Lunar Cratering Rates

We are currently engaged in a pilot study to measure the time history of impacts on the moon using soil samples collected by the Apollo astronauts. The soil contains many glassy "spherules," droplets of surface material that melted in an impact, were thrown from one to hundreds of kilometers or more, and solidified (by radiative cooling) before they landed on the lunar surface. We have 195 spherules from 1.9 grams of lunar soil that have been made available to us by NASA. Using startup funds from a small California Space Institute grant, we have obtained scanning electron microscope pictures of 52 spherules. We have also analyzed the chemical and mineral content of 18 spherules using an electron microprobe. Their ages will be determined using 40Ar/39Ar dating this summer. The Berkeley Geochronology Center has agreed to date 18 lunar spherules free of charge to determine how accurate the dates we obtain will be. In the second phase of our study, we hope to date 20 chemically dissimilar spherules to find the ages of 20 lunar craters. If these two preliminary studies demonstrate the viability of the technique, then the same procedure could be used for a much larger sample of several hundred to a thousand spherules. These measurements could determine whether the impact rate on the moon has been changing over the last three billion years and they could also identify brief periods of intense activity such as comet showers.

If you would like to see Scanning Electron Microscope (SEM) and Electron Microprobe data on some sample spherules, I have put lu09 and lu25 online for your viewing pleasure.

If you would like to see a more detailed description of our project, go here.

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