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Early accretion of water in the inner solar system from a carbonaceous chondrite–like source

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Science  31 Oct 2014:
Vol. 346, Issue 6209, pp. 623-626
DOI: 10.1126/science.1256717

History recorded in asteroid's water

Astronomers know that interstellar water is abundantly available to young planetary systems—our blue planet collected (or accreted) plenty of it. Still, the details of water's movement in the inner solar system are elusive. Sarafian et al. measured water isotopes in meteorite samples from the asteroid Vesta for clues to the timing of water accretion. Their samples have the same isotopic fingerprint of volatiles as both Earth and carbonaceous chondrites, some of the most primitive meteorites. The findings suggest that Earth received most of its water relatively early from chondrite-like bodies.

Science, this issue p. 623

Abstract

Determining the origin of water and the timing of its accretion within the inner solar system is important for understanding the dynamics of planet formation. The timing of water accretion to the inner solar system also has implications for how and when life emerged on Earth. We report in situ measurements of the hydrogen isotopic composition of the mineral apatite in eucrite meteorites, whose parent body is the main-belt asteroid 4 Vesta. These measurements sample one of the oldest hydrogen reservoirs in the solar system and show that Vesta contains the same hydrogen isotopic composition as that of carbonaceous chondrites. Taking into account the old ages of eucrite meteorites and their similarity to Earth’s isotopic ratios of hydrogen, carbon, and nitrogen, we demonstrate that these volatiles could have been added early to Earth, rather than gained during a late accretion event.

Hydrogen is vitally important in cosmochemical and geochemical processes. For example, water (H2O) plays a critical role in plate tectonics on Earth (1) and has likely shaped the surface of Mars (2). Despite the abundance of water on Earth and evidence of water on the Moon, Mars, and the asteroid 4 Vesta (24), these planetary bodies are often thought to have accreted dry (58). This prompts two key questions: Where did the water come from? And when was it present in the inner solar system? The answers may reveal information about accretion processes in terrestrial planets. Additionally, the extent and importance of lateral water transport throughout the history of the solar system are debatable (9, 10).

The source of water in planetary bodies can be investigated by measuring the ratio between the isotopes of hydrogen (deuterium, D or 2H, and hydrogen, 1H) because different regions of the solar system vary widely in measured D/H ratios (11). Thus, determining D/H ratios in meteorites with well-known ages and planetary origins can help constrain the timing of water delivery to accreting planetary bodies. Such analysis provides an ideal opportunity to study the provenance of water in bodies throughout the solar system, including Earth, the Moon, Mars, and the asteroid belt (1214).

Eucrites, a class of asteroidal basaltic meteorites, can provide unique information about the timing of water delivery to planets because most of these rocks crystallized 4559 to 4547 million years ago (Ma), or ~8 to 20 million years after the first solids in the solar system, calcium- and aluminum-rich inclusions (CAIs) (15, 16). Eucrites belong to the howardite-eucrite-diogenite group of meteorites (HEDs) that are derived from the asteroid belt, dominantly from the asteroid Vesta (1719). Determining the source of water in eucrites can constrain the time at which water existed in the inner solar system, because these ancient rocks are some of the oldest igneous rocks in the solar system. However, until recently it was believed that eucrites were completely anhydrous and therefore could provide no insight into the D/H ratio of Vesta (4, 6).

Here, we report the concentration and isotopic composition of structurally bound water in the mineral apatite [Ca5(PO4)3(OH,F,Cl)] from five different basaltic eucrite samples: Juvinas, Pasamonte, Cachari, Stannern, and Pecora Escarpment (PCA) 91078 (figs. S1 and S2). We measured the D/H and water contents simultaneously in situ using a Cameca 1280 ion microprobe. The water concentration in these apatites ranges from 668 to 2624 μg/g, which agrees with calculated water contents from electron probe measurements (table S1) (4, 20). We measured δD values ranging from –231 per mil (‰) to –37‰ (Table 1) (21). No systematic variation in D/H exists between samples or between different apatite grains in the same meteorite (Fig. 1). The weighted mean hydrogen isotopic composition for eucritic apatite from the five different investigated samples is δD = –162 ± 127‰ (2σ; n = 11).

