Last year, I wrote a feature story about the Late Heavy Bombardment, the time, roughly 3.9 billion ago, when the young bodies of the inner solar system were subjected to a beating by asteroids flung in from the outer solar system. The story was partly triggered by an abstract presented here last year: Herb Frey’s report that, using topographic data, he could identify some 92 likely impact basins bigger than 300 kilometres across — twice as many as contained in the canonical database. That meant that the moon — the ‘record plate’ for the bombardment, since the relic impact craters and basins are preserved better there than elsewhere — was hit harder than most thought. And the work somehow made me realize just how hellish the Earth was during that epoch — probably molten, oceans evaporated, asteroids the size of dinosaur killers casually striking the Earth every few decades or so.

And then in a talk today, Frey, of Goddard Space Flight Center in Greenbelt, Maryland, said that even that was lower bound. By using a crust thickness model, and identifying circular areas where the crust is thin, he can identify 50 more big impact basins unrecognized by topographic alone. That brings the number of basins greater than 300 kilometres up to about 140, he says, three times the standard number. “These should always be considered minimum numbers from now on,” he said.

image: Frey, GSFC