Long-lived magnetism from solidification-driven convection on the pallasite parent body

• Published in: Nature (London) Vol. 517; no. 7535; pp. 472 - 475
• Main Authors: Bryson, James F. J., Nichols, Claire I. O., Herrero-Albillos, Julia, Kronast, Florian, Kasama, Takeshi, Alimadadi, Hossein, van der Laan, Gerrit, Nimmo, Francis, Harrison, Richard J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 22-01-2015; Nature Publishing Group
• Subjects: 147/28; 704/445/123; 704/445/3928; 704/445/849; Accretion; Analysis; Chemical properties; Convection; Cooling; Cores; Humanities and Social Sciences; letter; Magnetic fields; Magnetism; Meteors & meteorites; multidisciplinary; Paleomagnetism; Reynolds number; Science; Silicates; Solidification
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature14114

Nanomagnetic imaging has been used to obtain a palaeomagnetic time series of two pallasite meteorites, revealing that their convection was driven by core solidification, which would have caused long-lived magnetic fields in the cores of early Solar System planetary bodies.
Magnetic activity in the early Solar System
Shortly after the birth of the Solar System, small planetary bodies became hot enough to segregate into a liquid metal core surrounded by rocky mantle. As the core cooled and froze, swirling motions of liquid metal, driven by the expulsion of sulphur from the growing inner core, generated a magnetic field. A class of meteorites known as pallasites preserves this phase of Solar System history as in the form of gem-quality crystals of the silicate mineral olivine embedded in a metallic matrix of iron–nickel alloy. James Bryson
et al
. use high-resolution magnetic imaging of the iron–nickel matrix of the Imilac and Esquel pallasite meteorites to derive a time-series record of magnetic activity on the pallasite parent body, encoded within nanoscale intergrowths of iron-rich and nickel-rich phases. This record captures the dying moments of the magnetic field generated as the liquid core solidified, providing evidence for a long-lasting magnetic dynamo driven by compositional convection.
Palaeomagnetic measurements of meteorites
1
,
2
,
3
,
4
,
5
suggest that, shortly after the birth of the Solar System, the molten metallic cores of many small planetary bodies convected vigorously and were capable of generating magnetic fields
6
. Convection on these bodies is currently thought to have been thermally driven
7
,
8
, implying that magnetic activity would have been short-lived
9
. Here we report a time-series palaeomagnetic record derived from nanomagnetic imaging
10
of the Imilac and Esquel pallasite meteorites, a group of meteorites consisting of centimetre-sized metallic and silicate phases. We find a history of long-lived magnetic activity on the pallasite parent body, capturing the decay and eventual shutdown of the magnetic field as core solidification completed. We demonstrate that magnetic activity driven by progressive solidification of an inner core
11
,
12
,
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is consistent with our measured magnetic field characteristics and cooling rates
14
. Solidification-driven convection was probably common among small body cores
15
, and, in contrast to thermally driven convection, will have led to a relatively late (hundreds of millions of years after accretion), long-lasting, intense and widespread epoch of magnetic activity among these bodies in the early Solar System.