Contribution of Major Basins to the Long-Wavelength Shape of the Moon from the Lunar Orbiter Laser Altimeter (LOLA)

Introduction: The long-wavelength shape of a planetary body can be used to infer information on spin state and orbital evolution [1]. The long-wavelength gravity field provides a measure of the shape of the gravitational potential that is in equilibrium with the rotation. But since the present-day gravity field need not have any correspondence with that at the time that the body cooled enough to “freeze in” its shape [2], the topography field may provide a more suitable proxy for the original long-wavelength structure. Applying this thought process to the Moon, we recognize that there are many long-wavelength contributions to the lunar topography, particularly difference in crustal thickness between the near and far sides [3] and most notably, major impact basins. In this study we begin to characterize the contribution of the largest lunar basins to the long-wavelength lunar shape using new observations from the Lunar Orbiter Laser Altimeter (LOLA), an instrument on the Lunar Reconnaissance Orbiter (LRO), as well as consideration of gravity. LOLA Observations: LOLA [4] is a 5-beam laser altimeter that ranges to the Moon continuously in the LRO mapping orbit at a rate of 28 Hz. The instrument has a precision of ~10 cm and the accuracy depends on orbit determination. Currently LOLA data processing is using mostly navigation orbits and the accuracy is at the few tens of meters level. Precision orbits of order 1 m should be achievable, with the help of the LRO laser ranging investigation, which consists of Earthbased laser ranging the LRO when the spacecraft is in view of Earth [5]. By long-wavelength standards the Moon is well sampled topographically, with the current map (cf. Fig. 1) containing nearly 800M observations as of December 2009 [6]. Lunar Basin Contributions: Major impact basins have imparted significant stochastic variations on the lunar shape. Since the depths of many major basins exceed the magnitude of the flattening [7] it is important to ascertain the extent that the flattening may be corrupted by impact-related signatures. The largest impact basin on the Moon, the 2200-km diameter South Pole-Aitken Basin (Fig. 1), is a degree 5 feature, indicating it can be represented adequately by a degree and order 5 spherical harmonic expansion [8]. SP-A occupies about 9% of the lunar surface area and is prominently placed between the south pole and the equator and centered at approximately 191 E longitude. The present volume of SP-A [8] is of order 10 m which, if filled with lunar crustal material (