Joshua P. Emery Earth & Planetary Sciences University of Tennessee Water in Asteroids Joshua P. Emery Earth & Planetary Sciences University of Tennessee On the plane, watched “Midnight in Paris”. The film isn’t very subtle – the hero begins the film pining away for the “glory days” of Paris in the 20s, but as it moves on, it beats you over the head with the message to appreciate the present. In this regard, I think we have it easy. It’s really an exciting time for planetary astronomy, and for the study of water in asteroids in particular.

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 No other condensation line (I think) with such a large, sudden contribution of mass  giant planet formation!

Nebular Snowline Location were Tmidplane= Tcond depends on nebula Lunine (2002) Perhaps not a static line, but migrating zone Lunine – top line is warm nebula, where Tsaturn ~80K (consistent with Saturn and satellites) Others cold, with and without accretion Dodson-Robinson et al. (2009)

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)

Differentiation Significantly different expressions in the presence of water Water keeps silicates from melting – buffering from latent heat and cooling by hydrothermal circulation

Intermediate Sizes Themis ~ 210 km Pallas ~ 545 km 390 – 450 km Three-dimensional topography model of Pallas based on images. The aspects are identical to those in Fig. 1. The approximate orientation of the spin axis and location of the south pole are shown. Shape consistent with hydrostatically relaxed, undifferentiated spheroid composed largely of hydrated silicates. Themis models – a) accretion at 3 Myr with water, b) accretion at 5 Myr with water, c) accretion at 5 Myr, no water (already hydrated) Castillo-Rogez and Schmidt (2010) Schmidt et al. (2009)

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

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

Ceres Ceres is a special case – strange 3-µm band Differentiated 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

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

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

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

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? Contours where T<145K

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)

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