Abstract
Enceladus is a target of future missions designed to search for existing life or its precursors. Recent flybys of Enceladus by the Cassini probe have confirmed the existence of a long-lived global ocean laced with organic compounds and biologically available nitrogen. This immediately suggests the possibility that life could have begun and may still exist on Enceladus. Here we will compare the properties of two proposed sites for the origin of life on Earth—hydrothermal vents on the ocean floor and hydrothermal volcanic fields at the surface—and ask whether similar conditions could have fostered the origin of life on Enceladus. The answer depends on which of the two sites would be more conducive for the chemical evolution leading to life's origin. A hydrothermal vent origin would allow life to begin in the Enceladus ocean, but if the origin of life requires freshwater hydrothermal pools undergoing wet-dry cycles, the Enceladus ocean could be habitable but lifeless. These arguments also apply directly to Europa and indirectly to early Mars. Key Words: Enceladus—Hydrothermal vents—Hydrothermal fields—Origin of life. Astrobiology 17, 834–839.
Highlights
To introduce our argument and guide the discussion, it is essential to make explicit a set of assumptions
A hydrothermal vent origin would allow life to begin in the Enceladus ocean, but if the origin of life requires freshwater hydrothermal pools undergoing wet-dry cycles, the Enceladus ocean could be habitable but lifeless
Barge et al (2017) explored thermodynamic aspects of chemical reactions that could occur in hydrothermal vents. ‘‘We argue that life only emerges when and where particular planetary-scale conditions of chemical disequilibria are produced through the interactions of the atmospherehydrosphere complex with fresh mafic to ultramafic oceanic crust continually replenished by active partial melting of the mantle.’’ Their argument focuses on the energy made available by chemical reactions that produce reducing power such as dissolved hydrogen, which in turn drives reactions related to putative metabolic pathways that could be incorporated into early life
Summary
To introduce our argument and guide the discussion, it is essential to make explicit a set of assumptions. We will begin by noting that microorganisms on Earth can survive in the deep, cold ocean in the absence of light energy. For these life-forms, the reducing power of hydrogen sulfide or dissolved hydrogen gas is used as a source of electrons. The plumes contain compounds consistent with the presence of hydrothermal sites on the ocean floor that are powered by chemical reactions. Hydrothermal sites are the only environments that can provide this suite of conditions Other proposed environments such as a global ice sheet (Bada et al, 1994), clay minerals (Ferris, 2002), and iron-sulfur minerals (Wachtershauser, 1992) lack one or more of the above properties
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