Thin section (crossed polars) of sample 10020 showing unshocked olivine and plagioclase.
Credit: Science/AAAS
Parent sample 10020,234 in the Lunar Sample Laboratory at the Johnson Space Centre in Houston.
Credit: Science/AAAS
DUBLIN: A powerful magnetic field shielded the early Moon, and was generated by a molten metal core that churned for much longer than previously thought, according to a new study.
The discovery stems from a re-examination of rocks collected during the first Moon landing on 20 July 1969. Previous studies of a 4.2 billion-year-old Moon rock revealed telltale signs of a magnetic field. Now a new analysis of another Moon rock has dated it to around 3.7 billion years ago, indicating that the Moon's churning metal centre persisted for far longer than scientists had assumed.
"We had an idea that [number] 10020 was a good sample for magnetic studies, but no idea it would be quite this good," said Erin Shea of Massachusetts Institute of Technology in the U.S., lead author of the study published in Science today. "Most lunar samples are very poor magnetic recorders."
The Moon's magnetic field
Today the Moon only has only a weak magnetic field and scientists in the past haven't been able to figure out why. In contrast to the Moon, the Earth has a dynamo-generated field maintained by a circulating electric current which protects us from solar wind blasts and gives direction to compasses.
According to Shea, evidence for a dynamo tells us amazing things about the interior of a planet, such as it being warm enough for molten metal. The persistence of the Moon's dynamo for 500 million years casts doubt on its core working like the Earth's, because our planet's dynamo is driven by cooling. The Moon's size means it is unlikely that simple cooling would keep the lunar dynamo running much past the 4.2-billion-year mark.
To study the magnetic field, scientists examined the direction of magnetisation in pieces of the rock. The uniformity of direction indicated a planetary force field, rather one generated locally by a meteorite impact.
Apollo 11 rock
Though it had long been suspected that a core-dynamo magnetic field once existed on the Moon, this study, which uses parts of 1 gm from a 450 gm chunk of Moon basalt brought back by Apollo 11, indicated that a strong dynamo and a molten core existed between at least 4.2 to 3.7 billion years.
"The most important implication is that we cannot assume that the lunar dynamo was powered in the same way that we think the dynamo is powered on Earth," said Shea. "It is possible other planets and asteroids once had a dynamo, though it had been assumed otherwise because they were so small. This observation is a powerful tool to help us in understanding what some of these large asteroids, such as Vesta, were like in the early solar system and what their interiors must be made of."
Planet-sized lab
"Knowledge of the existence and history of a lunar dynamo is a basic constraint on the early history and internal evolution of the Moon, which is our most accessible laboratory for understanding the formation and early evolution of the terrestrial planets," commented Lon L. Hood a planetary and atmospheric physicist at the University of Arizona in the U.S.
During the Apollo program, it was established that much of the lunar crust is magnetised, but lab studies and theoretical models indicated this could largely have been caused by shocking of lunar rocks during large-scale impacts on the Moon, he added.
"The estimated strength of the lunar field at 3.7 billion years is roughly 60 microTeslas (0.6 Gauss), which is comparable in intensity to the present-day Earth field at the surface. This is an unexpectedly strong field and challenges dynamo theory."
