1. 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.

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

3. 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)

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
Water keeps silicates from melting – buffering from latent heat and cooling by hydrothermal circulation

6. 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)

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

8. 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 (?)

9. 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

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
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

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?
Contours where T<145K

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