In the April issue, Michael Rampino wrote about cycles of comet collisions with Earth every 26 million to 30 million years. Is there evidence of cyclical bombardment on other planets or moons?

On Earth, the age of a crater can be measured in several ways, with varying degrees of accuracy. Dating the sediments that appeared earliest after the event, or the youngest rocks targeted by the impactor, gives a rough estimate of a crater’s age. Alternatively, some craters can be more accurately dated using the decay of radioactive elements in the rocks melted by the impactor.

The 26 million- to 30 million-year cycle noted is based on several analyses of different craters around the globe. We can date craters on Earth using these methods because we have direct access to them.

The impact cratering histories of the Moon and the other planets in the solar system are not known well enough to look for cycles. In one attempt to find cycles, samples of lunar soil were analyzed by scientists at the University of California, Berkeley. Their work was published March 10, 2000, in the journal Science. 

By dating tiny glass spherules — microscopic beads of glass produced from rocks melted by lunar impacts — in the soil, they determined that the rate of asteroid impacts on the Moon (and presumably on Earth as well) has increased in the last billion years, but they could not detect any cyclical changes.

Professor of Biology, 

New York University, New York