Jack is still actively studying the geology of the lunar surface, and when the Lunar Reconnaissance Orbiter which launched in 2009 took exceptionally high-resolution photographs of the lunar surface, he realised the true significance of some of the samples he had collected.
Rather than representing debris strewn across the Moon during meteorite impacts he realised that the boulders he had sampled, which are part of large-scale lava flows like those we see today in Hawaii, had barely moved since they cooled and crystallized over 3.5 billion years ago.
Prof. Claire Nichols, Associate Professor at the Department of Earth Science, studies planetary magnetic fields, and the Moon’s magnetic field history is particularly curious. Magnetic fields are driven in the dense iron-rich cores of planets by vigorous stirring of the electrically conductive, liquid metal.
Earth’s core is roughly half the radius of the planet, and the outer liquid part drives the bar-magnet-like field we observe around Earth today, responsible for the north and south magnetic poles aligned roughly along the planet’s rotation axis.
‘During the final manned mission to the Moon 49 years ago, Apollo 17 collected rocks from the lunar surface including samples which would turn out to be critical for our study nearly 50 years later.’
The Moon on the other hand has a tiny core, just one seventh of its radius. With such a tiny source of conductive material so far from the planet’s surface (the strength of the magnetic field drops off with distance from source cubed), we would never predict that the Moon generated a significant magnetic field. However, samples from the Apollo missions reveal that the Moon generated an intense magnetic field for at least 2 billion years.
Modellers have tried to simulate how such a strong and long-lived lunar magnetic field could be generated but have struggled to come up with plausible mechanisms. One issue is that observations have only revealed the strength of the magnetic field over time, rather than its geometry and orientation. Both observations are needed to be able to distinguish between the possible physical processes which stirred the molten part of the lunar core.
Almost all Apollo samples collected are loose material scattered across the lunar surface, and their original position and orientation are unknown. So when Jack realised he had collected samples whose original orientation could be recovered, he got in touch with the MIT Paleomagnetism Laboratory, to let us know he had just the samples we had been looking for.
After arriving on Earth, every sample from the Apollo missions was meticulously documented and photographed under lunar lighting conditions. In other words, light is shone onto the samples in such a way to exactly recreate the illumination and shadows when the sample was collected on the lunar surface.
‘The results suggest that the ancient lunar magnetic field had a very similar geometry to the magnetic field around the Earth today, which wasn’t what we had expected.’
We were able to borrow samples from the Johnson Space Center in Houston, Texas which were hammered off the rocky outcrops that Jack had determined must be close to their original position. We used the laboratory photographs to match up all the features with those in the astronaut photographs to work out the exact orientation the samples were in on the lunar surface when they crystallized 3750 million years ago.
Once we knew the orientation of the samples, we demagnetized them in the MIT Paleomagnetism Laboratory using a superconducting rock magnetometer to recover the strength – and for the first time – the direction of the ancient lunar magnetic field.
As previous studies had shown, we found evidence that the lunar magnetic field at this time was very intense – similar to the strength of Earth’s magnetic field today. We were also able to recover how steep the magnetic field was relative to the surface at Camelot Crater, where the samples were collected. This steepness can be used to determine where the magnetic north and south pole were relative to our sample’s location. We found that the magnetic poles were aligned along the present-day spin axis of the Moon (the axis around which the Moon rotates so that we always see the same side of it as it orbits around the Earth).
Our results suggest that the ancient lunar magnetic field had a very similar geometry to the magnetic field around the Earth today. This wasn’t what we had expected; given the strength of the Moon’s magnetic field, calculations show that it cannot have been generated by the same mechanism that drives Earth’s magnetic field. Additionally, because the Moon rotates incredibly slowly compared to the Earth - it rotates just once every 27 days - we wouldn’t expect the magnetic poles to be aligned along the spin axis. Instead, we would expect the magnetic field to be disorganized and have a much more complex pattern of magnetic field lines.
The alignment of the lunar magnetic poles along the spin axis is an important observation for understanding the interior dynamics of the Moon during its early history. Some model predictions have suggested that the mixing of molten iron in the centre of the Moon could have been physically driven by gravitational stirring. The rocky outer part of the Moon, the liquid metallic outer core and the solid metallic inner core all rotate at slightly different angles. This generates friction between the layers that could have generated a magnetic field – and given the alignment with the spin axis – seems the most likely candidate for driving the ancient field.
‘Samples from the Apollo missions reveal that the Moon generated an intense magnetic field for at least 2 billion years.’
The Moon is also thought to have shifted relative to its present-day spin axis over time. In other words, if we could travel billions of years back in time, we would be looking at a slightly different side of the Moon, and features such as craters and lava flows would be at different angles to us. This phenomenon happens when there are large-scale changes to the density distribution of a planet and is referred to as ‘true polar wander’. Independent studies have found that the Moon’s polar ice caps have shifted slightly over time due to this wander, and our prediction for the location of the magnetic north pole is consistent with that shift.
There are still many unanswered questions about the Moon’s geologic history, but our study has shown the power of working on oriented samples from other planets. This single observation at just one location on the lunar surface has significantly improved our understanding of how the ancient lunar magnetic field was generated.
We are about to enter a new dawn of planetary science with oriented samples currently being drilled on the Martian surface and due for return in the next decade, and the Artemis program aiming to send humans back to the Moon by 2024. We have shown the significance of collecting oriented samples collected in geologic context that have not moved since their formation. Our results can guide future sampling strategies so we can understand more about how the interior of the Moon has evolved over time.
The full paper, The palaeoinclination of the ancient lunar magnetic field from an Apollo 17 basalt, was published in Nature Astronomy.