Lunar Laser Altimeter Research Articles

Lunar laser altimetry in the SELENE project

Lunar laser altimeter (LALT) has been developed for the Japanese lunar exploration SELENE (SELenological and ENgineering Explorer) in 2003. Lunar laser altimetry and two other selenodetic missions are put together and called RISE (Researches In SElenodesy) project that has been developed mainly by National Astronomical Observatory of Japan. Main objective of this measurements is to construct a more global and accurate topographic model of the moon than Clementine's GLTM-2. The newly developed topography of the moon will make a great contribution to many problems of lunar geology, geophysics, and the reduction of lunar occultation data. In order to expand the coverage in latitude, LALT has a function of slant ranging in the direction about 40 degrees aside from nadir. LALT is designed to have a capability of detecting most of returned pulses from the lunar surface even for the slant ranging with proper threshold level.

Interpretation of lunar potential fields

Understanding of the internal dynamics of the Moon must start from the interpretation of the gravitational and magnetic fields, both present and past. It has been long known from the study of Cassini’s laws and its librations that the Moon substantially departs from hydrostatic equilibrium. This is confirmed by the second harmonic of the gravitational field determined by the tracking of orbiting satellites which also reveals anomalies (the mascons) clearly associated with the processes by which the circular mare formed. The mascons must be retained by the finite strength of the lithosphere, although there is evidence that they may have subsided by about 1 km by slippage along cylindrical fault systems around these mare, and these processes may be important in discussing moonquakes and the lunar transient phenomena. The analyses of the present figure of the Moon by the geometrical librations and by the lunar laser altimeter of Apollo 15 and 16 and other space determinations now seem essentially in agreement. The data gives evidence of the figure of the Moon prior to the filling of the mare, i.e. before about 3300 Ma and it can be concluded that the present non-hydrostatic low harmonics of the gravity field were not then present. Comparison between the present figure of the Moon and its gravity field show that there is a low harmonic variation in density in the deep interior. Both these conclusions point to thermal convection described by second degree harmonics as being the cause of the present non-hydrostatic shape of the Moon. The present lunar dipole magnetic field has been shown by successive analyses to be negligible, the most recent value being 0.05 nT at the surface. Yet magnetic anomalies near the surface of the Moon have been discovered: 1 nT at heights of 100 km and 10-30 nT with length scales of 10 km at the surface. These anomalies must arise from the magnetization of the crustal rocks as discovered in the returned samples. These various data conclusively show that the Moon between 4000 and 3200 Ma possessed a field of internal origin, probably dipolar, with an intensity which seemed to have diminished from over 1 Gat 4000 Ma to a few thousand nT at 3.200 Ma. Whether this field arose by dynamo processes in a small iron core of about 300 km radius, which was inferred from the convection theory and is compatible with the now known value of the moment of inertia factor, or whether it was a permanent magnetization of the deep interior produced by a primeval solar system magnetic field must await further understanding of the early thermal history of the Moon. Thermal convection is seen as an essential basis for understanding the thermal history of the Moon, the traces of tectonic evidence in the lithospheric shell and the history of the magnetic field.