In the 1970s, scientists subjected newly arrived samples of moon rocks to a barrage of tests. To their surprise, they discovered that some of the rocks were magnetic. When magma cooled and solidified into these rocks, the material had been exposed to a magnetic field. Which was strange because, as far as scientists knew at the time, the moon didn’t have a global magnetic field like the one that envelops Earth, then or ever. But the rocks suggested otherwise, and many studies of the samples since have shown that the moon indeed had a magnetic field billions of years ago, perhaps one as strong as Earth’s today.
Scientists don’t know exactly how long it lasted and why it disappeared. To investigate this mystery, they keep turning back to the moon rocks.
That’s how Sonia Tikoo found herself heating one of them to 1,436 degrees Fahrenheit (780 degrees Celsius) in a lab chamber.
The rock in question was collected by the Apollo 15 crew in 1971 from the southern rim of a crater called Dune. The rock, known as glassy regolith breccia 15498, is composed of basalt shards, partially covered in melted glass, and dates back 1 billion to 2.5 billion years. Igneous rocks, both on Earth and the moon, contain minerals that can preserve properties of an ambient magnetic field that exists at the time they form. When the lunar rock cooled from a molten state, grains of metallic iron inside recorded the magnetization around it. Heating up the rock again forces it to give up this hidden information.
“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.
Other scientists point to an ancient interaction between the moon’s liquid core and its mantle, the layer between the core and the surface. The core and the mantle may have rotated at slightly different angles, producing movement that swirled the core’s liquid metal. Over time, the angle narrowed and weakened the forces responsible for the dynamo, causing the magnetic field to disappear.
The study of magnetic fields beyond Earth has implications for the search for habitable worlds elsewhere in the universe. Without a magnetic field to protect a planet or a moon from solar wind and cosmic radiation, life as we know it on Earth could not exist. The invisible shield prevents solar wind—streams of charged particles from the sun—from eroding away Earth’s atmosphere and water. Mars once had its own magnetic field, as a young planet about 4 billion years ago, along with a thick atmosphere capable of supporting liquid water. But the shield ceased to exist as Mars cooled, and nearly all of its atmosphere was stripped away.
Nearly 50 years after they arrived on Earth, the Apollo moon rocks are still indispensable to lunar science, small representatives of a close neighbor researchers are still trying to understand. The rocks have both answered longstanding questions and revealed new mysteries about the moon’s origins and evolution. NASA receives dozens of applications for use of the samples in research each year. As their study continues, the rocks may give up more secrets still.
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