Lunar Transient Phenomena: What Do the Clementine Images Reveal?

Abstract

Lunar Transient Phenomena (LTP) have been reported for at least 450 years. The events range from bright flashes, to reddish or bluish glows, to obscurations. Gaseous spectra and photometric measurements of the events have been obtained. Several theories have been offered as explanations for LTP, including residual volcanic activity or outgassing, bombardment by energetic particles, and piezoelectric effects. As the first set of digital multispectral images of the entire Moon, the Clementine data offer a unique opportunity to couple inferences of compositional relationships with lunar geomorphology in the regions of LTP. We have selected 11 regions from which numerous reliable historical reports of LTP exist. Our analysis of the Clementine multispectral images shows that many events occur in regions of bright, spectrally reddish deposits, which may be characteristic of volcanic ejecta. The events may be associated with outgassing of volatiles collected in or beneath mare basalt flows. We find that LTP tend to occur near the edges of maria, in agreement with a suggestion originally made by Cameron (1972. Icarus 16, 339–387), and in other regions of crustal weakness. We also find that some of the reported events tend to be in craters with rims of distinctly different (bluer) composition. This compositional difference may result from recent slumping of the rim, accompanied by the appearance of fresher underlying material. In some cases, slumping may be triggered by the release of pockets of volatiles; in turn the slumping events may cause additional pockets of trapped material to be released. There are four instances in which Clementine multispectral images were acquired both before and after an event that was reported by a terrestrial team of amateur astronomers mobilized to observe the Moon during the mapping phase of Clementine. None of these four sets of images shows clear changes that could be attributed to the reported LTP.