Table 1. Hydrogen isotope and water analyses of apatite and epoxy.
View this table:
Fig. 1 Hydrogen isotopes versus water content of eucrite apatites.

Note the large variation in water content that contrasts with a small range in the hydrogen isotopic compositions. Error bars are 2σ; where error bars are not present, they are smaller than the symbol.

There are three processes that could have modified the hydrogen isotopic composition of the apatite we analyzed from that of the parental magma: (i) hydrous degassing before or during apatite crystallization, (ii) equilibrium stable isotope fractionation between hydrous phases and melt, and (iii) contamination or assimilation of exogenous H or D, presumably from the regolith of Vesta. Degassing is unlikely due to the small observed spread in δD with the relatively large spread of water contents of the analyzed apatite (22, 23). Apatite is the only hydrous mineral phase in eucrites, and stable isotope fractionation between apatite and melt is expected to be small (~20‰) at high temperature; thus, apatite-melt fractionation should be minor relative to the variation in δD observed (24). Contamination by exogenic H from solar wind and/or by D produced during spallation processes is unlikely, because apatites have young exposure ages, high water contents, and a small range of δD across exposure ages and metamorphic grades (20). We conclude that none of these processes have substantially affected the H isotopic compositions measured in the present study (20).

On the basis of the weighted mean δD of the apatites we analyzed, the δD value of eucrite magmatic water is –162‰, which is the value we adopt for bulk Vesta (20). This value is within the combined uncertainties of the value for the bulk Earth (~ –90‰) (22) and possibly the Moon (~ +90‰; Fig. 2) (3, 23). The fact that Earth and Vesta have indistinguishable δD values suggests that they have the same source of water. Our new D/H data, in combination with published bulk nitrogen (N) isotope data for eucrites, shows that the most likely common source of volatiles for Earth and eucrites (Vesta) is carbonaceous chondrites (13, 25). Note that N isotopes exclude the Jupiter family comets as a potential source of water for Earth and Vesta. Nitrogen isotopes also exclude Oort cloud comets as a source of water for the inner solar system planetary bodies as well. Kinetic isotope fractionation during both accretion and magmatic processes would not allow the H and N isotope systems as currently measured on Earth and Vesta to have evolved from the material implied by the signature of the Jupiter family comets or of the Oort cloud comets (Fig. 2). Although we propose that carbonaceous chondrites provided water for the inner solar system, there is evidence to show that at least Earth accreted from a heterogeneous mixture of hydrous and anhydrous materials, which rules out carbonaceous chondrites as the only terrestrial building blocks (8).

Fig. 2 Nitrogen isotopes versus hydrogen isotopic compositions of objects in the Solar System.

Note that the fields for the Moon and Mars are large; these represent all D/H data because of a lack of consensus about the hydrogen isotopic compositions for the bulk Moon and Mars. For carbonaceous chondrites, ice in equilibrium with bulk clays has δD values of ~ –400‰ to 100‰ (12). Early carbonaceous chondrite ice could be another source of water for the inner solar system. The degassing model line originating at protosolar values does not intersect Vesta, Earth, or the Moon. See (20) for details and data. [Figure modified from (3, 28)]

The hypothesis that inner solar system water came from a carbonaceous chondrite source is further supported by bulk carbon (C) isotope data. The C isotopic compositions of Earth and Vesta are very similar and overlap with that of carbonaceous chondrites but not with the solar composition (Fig. 3). Thus, the isotopic compositions of three different volatile elements as well as their concentration ratios for Earth and Vesta are consistent with the composition of carbonaceous chondrites.

Fig. 3 Carbon versus hydrogen isotopes.

Note that Earth, the Moon, and Vesta plot within the carbonaceous chondrite field. Because carbon isotopes vary from –250‰ to 200‰ in comets, C isotopes are not useful for excluding comets as a source of C for inner solar system bodies. The degassing line originating from the protosolar point assumes minimal C isotope fractionation and maximal H fractionation. See (20) for details and data.

