Moon once had internal dynamo

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 20 January, 2009

New analysis of an Apollo sample rock suggests that the Moon once had a liquid core with a dynamo that produced a strong magnetic field, adding fuel to a debate that has lasted 30 years.

The Rosetta Stone rock was returned from the Moon from the final lunar landing mission, Apollo 17, by the only geologist to have ever walked on the Moon’s surface, Harrison Schmidt. In total, from all six lunar landing missions, around 382 kilograms of rock were brought back to the Earth for scrutinisation, but they carried with them a mystery: what had produced the magnetisation found in many of those rocks?

Harrison Schmidt was one of 12 astronauts to walk the surface of the Moon and return rocks to the Earth for analysis. A 4.2 billion year old rock could hold the secrets to the Moon's magnetism. Image: NASA.

Schmidt’s 4.2 billion year old rock is the oldest of all the Moon rocks that has not been subjected to a major shock event from an impact, a process that tends to erase all evidence of earlier magnetic fields. “The type of rock is a troctolite, which is largely a mixture of plagioclase and olivine,” describes graduate student Ian Garrick-Bethell of MIT, and lead author of the Science paper published last week.  “The rock cooled slowly, perhaps 40 kilometres deep within the lunar crust, in contrast to the rocks that make up the dark mare plains that formed by extrusive volcanism.  This rock predates those plains, and unlike the dominantly grey or brown color of most lunar rocks, this rock has large green crystals of olivine. A very large impact event likely excavated it to the surface a long time ago.”

Garrick-Bethell, Ben Weiss — the Victor P. Starr Assistant Professor of Planetary Sciences in MIT's Department of Earth, Atmospheric and Planetary Sciences — and others carried out the most detailed analysis ever on the faint magnetic traces found in the rock. They used a commercial rock magnetometer that was specially fitted with an automated robotic system to take many readings. "This permitted us to study the magnetisation of the rock in much greater detail than previously possible," says Garrick-Bethell. “We actually analysed a number of different rocks, and finally homed in on this sample because its history is very well constrained.”

Indeed, previous studies of the rock suggest only two significant cooling events in the last 4.2 billion years. “Rocks acquire magnetisations as they cool, and since we measured two different magnetisations in the rock, we inferred that the two well-constrained cooling events resulted in the two magnetisations,” says Garrick-Bethell. “The first of these two cooling events lasted millions of years, while the second lasted for thousands of years. This rock has not been shocked by impact processes, since transient fields from impacts only last up to a day, but this rock cooled more slowly, and therefore acquired its magnetisation over timescales incompatible with impact-generated fields.”

The scientists used this information to infer a dynamo, since the rock must have remained in a magnetic environment for a long period of time — millions of years — and thereby solving the long standing mystery of the magnetism’s origin. This is not a new idea, but according to Weiss, is "one of the most controversial issues in lunar science." Until the Apollo missions, many prominent scientists were convinced that the Moon was born cold and stayed cold, never melting enough to form a liquid core. The Apollo missions proved that there had been massive flows of lava on the Moon's surface, but the idea that it has, or ever had, a molten core remained controversial. "People have been vociferously debating this for 30 years," says Weiss. The new analysis suggests that the magnetic field necessary to have magnetised the rock in question would have been about one-fiftieth as strong as Earth's is today.

Orbiting satellites have mapped the magnetic field strength across the surface of the Moon, but origin of the magnetism has long been debated. Image: Lunar Prospector.

The evidence is certainly growing for a liquid lunar core, but the evidence for a solid inner core is much more speculative.  “Right now there is some orbital and seismic evidence for a modern-day liquid core,” Garrick-Bethell tells Astronomy Now. “Our study infers an ancient liquid core, which has likely persisted. It is possible to have a liquid core with no dynamo, which is presumably the Moon’s present state. Our study does not directly address the issue of how much of the present core, if any, is solid. Also, our study does not tell us when the dynamo ceased. For that, we need more samples and/or new studies of existing samples.”

Garrick-Bethell suspects that there are many more samples out there that could teach scientists even more about lunar magnetism. “It would not be terribly difficult for a human mission to greatly improve our understanding of lunar magnetism, and thereby the thermal history of its core,” he says. “If you had a couple of days on the lunar surface dedicated to this goal, you would simply do what you do on the Earth: go to multiple, undisturbed volcanic bedrock units, which were observed by astronauts during Apollo, and drill out many small cores for paleomagnetic analysis. This type of activity is perfectly suited for humans, and would create a geographically oriented and statistically meaningful data set that would permit us to trace the detailed history of the lunar magnetic field in time and space, and tell us much about the Moon’s internal evolution.”

Humans have visited the surface of the Moon six times already, but the new study underscores how much we still don't know about our nearest neighbour in space — we’ve merely scratched the surface. With NASA’s pledge to return humans to the Moon in the 2020s, scientists could soon get their hands on the rocks they need to tell the full story of the Moon’s internal evolution.