Oceans in the Outer Solar System

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• Oceans in the Outer Solar System
• Adam Showman, Lunar Planetary Lab
• Some early ideas
• Evidence for oceans
• Understanding why (and where) oceans can exist
• Some astrobiological speculations

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History of pre-Galileo interior studies

• For Ganymede Callisto, early studies suggested
  an ocean, then reversed themselves. For Europa
  it was a little less decisive.
• Lewis 1971 ocean
• Consolmagno Lewis 1976 ocean
• Reynolds Cassen 1979 no ocean
• Cassen et al. 1979 ocean
• Cassen et al. 1980 no ocean?
• Squyres et al. 1983 ocean
• Ross and Schubert 1989 no ocean?
• Ojakangas Stevenson 1989 ocean

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Surprise the first solid evidence for an ocean
cameat a no-longer boring Callisto!

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Galileo Magnetometer data -- Callisto
Zimmer et al. (2000)
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Galileo magnetometer data -- Europa
Kivelson et al. (2000), Science
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How can icy satellites have oceans?Water phase
diagram is key
Kivelson et al. 2002 (left) and Showman et al.
1997 (right)



This differs drastically from terrestrial planets
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Galilean satellites
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Antifreeze helps too
Ammonia (Kargel 1992)
Sulfate salt (Kargel 1991)
Grasset et al. (2000)
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And convections not as efficient as we used to
think
Constant viscosity
Temperature-dependent viscosity stagnant lid
develops and inhibits heat escape
Showman and Han (2004)
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How does one quantify this?
Many numerical experiments have quantified how
convected heat flux varies with convective vigor.
They typically show that heat flux (expressed
as a Nusselt number) varies as Ra0.2-0.3, where
Ra is Rayleigh number
Mitri Showman (2005)
Given this cooling rate, and given an estimate of
the radiogenic/tidal heat production, one can
then determine the evolution, including whether
an ocean should exist.
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How does stagnant-lid convection affect the heat
flux?
Rayleigh number
and
Use Ganymede/Callisto parameters g1.3 m/s2,
a10-4/K, r103 kg/m3, k10-6 m2/sec, k3
W/m K, A26, h01014 Pa s, and F0.003 W/m2
corresponding to carbonaceous chondritic
radionuclide decay This implies h7 x 1019 Pa
sec and T180 K for constant-viscosity
convection But it implies h1.3 x 1015 Pa sec and
T245 K for stagnant-lid convection
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Evolution models of Ganymede confirm the rough
calculations Depending on the ice
viscosity, radiogenic heating alone may be able
to maintain oceans in Ganymede and
Callisto. Adding antifreeze makes it
relatively easy to explain oceans in these
bodies. Showman et al. (1997)
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Ganymede may have been tidally heated, which
makes it even easier to explain an ocean
Showman et al. (1997)
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Europa convection calculations by many authors
indicate that a modest (20-40 km-thick) shell
remains stable over an ocean
Mitri and Showman (2005)
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The satellite bestiary
8 bars
17 satellites (including Pluto) have diameters
exceeding 1000 km
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Source Wikimedia commons
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Hussmann et al. (2006)
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Rhea and Iapetus
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Dione diameter 1125 km, density 1.5
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Tethys diameter 1072 km, density 0.96
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Enceladus diameter 504 km, density 1.6
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Uranus satellites
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Ariel diameter 1158 km, density 1.7
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Miranda diameter 472 km, density 1.2
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Internal structures of icy bodies from Hussmann
et al. (2006)
These models assume a differentiated structure
and up to a few ammonia in the H2O layer
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Black smokers
Source University of Delaware, Craig Cary
Are these a good analog for icy-satellite
habitability? Not so fast
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Conclusions

• Evidence indicates that Callisto, Europa, and
  probably Ganymede contain oceans now. The
  water phase diagram, the inefficiency of
  convective heat loss, and antifreeze conspire to
  allow these oceans.
• Models indicate that, additionally, Titan, Rhea,
  Titania, Oberon, Triton, Pluto, Eris (2003
  UB313), Sedna, and Orcus (2004 DW) probably
  contain internal oceans at present if the ice
  contains small amounts of ammonia.
  Trans-Neptunian objects 2005 FY9 (Easter Bunny)
  and 2003 EL61 (Santa) may too, if their outer
  layers contain H2O. Thus, of known objects in
  the outer solar system, 14 may contain oceans
  now. At least a few objects big enough to have
  oceans probably remain undiscovered.
• These oceans have astrobiological potential,
  particularly in moons where the ocean abuts the
  silicate core. Europas ocean remains by far
  the easiest ocean to sample directly, although
  other oceans may be just as interesting.

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Ganymede
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Callisto
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