Earth and Vesta have indistinguishable H, C, and N isotopic compositions, and thus likely have the same volatile source. This source must have had an H isotopic composition similar to or lower than the values measured in our samples, because possible magmatic fractionation processes could only lead to a relative enrichment and not a depletion in deuterium (20). One important reservoir that has low D/H and low 15N/14N is the Sun (Fig. 2), which could theoretically be an alternative source of volatiles in the inner solar system, assuming that such a solar-derived reservoir would have been later altered by a secondary degassing process. Our model for degassing uses the most conservative input values, which lead to the largest isotope fractionations for N and H isotopes and reflect degassing of a 1:1 mixture of NH4 and H2. It is evident that degassing alone cannot realistically evolve solar H and N isotope ratios to values measured in Vesta (Fig. 2). Degrees of degassing in excess of 99.99% are required to evolve N and H isotope ratios of the Sun to ratios close to those of the inner solar system. Additionally, if this extreme degassing also involved volatile loss of C, then C isotopes would significantly disagree with a solar source of volatiles because the solar C isotopic composition would become heavier than that of Earth, the Moon, or Vesta (Fig. 3). It is highly unlikely that accretion and degassing would strongly fractionate H and N isotopes yet would allow C isotopes on Vesta to remain isotopically light. Consequently, we rule out a solar source of volatiles for the bodies in the inner solar system.

The H, C, and N isotopic similarities between eucrites, Earth, and potentially the Moon allow us to place important limits on the timing of water delivery to the inner solar system. Earth cannot provide timing of water delivery because it is currently geologically active. The Moon likely accreted its water at or before ~200 million years after CAIs, or around 4367 Ma (3, 23), but such a constraint is not very rigorous, given that all the planets in the inner solar system are thought to have fully accreted by this time. Eucrites provide a substantially earlier data point, which suggests that the source of Earth’s water was present in the inner solar system very early, ~8 to 20 million years after CAIs (15, 16). This evidence moves back the time at which the terrestrial water reservoir is thought to exist and have been available for accretion. Additionally, this reservoir was present between 1 and 2.4 AU and perhaps throughout the inner solar system. Late-stage addition of water to planets from outer parts of the solar system is therefore unlikely to have affected the water budgets of inner solar system bodies. Thus, the bulk of the highly volatile elements H, C, and N now present in Earth and the asteroid belt most likely arrived from a local source (i.e., carbonaceous chondrite–like material) very early in solar system history. The limited variation in δD over a large range of heliocentric distances (1 to 2.4 AU) supports the notion of a uniform source of water in the inner solar system. Our findings cannot preclude a late addition of water for Earth with a carbonaceous chondrite–like D/H, but the observation indicates that a late addition of water is not necessary.

Our geochemical results are in agreement with several dynamic solar system models for planetary water delivery. These models indicate that carbonaceous chondritic planetesimals delivered water during primary accretion of Earth (9, 26, 27). In addition, it is believed that Earth’s water condensed in the outer asteroid belt and giant planet regions and was then transported to Earth by dynamical processes related to giant planet migration (27). The implications are that (i) the migration of water in the inner solar system must have started by 8 to 20 million years after CAIs, or (ii) water was always present in the inner solar system, in which case no water migration is needed to satisfy the H isotopic composition of terrestrial planets (10).

Supplementary Materials

www.sciencemag.org/content/346/6209/623/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 and S2

Table S1

References (2881)

References and Notes

  1. See supplementary materials on Science Online.
  2. δD = {[(D/Hunknown)/(D/HVSMOW)] – 1} × 1000, where VSMOW = Vienna standard mean ocean water.
  3. Acknowledgments: We thank the anonymous reviewers; K. Righter, D. Ebel, C. Agee, and T. McCoy for sample allocations; and N. Shimizu and G. Gaetani for their immense help. Supported by NASA graduate fellowship NNX13AR90H (A.R.S.), an Andrew W. Mellon Foundation Award for Innovative Research (S.G.N.), and NASA Cosmochemistry Program award NNX11AG76G (F.M.M.). Secondary ion mass spectrometry analyses were made at the Northeast National Ion Microprobe Facility (NENIMF) at the Woods Hole Oceanographic Institution (WHOI). NENIMF acknowledges support from the NSF Instrumentation and Facilities Program, Division of Earth Sciences, and from WHOI. Data are presented in the main text of the manuscript and the supplementary materials.
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