Ferrovolcanism on metal worlds and the origin of pallasites

Abstract

As differentiated planetesimals cool, their cores can solidify from the outside-in1, as evidenced by paleomagnetic 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 Fe and Ni alloyed with lighter constituent elements, like sulfur, such inward core growth would likely be achieved by growth of solid FeNi dendrites4. Growth of FeNi dendrites results in interconnected pockets of residual melt that becomeprogressively enriched in sulfur up to a eutectic composition of 31 wt% sulfur as FeNi continues to solidify4. Here we show that regions of residual sulfur-enriched FeNi melt in the core attain sufficient excess pressures to propagate via dikes into the mantle. Thus, core material will intrude into the overlying rocky mantle or possibly even erupt onto the plantesimal's surface. We refer to these processes collectively as "ferrovolcanism". Our calculation 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 FeNi metal4. Ferrovolcanic eruptions may explain the observations that Psyche has a bulk density inconsistent with iron metorites5 yet shows evidence of a metallic surface composition6.

Observations, Meteorites, and Models: A Preflight Assessment of the Composition and Formation of (16) Psyche

“…An intact iron‐nickel metal core would have nearly twice the density of Psyche, as summarized in this paper. However, the density requirement is achieved if Psyche is either a rubble pile with significant porosity, or if it retains 25 to 80 km of silicate rock on its surface (Johnson et al, ), or if it is greatly enriched in sulfur. Having a fully silicate surface is likely precluded by the radar reflectance data, which implies that the interior is also at least partially metallic.…”

Asteroid (16) Psyche’s primordial shape: A possible Jacobi ellipsoid

“…Alternatively, Psyche's density appears compatible with that of stony-iron meteorites such as mesosiderites (Viikinkoski et al 2018) and pallasites (Elkins-Tanton et al 2020) as well as that of CB chondrites (Elkins-Tanton et al 2020). Recently, Johnson et al (2020) proposed that Psyche formed as a differentiated body and that ferrovolcanism could be at the origin of the high metal content of its surface (Shepard et al 2017). As of today, it is not understood whether Psyche formed as a differentiated or undifferentiated body.…”

Morphological Diversity of Impact Craters on Asteroid (16) Psyche: Insight From Numerical Models

“…The dunite layer was modeled as in scenario (a), while the iron core was modeled as either an intact core (scenario b) or a shattered, porous core (scenario c). Impacts into a dunite mantle covered by a thin iron layer. One hypothesis that may explain the observed elevated metal content on Psyche's surface is the presence of a thin iron layer covering the mantle as a result of ferrovolcanism (Abrahams & Nimmo, 2019; Johnson et al, 2019). During the cooling period of proto‐Psyche, compressional stresses produced in the cooling crust were relieved by faults, allowing Fe‐Ni rich material from the core to propagate to the surface in dikes.…”

“…Abrahams and Nimmo (2019) proposed that repeated ferrovolcanic eruptions on a fully metallic asteroid would be able to raise enough molten iron material to the surface to cover a more dense iron shell. Johnson et al (2019) showed that ferrovolcanic eruptions are also possible on iron asteroids covered by a thin mantle; however, an estimate of the amount of molten iron erupted was not given. Therefore, here we assumed that the impacts occurred after the iron melt cooled and formed a 50‐m layer covering the mantle.…”

“…As a result the morphology of small craters might differ substantially from those of large craters in the presence of small‐scale target heterogeneity. Ferrovolcanism has the potential to cause substantial small‐scale variations in near‐surface strength, density, and surface topography on Psyche (Abrahams & Nimmo, 2019; Johnson et al, 2019). As discussed previously, Johnson et al (2019) showed that repeated ferrovolcanic eruptions can produce a global iron layer on Psyche's surface.…”