Langrenus: Transient Illuminations on the Moon

The program for lunar surface texture analysis with imaging polarimetry conducted at Observatoire de Paris included the large crater Langrenus near the lunar limb. When making the relevant observations, bright features were discovered near the Langrenus central peak. They appeared in both the photographic images and the polarigraphic images, on which they evolved simultaneously. Very few cases of so-called lunar transient phenomena have been reported in the past; they were diverse and controversial. Most of the visual apparitions were described as short-duration flashes. None of them were suggestive of the slowly evolving effect presently observaed. The brightness enhancements produced a simultaneous increase of polarized light. Such a dependance is not compatible with reflectivity variations at the solid surface of the Moon, which should associate brightness increases with polarization decreases. It does not agree with incandescence and with flashing discharges, which do not produce polarized light. Specular reflection on properly oriented relief slopes may increase the brightness and the polarization simultaneously, but the process can hardly explain the amplitude of the enhancments observed. Particles erupted by volcanic processes such as fire fountaining should leave after the active phases, a deposition of dark material at the lunar surface, which is not observed. It is clusters of hills made of a bright material that are present at the event emplacements. The simultaneous increase of brightness and of polarization is consistant with light scattering on clouds made of separated grains. Small bright highland soil grains may be levitated above the lunar surface by outgassing from the lunar interior. The events occurred at the border of a mare basin, near the central peak of a large crater, in a terrain presumed to be particularly fractured and fissured. The intense radon emanation that was measured around the site with the Apollo orbital instruments indicates gas release from the lunar interior, and supports the interpretation of the brightening events as soil grains levitated by the degassing.