Enceladus and Titan: Prime targets in the search for life

Enceladus

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

Cassini-Huygens

INMS shows complex chemistry in the region above 900 km (probably below, too).

Cassini-Huygens

Surface mass spectrum from Huygens

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.

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Ligeia Mare:

One of Titan’s great seas

100 km

20092005

Ontario

Lacus

Hayes et al, 2009

Loss of liquid of ~ 86 km3 , av. depth 5 meters 5 m layer lost over 5 years

Moriconi et al 2010. Cassini VIMS data.

Titan’s Milankovitch cycles: precession of Saturn’s eccentric orbit

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-bondPolar 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

TiME: The Titan Mare ExplorerA Discovery Mission to an Extraterrestrial Sea

We must go back to the Saturn system...if we want to test whether the origin of life is a common cosmic phenomenon