Early lunar geology and geophysics

Abstract

Despite a number of human and robotic missions to the Moon, there are still important unanswered questions about its early evolution, and how it came to be the object we observe today. Here we use observational, experimental, and theoretical techniques to examine three important events that took place early in lunar history and have left a lasting signature. The first event is the formation of the largest basin on the Moon, the South Pole-Aitken Basin. We develop a systematic method to define the previously unknown boundaries of this degraded structure and quantify its gross shape. We also combine a number of remote sensing data sets to constrain the origin of heat producing elements in its interior. The second event we examine is the evolution of the lunar orbit, and the coupling between the Moon's early geophysical properties and the growth of orbital eccentricity. We use analytical models for tidal deformations and orbit evolution to show that the shape of the Moon suggests its early orbit was highly eccentric. However, we are also able to explain the presently high eccentricity entirely by traditional, secular tidal growth while the early Moon was hot. The third event we examine is the magnetization of lunar samples. We perform extensive paleomagnetic measurements of an ancient, deep-seated lunar sample, and determine that a long-lived magnetic field like that of a core dynamo is the most plausible explanation for its magnetic remanence. In sum, the earliest portion of lunar history has been largely obscured by later geologic events, but a great deal can still be learned from this formative epoch.