A long-lived magma ocean on a young Moon.

M Maurice,S Schwinger,T Kleine,N Tosi,D Breuer

doi:10.1126/sciadv.aba8949

M Maurice, S Schwinger + Show 3 more

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https://doi.org/10.1126/sciadv.aba8949

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Abstract

A giant impact onto Earth led to the formation of the Moon, resulted in a lunar magma ocean (LMO), and initiated the last event of core segregation on Earth. However, the timing and temporal link of these events remain uncertain. Here, we demonstrate that the low thermal conductivity of the lunar crust combined with heat extraction by partial melting of deep cumulates undergoing convection results in an LMO solidification time scale of 150 to 200 million years. Combining this result with a crystallization model of the LMO and with the ages and isotopic compositions of lunar samples indicates that the Moon formed 4.425 ± 0.025 billion years ago. This age is in remarkable agreement with the U-Pb age of Earth, demonstrating that the U-Pb age dates the final segregation of Earth's core.

Highlights

Most lunar formation models predict that the early Moon had a global magma ocean [1], which initially solidified from the bottom up via efficient radiative cooling to space at its hot liquid surface [2] or through a cold but very thin quenched crust [3]
It was initially proposed that this may result in an extended lunar magma ocean (LMO) lifetime of more than 200 million years (Ma) [5], a subsequent study by the same authors [6] has pointed out an error in the original model, which leads to a substantial overestimation of the LMO solidification time
Comparison to chronology of lunar samples The LMO solidification time scale of up to ~200 Ma inferred from our model seems inconsistent with the chronology of LMO products

Summary

Introduction

Most lunar formation models predict that the early Moon had a global magma ocean [1], which initially solidified from the bottom up via efficient radiative cooling to space at its hot liquid surface [2] or through a cold but very thin quenched crust [3]. The last ~20% of the LMO is thought to have crystallized on a longer time scale of 10 million years (Ma) [2] to 30 Ma [3]. It has been proposed that tidal dissipation in the crust [5, 6] or the magma ocean itself [7] may prolong the duration of the LMO solidification. Tidal dissipation mainly heats the uppermost layers of the crust and affects the magma ocean itself only on a reduced time scale (~104 years). Impacts of leftover debris from the Moon-forming event onto the lunar crust may have influenced the duration of LMO crystallization in two ways [3]: Holes punctured in the crust may have accelerated cooling, and deposition of the impactors’ kinetic energy may have slowed it down. The net result of these two opposing effects is not well constrained, but the most recent results indicate that holes in the crust are shortlived and, did not significantly enhance the cooling of the LMO [8]

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Conclusion