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Chang'e-6 impact glass yields gamma-iron grains that can hold a stable magnetic vortex
Chinese Academy of Sciences scientists found gamma-iron, never before seen in natural lunar material, as the main iron in two Chang'e-6 impact-glass samples. The mineral is a new candidate recorder of the Moon's lost magnetic field, though the team says further work must show how much it can reveal.
The Scientist · Science desk
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What happened
- Off-axis electron holography showed the relatively large gamma-iron grains settling into a stable single-vortex magnetic state that held a consistent response under an applied field.
- The team proposes that traces of carbon and other elements, fast cooling of impact melt and shielding by the surrounding glass kept the normally high-temperature phase from converting.
- Focused ion beam sections, transmission electron microscopy and chemical analysis revealed numerous nanoscale iron particles embedded throughout the glass.
- The study, led by Haifeng Du of the High Magnetic Field Laboratory at the Hefei Institutes of Physical Science, appeared in PNAS on September 16.
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Why it matters
- capability If gamma-iron and alpha-iron each lock in a signal at a different stage of an impact, one particle of lunar glass could hold two records of the same event.
- constraint Until the grains are dated and shown to have kept their magnetisation since they formed, gamma-iron is a possible carrier for lunar paleomagnetism, but it does not yet measure the field itself.
- decision Groups interpreting magnetisation in lunar impact glass now have a second iron phase to separate out before they attribute a signal, because the two phases behave differently magnetically.
The Moon has no global magnetic field now. What is known about the one it once had comes from magnetic minerals preserved in its rocks and soil [12]. Gamma-iron is an odd addition to that set. Under normal conditions it is stable only at high temperatures, and as it cools it typically rearranges into another form, alpha-iron [6]. Its survival at the lunar surface is the first puzzle, and the stabilisers the team proposes are hypotheses at this stage [7].
The study then asks what the grains do. After focused ion beam sections and electron microscopy fixed the phase [5], the team used off-axis electron holography on individual gamma-iron particles [8]. Working grain by grain is what makes the result specific. Only some of the iron particles were gamma-iron, even though it was the dominant form [2]. A magnetic measurement of a whole sample would blend the behaviour of every phase present. A single-particle map ties the vortex state to gamma-iron itself.
According to the Chinese Academy of Sciences release [14], that stability means gamma-iron could act as a previously unrecognised recorder of magnetic information in lunar material [9]. A grain that settles reliably into one magnetic state meets the first condition for a recorder. The study does not show whether these grains hold a signal from the Moon's past. In the lab, a consistent response to an applied field tests behaviour over the length of an experiment. Paleomagnetism asks for more: a grain that took on its magnetisation in an ancient field and has kept it ever since. The release does not give grain sizes, the number of particles imaged, an age for the glass or any estimate of field strength.
The finding rests on two samples [2], and it is not known whether gamma-iron is common in lunar impact glass.
I think the more promising idea is the pairing. Gamma-iron and alpha-iron form under different conditions and behave differently magnetically, so the team suggests each could record a separate stage of an impact [10]. The release does not report a test of that proposal, but the work already widens the known set of magnetic minerals in lunar samples [15].
"This tiny magnetic fossil may help us better understand the Moon's ancient magnetic history," said Dr. Long Li of the Hefei Institutes of Physical Science, a member of the team [4]. The release says future research is needed to determine how much these minerals can reveal about the ancient field and how it evolved [11].
What to watch
- A paleointensity estimate or a dated magnetisation from these gamma-iron grains would turn a candidate recorder into an actual reading of the Moon's field.
- Surveys of impact glass beyond these two samples would show whether gamma-iron is common in lunar soil or particular to these melts.
- Experiments that vary carbon content, quench rate and glass confinement would test which of the proposed stabilisers keeps gamma-iron from converting.