The origin of inner Solar System water

Abstract

Of the potential volatile sources for the terrestrial planets, the CI and CM carbonaceous chondrites are closest to the planets' bulk H and N isotopic compositions. For the Earth, the addition of approximately 2-4 wt% of CI/CM material to a volatile-depleted proto-Earth can explain the abundances of many of the most volatile elements, although some solar-like material is also required. Two dynamical models of terrestrial planet formation predict that the carbonaceous chondrites formed either in the asteroid belt ('classical' model) or in the outer Solar System (5-15 AU in the Grand Tack model). To test these models, at present the H isotopes of water are the most promising indicators of formation location because they should have become increasingly D-rich with distance from the Sun. The estimated initial H isotopic compositions of water accreted by the CI, CM, CR and Tagish Lake carbonaceous chondrites were much more D-poor than measured outer Solar System objects. A similar pattern is seen for N isotopes. The D-poor compositions reflect incomplete re-equilibration with H₂ in the inner Solar System, which is also consistent with the O isotopes of chondritic water. On balance, it seems that the carbonaceous chondrites and their water did not form very far out in the disc, almost certainly not beyond the orbit of Saturn when its moons formed (approx. 3-7 AU in the Grand Tack model) and possibly close to where they are found today.This article is part of the themed issue 'The origin, history and role of water in the evolution of the inner Solar System'.

Keywords: Grand Tack; asteroids; chondrites; terrestrial planets; volatiles; water.

Figure 1.

Comparison of the estimated bulk H and N isotopic compositions of the major inner Solar System bodies with those of the average Jupiter family (JFC) and Oort Cloud (OCC) comets and individual members of the most volatile-rich chondritic meteorites. The H isotopic compositions of Mars and the Moon are probably upper limits. The isotopic composition of N in Venus's atmosphere is roughly terrestrial, but its H isotopes have been hugely fractionated by the loss of H to space. The sources for the isotopic compositions are given in tables 1 and 2, and Alexander et al. [23].

Figure 2.

The CI chondrite-normalized estimated abundances of H, C, N, noble gases and volatile lithophiles in the bulk Earth. Within the uncertainties, the volatile element abundances are consistent with a 2–4% CI contribution to a largely volatile-depleted proto-Earth. The depletions in N and Xe could be due to an underestimate of the N content of the Earth and loss of Xe from the atmosphere to space, respectively (see the text for details). The terrestrial abundances of H, C, N and noble gases are from Marty [47], and the volatile lithophile abundances are from Palme & O'Neill [51] except for I that is from Deruelle et al. [52]. The CI chondrite H, C and N abundances are from Alexander et al. [23], while all other elements are from Lodders [53].

Figure 3.

Estimates of the accretion times of the chondrite parent bodies (modified after [72]). All the chondrites have avoided melting, but they experienced sufficient lithification as a result of metamorphism and/or aqueous alteration to be robust enough to survive impacts and atmospheric entry. Hence, the chondrites must have formed in a window of time between when there was not quite enough short-lived radioactivity to melt and differentiate planetesimals, and when there was not enough radioactivity to even melt water ice. With the exception of the highly reduced enstatite chondrites (E) and possibly the COs, all the chondrites accreted at least some water, presumably as ice, implying accretion temperatures that were below the sublimation temperature of water ice in the nebula (150–170 K). (Online version in colour.)

Figure 4.

The bulk H isotopic compositions versus the bulk C/H ratios of CI, CM, CR and Tagish Lake carbonaceous chondrites (after [23]). The linear arrays displayed by the CM and CR chondrites reflect mixing between relatively D-poor water/OH and D-rich organic matter (δD ≈ 3500‰). The y-axis intercepts of the best-fit lines to the CM and CR trends give their water/OH compositions. The open red squares are CM chondrites that were not used in the fit for various reasons (see [23]). (Online version in colour.)

Figure 5.

Comparison of the estimated H isotopic compositions of water in various chondrites with those for water in comets and Saturn's moon Enceladus (after [23]). The CI, CM and Tagish Lake isotopic compositions are clearly distinct from those measured for outer Solar System objects. The CR composition overlaps with those of the most D-poor comets. Surprisingly, the water in ordinary and R chondrites has H isotopic compositions that resemble those of most comets and Enceladus. However, this does not necessarily mean that the ordinary and R chondrites come from the outer Solar System. It is more likely that the H isotopic compositions of their water has been fractionated by parent body processes (see the text for details). This parent body fractionation may also have affected the carbonaceous chondrites, particularly the CRs.

Figure 6.

The H isotopic compositions of water in various objects as a function of radial distance from the Sun. The H isotopic composition of methane in Titan's atmosphere is also shown, although it is not known how closely it reflects that of the water that Titan accreted. The asteroidal parent bodies of the carbonaceous chondrites are not known and so they have been given nominal radial distances of 3 AU. Saturn's moons Titan and Enceladus have been given their current orbital distances, although if there was a Grand Tack they may have formed between approximately 3 AU and approximately 7 AU (arrowed). The formation locations of the comets are unknown, but are thought to have been between approximately 20 AU and approximately 30 AU (e.g. [92]). With the possible exception of the CRs, the carbonaceous chondrite with the most D-rich water, the carbonaceous chondrites have compositions that are distinct from any measured outer Solar System body. Sources are given in tables 1 and 2, and Alexander et al. [23].

Figure 7.

The same as for figure 6 but for N isotopes. The N isotopic compositions of individual comets are generally very consistent but the uncertainties can be large, so they have been averaged (table 2), except for the JFC outlier comet 73P. Titan's atmospheric N isotopic composition is thought to reflect that of the NH₃ it accreted [95]. The chondrite compositions (carbonaceous in blue, ordinary in red) are for bulk meteorites. However, amino acids and other N-bearing soluble organic compounds (blue double arrow) probably formed from HCN/NH₃ and exhibit a similar range of isotopic compositions to the bulk meteorites (see the text for details).

Figure 8.

Comparison of the CI chondrite-normalized estimated volatile abundances in Earth and Venus. The Venus abundances assume that its interior has entirely degassed. The relative depletions in Venus's C and ⁴⁰Ar (from decay of ⁴⁰K) suggest that this was not always the case. The large relative enrichments in Ne and ³⁶Ar indicate that Venus did not have the same mix of sources as Earth (see the text for discussion). The data for the Earth are from Marty [47], and those for Venus from Fegley [113] except for Xe which is from Bogard [114] but its value is very uncertain.