Preliminary Measurement of Meter-scale Roughness on the Moon from Lunar Orbiter Laser Altimeter (LOLA) Pulse Spreading

The Lunar Orbiter Laser Altimeter (LOLA) transmits short (~ 5 ns) pulses from its redundant 1064-nm lasers. Backscattered pulses are spread in time due to interaction with the lunar surface. In the absence of a planetary atmosphere or large incidence angles, the backscattered pulses provide a measure of the root mean square (rms) roughness of the surface at the scale of the laser footprint. Scattering of returns from heights varying by >30 cm (rms) extends the pulsewidth significantly from those over level ground. In the nominal 50-km mapping orbit, the effective footprint is a 2.5-m-diameter circular spot reflecting 50% of the laser photons. The interpretation of pulses is complicated by the Lunar Reconnaissance Orbiter’s ~30 km x 200 km commissioning orbit which leads to varying surface spot size and pulse amplitude. LOLA monitors the pulse width at the threshold crossing of the backscattered pulse, and this parameter may also be used to infer the transmitted laser-pulse shape. The relationship between the LOLA measured pulse width and the threshold value for Laser 1 and 2 was measured prior to launch for calibration purposes. Pulse widths measured during the first month of commissioning orbit indicate returned pulses spread to as wide as 30 nanoseconds. While these observations require additional corrections, preliminary results show that pulse widths are visibly widened by steep slopes, the aprons of some impact structures and by South Pole-Aitken massifs, among other structures. Analysis is underway to calibrate pulse widths against threshold, energy and other instrument parameters to provide a globally consistent quantitative measure of the roughness of the Moon at the scale of a few meters.