Moon Rock Reveals Hot Molten Core | WIRED

Moon Rock Reveals Hot Molten Core
A moon rock brought back by astronauts from the last Apollo mission in 1972 has finally proven its worth by revealing that the moon used to have a hot, convecting molten core that generated a magnetic field. The moon’s magnetic field was probably about one-fiftieth as strong as Earth’s current field. The discovery is consistent […]
A moon rock brought back by astronauts from the last Apollo mission in 1972 has finally proven its worth by revealing that the moon used to have a hot, convecting molten core that generated a magnetic field.
The moon’s magnetic field was probably about one-fiftieth as strong as Earth’s current field. The discovery is consistent with the theory that our moon was born when a giant asteroid barreled into Earth and broke off chunks that clustered together to become an orbiting satellite.
The rock is a special sample because it dates from about 4.2 billion years ago, but somehow managed to avoid being subjected to major shocks from asteroid impacts, which tend to erase evidence of any magnetic fields.
“It is one of the oldest and most pristine samples known,” said MIT graduate student Ian Garrick-Bethell, lead author of a study published Thursday in Science. “If that wasn't enough, it is also perhaps the most beautiful lunar rock, displaying a mixture of bright green and milky white crystals.”
Thanks to this super specimen, scientists were able to detect magnetic traces in it that suggest the moon used to have a magnetic field in ancient times. The field was likely caused by an active hot liquid core, like the one inside Earth today.
"We believe the moon's core is still molten now, but the difference is that it doesn't have this dynamo, this convecting current that supports and generates a magnetic field," Garrick-Bethell told Wired.com. "It's possible that it stopped because the moon is much cooler now."
Using an instrument called a rock magnetometer, the researchers took many highly detailed measurements of the rock’s magnetism, which allowed them to rule out the possibility that the magnetic traces were left over from temporary fields caused by major asteroid impacts on the moon. Instead, the evidence showed that the magnetic field must have been in place for millions of years, which means it must have been caused by a molten dynamo core.
The idea of a magnetic moon isn’t new, but has been “one of the most controversial issues in lunar science,” said MIT planetary scientist and co-author Ben Weiss. “People have been vociferously debating this for 30 years.”
The finding also supports the argument that it’s important to send trained humans, and not just robots, to visit alien worlds. It was astronaut Harrison “Jack” Schmitt, the only geologist ever to walk on the moon, who hand-selected the specimen that led to this discovery.
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Emily Mullin
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