Seismic noise in the moon

For many years, scientists have used microseismic noise—mild seismic waves generated by ocean waves and atmospheric processes—to image Earth’s internal geological makeup. Researchers from France, Denmark, and the US are now using the technique to explore the Moon and, perhaps some day, other planets. In 1972, astronauts on the Apollo 17 mission planted an array of four geophones on the Moon that passively recorded its seismicity, and the record was radioed to Earth. The geophones were again activated for more than eight months in 1976-77, and that data set is what has now been analyzed. The researchers looked at noise correlations between pairs of geophones, from which they extracted faint activity signals. The signals grew as the array entered sunlight and declined as it entered shadow—with a precise 29.5-day period. They concluded that the low-level seismic activity arose as surface material cracked when heated by the Sun from −170 °C to +110 °C. The analysis also corroborated the determination of subsurface geology by other, more established experimental methods during the Apollo program. The scientists say that microseismic noise can be used to explore other planets, notably Mars, that have different sources of seismic noise. (E. Larose et al., Geophys. Res. Lett. 32 , L16201, 2005 http://dx.doi.org/10.1029/2005GL023518 .)