“Because magnetization is acquired by temperature-related processes, it follows that it can also be removed by temperature-related processes,” Tikoo, a planetary science professor at Rutgers University in New Jersey, explained in an email. “We can heat the rocks progressively in a zero magnetic field environment to increasing temperatures and measure how the rock's magnetization changes after each heating step.”
As the temperature crept higher, Tikoo and her fellow researchers tracked how the intensity of the magnetization changed using an instrument called a magnetometer. They stopped at 1,436 degrees Fahrenheit, which is the maximum temperature at which metallic iron can preserve magnetism. Anything higher, and the rock loses its magnetization completely.
Researchers have exposed lunar rocks to extreme heat in the past, a process that, if things go awry, could lead to chemical changes in the samples, Tikoo said. One of the researchers built a heating environment that mimicked closely the conditions under which the rock formed to prevent any damaging effects. Still, they were nervous.
“We were very concerned about accidentally destroying the sample and would monitor our heating experiments very closely to make sure all aspects of the heating process were taking place smoothly,” Tikoo said.
Scientists estimate the moon’s magnetic field existed between 4.25 billion years ago and 3.56 billion years ago, when the solar system was young. Tikoo and her fellow researchers say that their new analysis of the Apollo moon rock’s magnetic intensity suggests that the field may have persisted for 1 billion to 2.5 billion years longer than that. Their findings are described in a paper published Wednesday in Science Advances.
Scientists don’t know when the moon’s magnetic field vanished, nor do they fully understand what powered it in the first place. Earth’s magnetic field, which stretches several tens of thousands of miles into space, is generated by the constant churning of liquid metal at the planet’s molten core. This constant motion is known as a dynamo. The existence of a magnetic field on the moon means a similar dynamo was at work. Scientists believe that because of the moon’s small size relative to Earth’s, a molten core would have cooled off quickly. But if it were powered by this process alone, the lunar dynamo would have shut down in a matter of a few hundred million years—not a billion or more years, like the moon rocks suggest.
So scientists must consider alternative causes for a lunar dynamo. Tikoo said the moon’s core, which is mostly iron, may contain other, lighter elements—like sulfur and carbon—that could have contributed to the dynamo’s power source and its duration. As the moon’s molten core cooled and solidified over billions of years, leftover liquid kept swirling, buoyed by light elements. The heat that escaped the core during the cooling process, combined with the rising of this remaining liquid, could have powered the magnetic field for a longer period.