Research ArticleSPACE SCIENCES

New clues to ancient water on Itokawa

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Science Advances  01 May 2019:
Vol. 5, no. 5, eaav8106
DOI: 10.1126/sciadv.aav8106
  • Fig. 1 Thermal diffusion model to quantify water loss during thermal metamorphism at temperatures of 600° to 800°C.

    The model is based on the analytical solution to the diffusion equation discussed by Ingrin and Blanchard (22). A sphere 25 km in radius with pyroxene composition was heated to temperatures of 600° and 800°C for a duration of 10 Ma. The water contents in pyroxene grains at distances of 100 m (A), 1 km (B), and 10 km (C) from the top surface of the sphere are plotted against the duration of thermal metamorphism. Dashed lines indicate the measured concentrations of water in two Itokawa grains. Higher temperatures and shallower depths result in larger diffusivity and enhanced loss of water. At a distance of 100 m, heating at 800°C would result in a loss of up to ~213 ppm water. When the distance from the surface is larger than 1 km, water loss is less than 25 ppm.

  • Fig. 2 Modeling of dehydration due to an impact event.

    It shows the residual water contents in pyroxene grains at distances of 100 m (A), 1 km (B), and 10 km (C) from the surface of a sphere with a radius of 25 km. The same diffusion model has been used as in Fig. 1, although different input parameters, e.g., post-shock temperatures (800°, 1000°, and 1200°C) and cooling duration (2 ka), are subjected to the sphere. Dashed lines indicate the measured concentrations of water in two Itokawa grains. When temperature is lower than 1000°C, the loss of water is limited to be <25 ppm for all cases. The highest post-shock temperature (1200°C) causes ~80 ppm loss of water at a distance of 100 m. The loss of water is insignificant if the pyroxene occurs at a distance that is larger than 1 km from the surface.

  • Fig. 3 Hydrogen isotope compositions of objects in the solar system expressed as δDSMOW.

    Itokawa LPx grains (−79 ± 70‰ for RA-QD02-0057 and −53 ± 69‰ for RA-QD02-0061) are shown in blue diamonds. Except for Itokawa grains, no errors have been plotted for clarity. The gray shaded region represents the range of terrestrial samples. Jupiter family comets (JFCs) are in the Kuiper Belt that extends from 30 to 50 AU; Oort cloud comets (OCCs) occupy a vast space from 2000 to 50,000 AU from the Sun. We have plotted all Jupiter family comets and Oort cloud comets at 50 and 50,000 AU, respectively, instead of their current locations. The measured phases from OCs are separated into D-rich matrices, D-rich bulk rock, and low-δD NAMs. The high δDSMOW values for the martian glass phases and apatite likely reflect interaction with the D-rich martian atmosphere and are not relevant while determining the bulk mantle water on Mars. The inset shows the δDSMOW of samples from the asteroid belt, such as eucrites, ordinary chondrites (OCs), and carbonaceous chondrite (CCs). Our measurements show that the δDSMOW values of two Itokawa grains, apatite from Vesta, most NAMs from OCs, and some of the bulk carbonaceous chondrites fall within the range of terrestrial samples. IOM, insoluble organic matter (see Supplementary Text for data and references).

  • Fig. 4 δDSMOW (‰) of pyroxene and olivine from LAR 12036 and Bishunpur, and matrices from ordinary chondrites.

    The black diamonds indicate the median δDSMOW of NAMs from ordinary chondrites. The errors are 2 SM. The δDSMOW values of pyroxene and olivine from ordinary chondrites fall in a range of −250 to +150‰. The median δDSMOW values of NAMs from ordinary chondrites are within a range of −100 to 0‰ and fall in the range of terrestrial samples, which is indicated by the gray shaded area. The δDSMOW values of phases in the matrices of ordinary chondrite have a range of +171 to +4562‰, which are higher than that of NAMs from ordinary chondrites and terrestrial samples (see Supplementary Text for data). Py, pyroxene; Ol, olivine.

  • Fig. 5 Water contents (ppm) of two measured Itokawa grains and the concentrations of water in pyroxene and olivine from LL6 LAR 12036, LL3.1 Bishunpur ordinary chondrite, terrestrial rocks, and angrite.

    The black diamonds indicate the median water contents of NAMs in each group. The errors are 2 SM, and some errors are smaller than the size of the symbols. Pyroxenes from Itokawa, LAR 12036, and Bishunpur ordinary chondrites have comparable water contents, and their median water contents are higher than those of terrestrial pyroxenes. The olivines from ordinary chondrites and terrestrial samples have low water contents. The angrite samples are drier (see Supplementary Text for data).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/5/eaav8106/DC1

    Supplementary Text

    Fig. S1. Itokawa particles, overview of LAR 12036 ordinary chondrite, and sample holder for NanoSIMS measurements.

    Fig. S2. Reported oxygen isotopic compositions of Itokawa minerals [modified after 14].

    Fig. S3. SIMS craters on Itokawa particles and ion intensities during their NanoSIMS measurements.

    Fig. S4. Representative reflected-light images of the analyzed spots after Cameca IMS 6f analyses and the H counting rates during the analyses.

    Fig. S5. Calibrations of water contents and hydrogen isotope compositions using terrestrial standards.

    Fig. S6. The onion shell model of parent body used for expected water loss calculation.

    Table S1. Water concentration and D/H ratios of reference standards and Itokawa grains.

    Table S2. D/H ratios of reference standards and adopted pyroxene from LAR 12036 ordinary chondrite.

    Table S3. Simulated water loss caused by thermal metamorphism and impact events using the thermal diffusion model.

    Table S4. Estimated average water loss of the assumed 25-km-radius parent body caused by thermal metamorphism and impact events.

    References (4698)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. Itokawa particles, overview of LAR 12036 ordinary chondrite, and sample holder for NanoSIMS measurements.
    • Fig. S2. Reported oxygen isotopic compositions of Itokawa minerals modified after 14.
    • Fig. S3. SIMS craters on Itokawa particles and ion intensities during their NanoSIMS measurements.
    • Fig. S4. Representative reflected-light images of the analyzed spots after Cameca IMS 6f analyses and the H counting rates during the analyses.
    • Fig. S5. Calibrations of water contents and hydrogen isotope compositions using terrestrial standards.
    • Fig. S6. The onion shell model of parent body used for expected water loss calculation.
    • Table S1. Water concentration and D/H ratios of reference standards and Itokawa grains.
    • Table S2. D/H ratios of reference standards and adopted pyroxene from LAR 12036 ordinary chondrite.
    • Table S3. Simulated water loss caused by thermal metamorphism and impact events using the thermal diffusion model.
    • Table S4. Estimated average water loss of the assumed 25-km-radius parent body caused by thermal metamorphism and impact events.
    • References (4698)

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