The Moon: Dead or Alive?

Peter H. Schultz
Geological Sciences, Brown University, Providence, RI

The Moon has long been considered dead, with only occasional impacts. There is new evidence, however, that it is still alive, although barely. Several features (first noted during the Apollo missions) are now believed to indicate that gas is being episodically released from the deep interior. Continuous impact bombardment has created a regolith covering the Moon to depths of 3m (3 Ga) to 6 m (3.7 Ga), with older surfaces accumulating more than 10m. Very few surfaces still have exposed bedrock. In a few small areas, however, this regolith has been stripped away. One particular depressed region called Ina (about 3km across) contains well-defined scarps (5-10m) and only two impact craters, indicative a very young surface. Moreover, spectral reflectance data from the Clementine mission reveal little evidence for space weathering, a process that gradually darkens the surface. While first recognized as an unusual feature by the Apollo astronauts and interpreted as a young caldera by researchers, it was not appreciated how young. At Ina, the regolith must have been removed no more than 10 million years ago. Ina is one of at least four other similar features, all situated along an ancient system of radial and concentric faults covering much of the lunar nearside. It is suggested that that these faults provide the conduits for gas escaping from the deep interior, even today. Forty years ago, such an interpretation would have been heresy. Instruments on recent lunar orbiters, however, also detected radiogenic gases with short half-lives, thereby indicating that gas is still escaping at certain locations, especially young impact craters. But the Ina structure was not caused by an impact. While the last effusions of lavas ceased about 1 billion years ago, small outbursts of gas may yet provide clues about a cooling core.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas