The crusts of the Moon, Mercury and many small bodies in the Solar System are magnetized. Although the magnetizing field is commonly attributed to that of an ancient core dynamo, a long-held hypothesis is amplification of the interplanetary magnetic field and induced crustal field by plasmas generated by meteoroid impacts. A new study, published in the journal Science Advances, demonstrates that although impact plasmas can enhance the field inside the Moon, the resulting fields are at least three orders of magnitude too weak to explain lunar crustal magnetic anomalies.
Early Moon. Image credit: NASA.
The Moon presently lacks a core dynamo magnetic field.
However, it has been known since the Apollo era that the lunar crust contains remnant magnetization, with localized surface fields reaching up to hundreds of nanoteslas or higher and spanning up to hundreds of miles.
Magnetic studies of Apollo samples and the lunar crust indicate that the magnetizing field likely reached tens of microteslas before 3.56 billion years ago.
The origin of the strongest lunar crustal anomalies and the source of the field that magnetized them have been longstanding mysteries.
“There are two long-term hypotheses associated with why the Moon’s crust might be magnetic,” said co-author Dr. Katarina Miljkovic, a researcher in the Curtin Space Science and Technology Centre in the School of Earth and Planetary Sciences at Curtin University.
“One is that the magnetization is the result of an ancient dynamo in the lunar core, and the other is that it’s the result of an amplification of the interplanetary magnetic field, created by meteoroid impacts.”
“Our research is a deep numerical study that challenges the second hypothesis — the impact-related magnetization — and it essentially ‘debunks’ it.”
“We found that meteoroid impact plasmas interact much more weakly with the Moon compared to the magnetization levels obtained from the lunar crust.”
“This finding leads us to conclude that a core dynamo is the only plausible source of the magnetization of the Moon’s crust.”
For the study, Dr. Miljkovic and colleagues estimated the vapor formation that occurred during large meteoroid impact bombardment of the Moon approximately 4 billion years ago.
“When we look at the Moon with the naked eye, we can see these large craters caused by ancient meteoroid impacts,” Dr. Miljkovic said.
“They are now filled with volcanic maria, or seas, causing them to look darker on the surface.”
“During these impact events, the meteoroids hit the Moon at a very high speed, causing displacement, melting, and vaporization of the lunar crust.”
“We calculated the mass and thermal energy of the vapor emitted during these impacts. That was then used as input for further calculations and investigation of the behavior of the ambient magnetic field at the Moon, following these large impact events.”
“Basically, we made a much more inclusive, high fidelity and high-resolution investigation that led to debunking of the older hypothesis.”
“The impact simulations, combined with plasma simulations, harness the latest developments in scientific codes and computing power and allowed our team to perform the first simulations that could realistically capture and test this long-proposed mechanism,” said lead author Dr. Rona Oran, a researcher in the Department of Earth, Atmospheric and Planetary Sciences at MIT.
Using such tools was key to allowing the team to look at many different scenarios, and in this way to rule out this mechanism under any feasible conditions that could have existed during the impact.
This refutation could have important implications to determine what did magnetize the Moon, and even other objects in the solar system with unexplainable magnetized crusts.
“In addition to the Moon, Mercury, some meteorites, and other small planetary bodies all have a magnetic crust,” Dr. Oran said.
“Perhaps other equivalent mechanical dynamo mechanisms, such as those we now believe to have been in operation on the Moon, could have been in effect on these objects as well.”
_____
Rona Oran et al. 2020. Was the moon magnetized by impact plasmas? Science Advances 6 (40): eabb1475; doi: 10.1126/sciadv.abb1475
