When molten rock gradually cools, magnetic minerals within the melt align with the background magnetic field, preserving a signature of the direction and inclination of the magnetic field at the time of cooling. The temperature threshold at which magnetic signatures become stamped into the rock is called the blocking temperature.
Scientists believe that a similar process has occurred naturally on the surface of Earth’s Moon. Satellite data on the magnetic field of the Moon show that the lunar surface has many magnetic anomalies. Although some of these anomalies could be explained by internal lunar processes (e.g., volcanism or iron-rich dikes), others exist in places where the amount of metal contained in Moon rocks could not produce such strong magnetic signals. Some of these hard-to-explain magnetic anomalies are located in large basins created by past impacts.
Oliveira et al. explored the idea that throughout the geologic history of the Moon, asteroids containing iron hit its surface. The heat of the impact would have heated the projectiles and the targeted Moon’s surface materials until they melted, creating pools in the craters the impacts left behind. As this molten material cooled down in its new environment, the Moon’s magnetic field could have magnetized it. Could this pathway have caused the observed magnetic anomalies on the Moon’s surface?
The team looked at magnetic field data collected by satellites scanning the Moon to estimate the magnetization of the melt sheets on craters’ floors using a unidirectional magnetization model. To calculate the amount of iron metal contained in the impact melt sheets, the researchers developed a technique based on lab experiments that relates the abundance of metallic iron to magnetization, the magnetizing field strength, and rock magnetic properties.
Assuming the magnetized portion of the Moon’s large basins is roughly a kilometer thick, the team found that, on average, the melt sheets contain roughly 0.11%–0.45% of iron metal that might have been delivered by the projectiles that formed the impact basins. These findings are consistent with past studies of Moon rocks and meteorites that have landed on the Moon and Earth, respectively.
Not only does this study help scientists better understand the possibility that magnetic anomalies on the Moon were caused by iron-impregnated projectiles, it also opens up the possibility that the magnetic fields of other space bodies—such as Mercury—could have been caused by external objects. By illuminating the general composition of these projectiles, the process of their impacts, and their evolution after hitting the Moon’s surface, the team’s results will hopefully improve future studies of planetary surfaces and structures. (Journal of Geophyscial Research: Planets, https://doi.org/10.1002/2017JE005397, 2017)
—Sarah Witman, Freelance Writer