Arguments for the non-existence of magma oceans in asteroids

Wilson, Lionel and Keil, Klaus (2017) Arguments for the non-existence of magma oceans in asteroids. In: Planetesimals : Early Differentiation and Consequences for Planets. Cambridge University Press, Cambridge, pp. 159-179. ISBN 9781107118485

Abstract

Introduction: The Ambiguity of the Term “Magma Ocean” Applied to Asteroids The suggestion that Earth’s Moon (e.g. Warren, 1985; Taylor and Norman, 1991) and Earth itself (e.g. Agee and Longhi, 1992) had magma oceans in their very early histories, coupled with the realization that 26 Al present in early-formed asteroids provided a heat source for partial and perhaps total melting (e.g. Hevey and Sanders, 2006; McCoy et al., 2006a; Wadhwa et al., 2006; Schiller et al., 2010), prompted study of the magma ocean concept on asteroids. However, this use of the term magma ocean in relation to melting in asteroid mantles is very different from its original use to describe the molten outer part of the Moon immediately after its formation. The Moon is commonly inferred to have experienced melting of its outer several hundred kilometers as a result of its very rapid accretion from material left in Earth’s orbit after the impact of a Mars-sized body on the proto-Earth (Canup, 2004), though many geochemical and isotopic problems with this scenario have emerged (Asphaug, 2014). Other terrestrial planets may also have had near-surface magma oceans, produced by less frequent but higher-energy impacts of planetesimals that were the last contributors to the growth of these bodies (e.g. Rubie et al., 2015). In all these cases, the magma ocean would have formed all of the outer part of the body and covered a much cooler interior. In contrast, asteroid magma oceans, of the kind currently generally implied in the literature, would have been produced in the interior of bodies by the accumulation of heat from the decay of, predominantly, the short-lived but very energetically radioactive isotope 26 Al (e.g. McCoy et al., 2006a), present at the time of solar system formation. If an asteroid formed sufficiently early, its interior could have reached a temperature well above the solidus of all common minerals (Hevey and Sanders, 2006; McCoy et al., 2006a; Wadhwa et al., 2006; Schiller et al., 2010). If melts did not migrate after formation, a very large fraction of partial melting - up to complete silicate melting - below an outer, cool crustal layer is theoretically possible. Such a body of liquid silicate would clearly qualify as a subcrustal magma ocean.