The direction of core solidification in asteroids: Implications for dynamo generation

IF 2.5 2区 物理与天体物理 Q2 ASTRONOMY & ASTROPHYSICS
K.H. Dodds , J.F.J. Bryson , J.A. Neufeld , R.J. Harrison
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引用次数: 0

Abstract

Paleomagnetic studies of meteorites over the past two decades have revealed that the cores of multiple meteorite parent bodies, including those of certain chondritic groups, generated dynamo fields as they crystallised. However, uncertainties in the direction and mode of core solidification in asteroid-sized bodies have meant using the timings and durations of these fields to constrain parent body properties, such as size, is challenging. Here, we use updated equations of state and liquidus relationships for Fe-FeS liquids at low pressures to calculate the locations at which solids form in these cores. We perform these calculations for core-mantle boundary (CMB) pressures from 0–2 GPa, and Fe-FeS liquid concentrations on the iron-rich side of the eutectic, as well as two values of iron thermal expansivity that cover the measured uncertainties in this parameter, and adiabatic and conductive cooling of these cores. We predict inward core crystallisation from the CMB in asteroids due to their low < 0.5 GPa pressures regardless of the uncertainties in other key core parameters. However, due to low internal pressures in these cores, remelting of any iron snow, as proposed to generate Ganymede’s present-day field, may be unlikely as the cores are approximately isothermal. Therefore a different mode of inward core solidification is possibly required to explain compositionally-driven dynamo action in asteroids. Additionally, we identify possible regimes at higher > 0.6 − 2 GPa pressures in which crystallisation can occur concurrently at the CMB and the centre.
小行星内核凝固的方向:对发电机产生的影响
过去二十年来对陨石进行的古地磁研究显示,多个陨石母体(包括某些软玉群的母体)的内核在结晶过程中会产生动力场。然而,小行星大小的陨石核心凝固方向和模式的不确定性,意味着利用这些场的时间和持续时间来约束母体的属性(如大小)是具有挑战性的。在这里,我们使用低压下 Fe-FeS 液体的最新状态方程和液相关系来计算这些内核中固体形成的位置。我们针对 0-2 GPa 的岩心-岩幔边界(CMB)压力、共晶富铁一侧的 Fe-FeS 液体浓度、涵盖该参数测量不确定性的两个铁热膨胀率值以及这些岩心的绝热和传导冷却进行了计算。由于小行星的内压较低,我们预测它们的内核会向内结晶,与其他关键内核参数的不确定性无关。然而,由于这些内核的内部压力较低,任何铁雪的重熔(如产生木卫二现今磁场的提议)都不太可能发生,因为这些内核大致处于等温状态。因此,可能需要一种不同的内核凝固模式来解释小行星中由成分驱动的动力作用。此外,我们还确定了在较高的 > 0.6 - 2 GPa 压力下的可能状态,在这种状态下,结晶可在 CMB 和中心同时发生。
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来源期刊
Icarus
Icarus 地学天文-天文与天体物理
CiteScore
6.30
自引率
18.80%
发文量
356
审稿时长
2-4 weeks
期刊介绍: Icarus is devoted to the publication of original contributions in the field of Solar System studies. Manuscripts reporting the results of new research - observational, experimental, or theoretical - concerning the astronomy, geology, meteorology, physics, chemistry, biology, and other scientific aspects of our Solar System or extrasolar systems are welcome. The journal generally does not publish papers devoted exclusively to the Sun, the Earth, celestial mechanics, meteoritics, or astrophysics. Icarus does not publish papers that provide "improved" versions of Bode''s law, or other numerical relations, without a sound physical basis. Icarus does not publish meeting announcements or general notices. Reviews, historical papers, and manuscripts describing spacecraft instrumentation may be considered, but only with prior approval of the editor. An entire issue of the journal is occasionally devoted to a single subject, usually arising from a conference on the same topic. The language of publication is English. American or British usage is accepted, but not a mixture of these.
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