Magnetism on the Moon has always been a bit confusing. Remote sensing probes have noted there is some magnetic signature, but far from the strong cocoon that surrounds Earth itself. Previous attempts to detect it in returned regolith samples blended together all of the rocks in those samples, leading to confusion about the source - whether they were caused by a strong inner dynamo in ages past, or by powerful asteroid impacts that magnetized the rocks they hit. A new study from Yibo Yang of Zhejiang University and Lin Xing of the Chinese Academy of Sciences, published recently in the journal Fundamental Research, shows that the right answer seems to be - a little of both.
Their research was based on analyzing the magnetic signature of a single grain of lunar dust, using a type of advanced diamond sensor doped with nitrogen-vacancy centers that made it capable of acting as a quantum-scale magnetic sensor. They used a technique called Optically Detected Magnetic Resonance (ODMR) to monitor the fluorescence given off by the diamond as a laser was shone through it. Changes in that fluorescence indicated changes in the local magnetic field strength of the particle.
The system they designed was significantly more powerful than commercially available quantum magnetic sensors, allowing them to analyze dust grains returned from the Chang’e 5 mission in greater detail than previously possible. Most importantly, they were able to see what individual parts of the grain was causing the field variations - whether that was a nano-particle sized piece of iron or a fracture in the grain itself. What they found was a very clear split based on the type of dust they were analyzing.
Video describing how quantum diamond sensors work. Credit - 632nm YouTube ChannelBasalt is a type of rock that forms from cooled magma. Under the new quantum magnetic field sensor, basaltic grains of dust showed relatively weak magnetic signals, but with a highly uniform orientation. Most of its magnetism seems to have come from iron native in the rock, or a type of iron-sulfide mineral called troilite. But the orientation here is what matters - it appears that, as the magma that formed the basalt cooled, there was a lunar magnetic field pushing them to align in the same direction. Most likely, that was caused by an active lunar “dynamo”, which appears to have been the case up to at least 2 billion years ago when these grains were formed.
That provides some additional evidence for whether there was an active dynamo or not. But there was another type of dust grain analyzed by the researchers - breccia. These grains were formed by fusing broken fragments of other rocks together, most likely by the heat from asteroid impacts. They exhibited much stronger magnetization, and with completely random distributions. Likely a result of a process called Shock Remnant Magnetization, their varied magnetic signatures are caused by iron-nickel alloys or nano-phase iron created by the meteorite impacts that created the dust grain itself.
Some cracks in the rock had noticeable magnetic “stripes” that formed in perfect alignment with them as well. The researchers believe these might be caused by either the solar wind or micrometeoroid impacts that chemically altered the material inside the cracks, long after the rock they were held in was formed. If true, this would directly show the process of space weathering in a visible form.
Video describing how the Moon lost it’s magnetic field. Credit - TomoNewsUS YouTube ChannelThe types of quantum-level magnetic sensors used in this work are becoming more commonplace in geology, so this will undoubtedly not be the last paper utilizing this technique to analyze space rocks. But for now, given that Chang’e’s samples are the youngest ever returned from the Moon, this is the best evidence yet of the magnetic history of our nearest neighbor - time to update the theories once again.
Learn More:
Keai Communications Co, Ltd / EurekaAlert - Microscopic magnetic-field imaging of a single lunar dust grain
Y. Yang et al. - Microscopic magnetic-field imaging of a single lunar dust grain
UT - Did a Large Impact on the Moon Make its Rocks Magnetic?
UT - New Study Says Moon's Magnetic Field Existed 1 Billion Years Longer Than We Thought