikes on the inner solar system might be a fiction. About 3.9 billion years ago, in a period called the Late Heavy Bombardment, the inner planets were deluged by these objects – so we thought. The strongest evidence for this is that rocks brought back from the moon by Apollo astronauts seem to cluster around this date, suggesting a cataclysmic battering melted the lunar surface around that time and locked in those ages. But multiple strikes could muddy the record, says Patrick Boehnke at the University of California, Los Angeles. He and Mark Harrison, also at UCLA, simulated what it would look like if a decreasing number of asteroids and comets battered the lunar surface over time. They found this could produce an illusory uptick at 3.9 billion years. Rather than a sudden deluge, the moon might have seen a slow, steady rain (PNAS, doi.org/bqp2). Although it has been a bedrock assumption in planetary science, other indications, such as Earth samples, also point away from a bombardment in the solar system. “How can one line of evidence say this was a hellish, horrible time period, and this other that maybe it was quite nice?” asks Boehnke. This article appeared in print under the headline “Asteroid assault, or a gentle massage?