Enceladus and Titan: Prime targets in the search for life.
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Transcript of Enceladus and Titan: Prime targets in the search for life.
Enceladus is a moon of Saturn, 1000 times smalller than Europa, with jets emanating from hot crevasses. The jets are icy particles frozen from salty water.
Titan
2nd largest moon in the sol sys
Density 1.88 g/cm3 --> rock+ice
Dense N2-CH4 atmosphere. Surface 89-94 K; pressure 1.5 atm.
Titan’s atmosphere
NB – 1D radiative transfer codes are able to produce matching temperature profiles by including what we know about Titan’s composition
methane
ethane
acetylene
nitrogen HCN
Photochemistry at the heart of Titan’s surface-atmosphere evolution
λ < 1450 Å
λ ≈ 2000 Å
Quantification of the methane consumption (simplified)
Net photochemical destruction rate of CH4 (Lavvas, et al. 2008) p-l= -3.5 x 10-13 g cm-2s-1
referred to the surface
Mass per area of methane in atmosphere ~ P/g = 370 g cm-2
Lifetime of atmospheric methane: -P/g(p-l) = 35 million years
Amount of methane that must be supplied to the surface over time
Need 4.5 x 109 years /3.5 x 107 years ~ 100 x present atmospheric methane overthe age of the solar system.
or 100P/g = 3.7 x 104 g cm-2 / (0.7, 0.4 g cm-3) = (0.5, 0.9) km thick layer of (C2H6, CH4)over the age of the solar system
Methane must be resupplied through time by outgassing from the interior.
Cycling of lakes/seas on Titan
Seasonal30 years
Methane-rich
Ethane/propane-rich
Croll-Milankovitch50,000 years
Ethane/propane-rich
Precipitates (C2H2, HCN…)
Lower viscosity waves
Higher viscosity smooth
“Hydration/dehydration” cycling
Seasonal Croll-Milankovitch
No analog exists in Earth’s hydrologic cycle for this kind of complexity
Cassini surface mysteries
There is a sink of hydrogen at the surface
Acetylene is underabundant in surface deposits
Benzene is present in surface deposits
Ethane is 100-1,000 times less abundant than expected on the surface.
MethanogenicC2H2+3H22CH4
Chemistry3C2H2C6H6
Titan surface reactions?
+ 334 kJoules/mole (McKay and Smith, 2005)
Could this Titan host life on its surface? At -180 C liquid water is not stable at the surface. However, should one rule out a priori a form of life that exists in ethane and methane….?
What’s right with water as the host
liquid for biochemistry
Excellent solvent for salts
Polar
Provides an “inside” and “outside” for biopolymers
Could methane and ethane act as
a liquid medium for life?Allows organic molecules to hydrogen-bond
Polar hydrocarbons associated with the liquid might create “insides” and “outsides” in liquid ethane/methane
“Biological” molecules would be dominated by C-N bonds rather than C-O as on Earth.
Some C-O compounds resulting from interaction with water-ice bottom.
Steve Benner and colleagues
Life in ethane-methane seas on Titan is a far more stringent test of a separate second origin than life found on Mars, Europa, or Enceladus.
Impact exchange of material between bodies in our solar system implies a fundamental difficulty in proving that life “like us” had a separate origin.
Life in a hydrocarbon sea is so fundamentally different-- at the molecular level—that it immediately would imply a second, separate origin of life.
Titan has a big advantage over other astrobiological targets
? Titan
No planetary protection issues: landing in the hydrocarbon lakes sterilizes the probe.
Easy access to the lakes: no radiation, no drilling, just repeat Huygens mission....
LPL/JPL/NASA/USGS
“ If life is an intrinsic property of chemical reactivity, life should exist on Titan.
“Indeed, for life not to exist on Titan, we would have to argue that life is not an intrinsic property of the reactivity of carbon-containing molecules under conditions where they are stable.”
Limits of Organic Life, National Research Council 2007