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Ferrovolcanism on metal worlds and the origin of pallasites


As differentiated planetesimals cool, their cores can solidify from the outside in1, as evidenced by palaeomagnetic measurements and cooling-rate estimates of iron meteorites2,3. The details of outside-in solidification and fate of residual core melt are poorly understood. For a core primarily composed of iron and nickel alloyed with lighter constituent elements such as sulfur, this inward core growth would probably be achieved by growth of solid iron–nickel dendrites4. Growth of iron–nickel dendrites results in interconnected pockets of residual melt that become progressively enriched in sulfur up to a eutectic composition of 31 wt% sulfur as iron–nickel continues to solidify4. Here, we show that regions of residual sulfur-enriched iron–nickel melt in the core attain sufficient excess pressures to propagate via dykes into the mantle. Thus, core material will intrude into the overlying rocky mantle or possibly even erupt onto the planetesimal’s surface. We refer to these processes collectively as ferrovolcanism. Our calculations show that ferrovolcanic surface eruptions are more likely on bodies with mantles less than 50 km thick. We show that intrusive ferromagmatism can produce pallasites, an enigmatic class of meteorites composed of olivine crystals entrained in a matrix of iron–nickel metal4. Ferrovolcanic eruptions may explain the observations that asteroid 16 Psyche has a bulk density inconsistent with iron meteorites5 yet shows evidence of a metallic surface composition6.

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Fig. 1: Structure of a differentiated planetesimal that may experience ferrovolcanic eruptions of sulfur-rich FeNi melt.
Fig. 2: Excess pressure associated with the buoyancy of sulfur-enriched iron melts surrounded by dense solid iron as a function of core radius.
Fig. 3: Mantle penetration height of ferromagmatic dykes as a function of sulfur content.
Fig. 4: Modelled density of Psyche, assuming a two-layer structure of a metal core and a rocky mantle compared with the observed density.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


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We thank H. J. Melosh, F. Nimmo, J. N. H. Abrahamson, E. R. D. Scott and M. Caffee for discussion and comments on this work.

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Authors and Affiliations



B.C.J. conceived this study and the application of ferrovolcanism to pallasites and the Psyche observations, which M.M.S. had noted. B.C.J. produced the excess pressure and mantle penetration calculations, with input from all authors. M.M.S. produced the Psyche density calculations. All authors contributed to preparation of the manuscript and the conclusions presented in this work.

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Correspondence to Brandon C. Johnson.

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Johnson, B.C., Sori, M.M. & Evans, A.J. Ferrovolcanism on metal worlds and the origin of pallasites. Nat Astron 4, 41–44 (2020).

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