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Water in Asteroids

Water in Asteroids. Joshua P. Emery Earth & Planetary Sciences University of Tennessee. Why is H 2 O in Asteroids Interesting?. A lot of mass in H 2 O Big effect on accretion where condenses Significant impact on geochemical evolution Latent heat energy buffer Heat from serpentinization

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Water in Asteroids

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  1. Water in Asteroids Joshua P. Emery Earth & Planetary Sciences University of Tennessee

  2. Why is H2O in Asteroids Interesting? • A lot of mass in H2O • Big effect on accretion where condenses • Significant impact on geochemical evolution • Latent heat energy buffer • Heat from serpentinization • Resulting mineralogies • Potential source of terrestrial volatiles • Asteroids retain a record of the initial H2O distribution and evolutionary events

  3. Nebular Snowline • Location were Tmidplane= Tcond depends on nebula Lunine (2002) • Perhaps not a static line, but migrating zone Dodson-Robinson et al. (2009)

  4. Internal Temperatures • Heating from 26Al • Less 26Al at larger distances due to slower accretion • Latent heat (melting) • Serpentinization • Hydrothermal flow • Hydraulic fracturing • Others. . . McSween et al. (2003)

  5. Differentiation • Significantly different expressions in the presence of water

  6. Intermediate Sizes • Themis ~ 210 km • 390 – 450 km • Pallas ~ 545 km Castillo-Rogez and Schmidt (2010) Schmidt et al. (2009)

  7. Surface Expressions • Current ice • Hydrated phases • Phyllosilicates • Carbonates • Oxides & Hydroxides • Salts • Geological features

  8. Hydrated C-types • Majority (~2/3) are hydrated • Anti-correlation with distance (?) Rivkin et al. (2011) Lebofsky et al. (1990) Vilas (1996) Fraction with 0.7 µm band Jones et al. (1990) Carvano et al. (2003)

  9. Ceres • Ceres is a special case – strange 3-µm band • Fe-rich phyllosilicates + brucite +carbonates • Water ice • Ammoniated clays • Differentiated • Liquid H2O mantle? • OH emission? • Albedo variations • 0.02 to 0.16 • Geology? Milliken & Rivkin (2009) • Other asteroids with similar surface compositions? • e.g., 10 Hygeia

  10. Hydrated M- and E-types • M- and E-type asteroids with 3-µm band • ~33 to 50% of those observed • Correlated with size (larger more likely hydrated) • Generally do not show 0.7-µm band Rivkin et al. (2000) • NIR spectra, radar, thermal confirm non-metallic (silicate) nature of many M-types

  11. Main Belt Comets • Asteroidal orbits – cannot derive from comets • Dust released by ice sublimation • perhaps following recent impact • Too faint for direct NIR spectral search for ice Hsieh (2008)

  12. Themis, Cybele, etc. • Rounded band, centered ~3.1 µm • H2O frost/coating • Goethite 24 Themis (a~3.13 AU) 65 Cybele (a~3.43 AU) Licandro et al. (2011) Rivkin & Emery (2010) • Detected on several more outer belt asteroids

  13. Stability and Supply of Ice • Ice stability • Exposed ice sublimates at rate ~1 mm/105yr at 120K • Buried ice can last 4.5 Gyr • Fanale & Salvail (1989), Schorghofer (2008) Schorghofer (2008) • Requires a mechanism to bring ice to surface • Recent impacts? Why no tail like MBCs? • Vapor diffusion driven by underlying activity? • If H2O at surface, must be sublimating • should see OH emission • How pervasive?

  14. Trojan Asteroids • Widely thought to contain ice, but none detected • Also no sign of hydrated silicates • Ennomos → suggested high albedo, but not confirmed • Refractory mantle? Yang & Jewitt (2007) Emery & Brown (2004) • Densities • Hektor ~ 2.2 g cm-3 • Patroclus ~ 1.0 g cm-3 Marchis et al. (2006)

  15. Summary • Widespread evidence for the presence of H2O in the asteroid belt • Past & present • Current state of that H2O provides strong constraints on evolution • Geochemical and dynamical • Surface geology and composition also heavily influenced by H2O • Detailed views from spacecraft • Telescopic surveys

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