Highly siderophile elements in Earth’s mantle as a clock for the Moon-forming impact


According to the generally accepted scenario, the last giant impact on Earth formed the Moon and initiated the final phase of core formation by melting Earth’s mantle. A key goal of geochemistry is to date this event, but different ages have been proposed. Some1,2,3 argue for an early Moon-forming event, approximately 30 million years (Myr) after the condensation of the first solids in the Solar System, whereas others4,5,6 claim a date later than 50 Myr (and possibly as late as around 100 Myr) after condensation. Here we show that a Moon-forming event at 40 Myr after condensation, or earlier, is ruled out at a 99.9 per cent confidence level. We use a large number of N-body simulations to demonstrate a relationship between the time of the last giant impact on an Earth-like planet and the amount of mass subsequently added during the era known as Late Accretion. As the last giant impact is delayed, the late-accreted mass decreases in a predictable fashion. This relationship exists within both the classical scenario7,8 and the Grand Tack scenario9,10 of terrestrial planet formation, and holds across a wide range of disk conditions. The concentration of highly siderophile elements (HSEs) in Earth’s mantle constrains the mass of chondritic material added to Earth during Late Accretion11,12. Using HSE abundance measurements13,14, we determine a Moon-formation age of 95 ± 32 Myr after condensation. The possibility exists that some late projectiles were differentiated and left an incomplete HSE record in Earth’s mantle. Even in this case, various isotopic constraints strongly suggest that the late-accreted mass did not exceed 1 per cent of Earth’s mass, and so the HSE clock still robustly limits the timing of the Moon-forming event to significantly later than 40 Myr after condensation.

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Figure 1: The late-accreted mass relative to each synthetic Earth-like planet’s final mass as a function of the time of the last giant impact.
Figure 2: The likelihood that a planet suffering a last giant impact within a specific range of times has a late-accreted mass less than or equal to the chondritic mass of 4.8 ± 1.6 × 10−3.


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S.A.J., A.M., D.P.O’B. and D.C.R were supported by the European Research Council Advanced Grant ‘ACCRETE’ (contract number 290568). D.P.O’B. was also supported by grant NNX09AE36G from NASA’s Planetary Geology and Geophysics research programme. S.N.R. acknowledges support from the NASA Astrobiology Institute’s Virtual Planetary Laboratory lead team, supported by NASA under cooperative agreement NNH05ZDA001C.

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A.M., K.J.W., S.N.R. and D.P.O’B. initiated a study of numerically simulating terrestrial planet formation. A.M., S.A.J., S.N.R., D.P.O’B. and K.J.W. ran N-body simulations and provided reduced data. S.A.J. analysed results. S.A.J., A.M. and D.C.R. discussed the meaning of the discovered relationship. S.A.J. wrote the paper with guidance from A.M. All authors commented on the manuscript.

Correspondence to Seth A. Jacobson.

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Extended data figures and tables

Extended Data Figure 1 Four panels comparing the classical (triangles) and Grand Tack (circles) scenarios.

a, The mass of each planet in every simulated Solar System as a function of the semi-major axis. Planets within the grey zone are considered to be Earth-like. Planets at the distance of Mars from the Sun are too massive in classical simulations but are correct in Grand Tack simulations. b, c and d show the relative Late Accretion mass as a function of different parameters and the chondritic mass and its uncertainty MC = 4.8 ± 1.6 × 10−3 as a dashed line and grey region. b is the same as Fig. 1 of the main text, but with a broader timescale. c and d show the relative Late Accretion mass and two orbital structure statistics: concentration37 Sc and angular momentum deficit37,76 Sd. The Solar System has Sc = 89.9 and is marked with a vertical dashed line in c. Grand Tack simulations are as, or more, concentrated than classical simulations. The Solar System has Sd = 0.0018 and this is marked by a vertical dashed line in d. A late giant planet instability (about 500 Myr after the condensation of the first solids in the Solar System) probably excites the inner Solar System; given that these simulations end before the instability, they should possess an angular momentum deficit that is only a fraction of the current value77. Grand Tack simulations are either as dynamically excited or colder than classical simulations, although they are in some cases hotter than the current terrestrial planets.

Extended Data Figure 2 Figure 1 of the main text is reproduced in each panel for each scenario within the classical category.

For reference, the horizontal dashed line and enclosing darkened region are the best estimate and the 1σ uncertainty of the Late Veneer mass MC = 4.8 ± 1.6 × 10−3. Each panel shows the Earth-like planet’s relative late-accreted mass as a function of last giant impact time for different initial ratios of the total mass in embryos to planetesimals, as follows. 1:1 in a, 2:1 in b, 4:1 in c, and 8:1 in d. The mean time of the last giant impact increases and the relative Late Accretion mass decreases as the initial ratio of total embryo mass to planetesimal mass increases.

Extended Data Figure 3 The late-accreted mass MLA shown as greyscale contours of the two parameters X and Y.

The blue dashed contours indicate the location of a few specific late-accreted masses and are labelled in the framed boxes. For this figure, we assumed that Earth receives 52 times as much mass in small projectiles than the Moon (σ = 52). The red region is inaccessible, given that the measured chondritic mass in the lunar mantle requires a minimum flux of small planetesimals onto Earth's mantle.

Extended Data Figure 4 Each panel is like Extended Data Fig. 3 but showing additional constraints.

The constraint X ≥ 0.53, deduced by a reanalysis of Rudge et al.23 with the Hf/W ratio updated from Konig et al.15, is shown in light red in each panel. In bd, the solid green contours indicate the set of X, Y parameters that reproduce the nominal difference in W-isotope composition between the Moon and Earth (0.09 in units; see Touboul et al.4). The difference among these panels is in the assumed Hf/W and W-isotope composition for the mantle of the differentiated projectiles: b assumes values typical of eucrites; c is for aubrites and d is for Mars. The green dashed contours are the 1σ uncertainties on X, Y related to the uncertainty on the difference between the W-isotope composition between the Moon and Earth: ±0.1 in units. The areas exterior to these 1 − σ uncertainties have been coloured red.

Extended Data Table 1 Chondritic concentrations for different HSEs in Earth's mantle 13 and chondritic meteorites14 and the ratio of the two
Extended Data Table 2 Tungsten abundances and ratios for the mantle materials: basaltic eucrites75, aubrites78, and Mars75

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Jacobson, S., Morbidelli, A., Raymond, S. et al. Highly siderophile elements in Earth’s mantle as a clock for the Moon-forming impact. Nature 508, 84–87 (2014). https://doi.org/10.1038/nature13172

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