High resolution observations of the sodium emission from the Moon

Abstract

High resolution spectroscopy of the Moon's atmosphere allows to analyze the closest layers to the lunar surface giving the possibility to get information on the source and the evolution of the atmospheric atoms and their distribution with respect to the latitude. The sodium emission, that is one of the main components of the atmosphere, is easier to study thanks to the good efficiency of the solar resonant scattering. A long term program of observations of the Moon's sodium atmosphere has been performed at the 182cm telescope of the Astronomical Observatory of Padova, using an echelle spectrograph, allowing to get spectra in different periods of the year and lunar phases and at different latitudes. The slit was placed parallel to the lunar equator, with a spatial coverage of 150 km few tens of km above the surface, at the east and west sides (bright and dark limbs) and perpendicular to the equator at the north and south sides. In this way it has been studied the behaviour of the column densities, the scale heights and temperatures with respect to the latitude and the lunar phase. We report also one spectrum obtained during the total eclipse of April 1996. Moreover it has been possible to get spectra using the WHT-IAC Canarian telescope with a very high resolution spectrograph and a larger spatial coverage in order to discriminate better the thermal and suprathermal components assumed to characterize the sodium distribution in the atmosphere. Our data seem to emphasize the micrometeorite impacts mechanism that is not so neglible looking at the asimmetry of the emission intensities measured during one full Moon night, their high value agree with Potter and Morgan (1991). A further comparison with a spectrum obtained by Sprague et al. (1991) at about the same lunar phase shows an higher temperature for our spectrum that could be due to a competing release mechanism involving also the micrometeorite impacts.