Elongated patterns of the lunar magnetic anomalies: Implication for paleo-stress field of the lunar crust

The study on the lunar magnetic anomalies can give crucial constraint on the evolution of the lunar interior, in particular, the crust and core of the early Moon. There are two main problems concerning the origin of the lunar magnetic anomalies: the ambient field recorded as the magnetization and the process of magnetization acquisition. Recent observations and analyses on the lunar magnetic anomalies suggest that the lunar magnetic anomalies are likely to have been magnetized in the ancient dynamo field. The early lunar dynamo are supported by the paleointensity study of the returned sample,the dynamo simulation study, and the seismological study of the lunar core size. For the magnetization acquisition, two possible mechanisms have been proposed so far: shock and/or pressure remanent magnetization caused by a large impact, and thermoremanent magnetization by some igneous activity or impact-related heating. When modeling the origin of the lunar magnetic anomalies, the model should satisfy the following observation results: (1) there are several regions of strong magnetic anomalies, especially, in the highland region, (2) a most intensive anomaly cluster is located at northwest part of the South Pole-Aitken basin, (3) magnetization intensity of the anomaly source is predicted to be two to three order of magnitude as large as that of the returned samples, (4) relatively weak magnetic anomalies are widely distributed over the Moon, and (5) almost all of the magnetic anomalies are more or less elongated. In the present study, we focus on the elongation of anomalies since it could reflect characteristic configuration of the anomaly source. We have developed a new method for mapping three components of the lunar magnetic anomaly field at the surface from satellite observations. This method has been applied to some regions using the low altitude observations by magnetometers of Kaguya and Lunar Prospector. Regional maps of three components from the two datasets show good agreement and indicate that clear elongation of the anomaly pattern with a width of 2 - 3 degree wavelength. These patterns suggest that the anomaly source may be thermoremanent magnetization of dike-like intrusion suggested by Purucker et al. [2012]. If it is a case, the elongation direction is parallel to a direction of the horizontal maximum stress field in the early lunar crust like the Earth's dikes. The paleo-stress field of the Moon would be controlled mainly by the Earth's tidal force and the centrifugal force of the Moon's rotation. If the two-dimensional sheet is uniformly magnetized, its magnetic field is perpendicular to the elongation direction. Thus we can estimate the elongation direction from two horizontal components of the magnetic anomaly fields. A parallel direction on a spherical surface corresponds to small circles around some pole or great circles near the equator. Thus, the coordinate system is rotated, and the north and east components are recalculated tofind good coordinates. From the global mapping result at 30km altitude, nearly parallel pattern of the magnetic anomalies can be found in a few regions. Analyzing several regions, we will discuss a possibility to estimate the elongation direction of the anomaly source and paleo-stress field of the Moon.