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Title:
Organic History and Ice-Rock Decoupling on Enceladus
Authors:
Zolotov, M. Y.
Affiliation:
AA(School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States ; )
Publication:
American Geophysical Union, Fall Meeting 2007, abstract #P21B-0540
Publication Date:
12/2007
Origin:
AGU
AGU Keywords:
1055 Organic and biogenic geochemistry, 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5220 Hydrothermal systems and weathering on other planets, 6280 Saturnian satellites
Abstract Copyright:
(c) 2007: American Geophysical Union
Bibliographic Code:
2007AGUFM.P21B0540Z

Abstract

The Cassini detection of methane, propane and acetylene in the Enceladus plume, and condensed organic compounds (OC) on the south polar region imply an organic-bearing interior of the moon. At least a few wt. % of C is expected in rocks from which Enceladus accreted. By analogy with carbonaceous chondrites, the majority of accreted OC was in a polymer in which polyaromatic groups are linked by O-, N-, and S-bearing aliphatic units. If accreted, cometary-type materials also delivered CO2, CO(?), methanol, ethane, ethene, acetylene, and condensed OC. Subsequent water ice melting and hydrothermal processes driven by decay of short-lived radionuclides led to dissolution of CO, CO2 and methanol in water and transformations of the polymer and cometary OC. CO converted to formic acid, carbonate species, methanol and methane. Hydrous pyrolysis and oxidation of the polymer partially liberated aromatic molecules and led to the formation of O-bearing OC (carboxylic and amino acids, alcohols). Increase in temperature favored oxidation of OC to carbonate species and
N2, and led to graphitization of the polymer. Despite net oxidation of OC driven by H2 escape, mineral- catalyzed Fisher-Tropsch like synthesis of hydrocarbons and methane occurred in H2-rich niches. As a result, an array of aromatic, aliphatic, and N-, O-, S-bearing OC, and methane was delivered into a primordial water ocean in hydrothermal fluids. Highly soluble OC (acids, alcohols) made multiple passes through hydrothermal systems causing further oxidation of OC in rocks and solutions. In contrast, hydrocarbons exolved from cold oceanic water and formed an organic layer below the ice shell. Subsequent cooling of ocean-entering fluids and ocean freezing from above led to further separation and accumulation of OC. Some OC was trapped in ice, and methane formed clathrates. After freezing of salt eutectic brines, the light oil (a solution/mixture of ethane, propane, butane, ethene, acetylene, methanol, toluene etc.) remained unfrozen and decoupled the ice shell from underlying salt deposits and rocks. Even after oil solidification, if it occurred, the organic layer had a lower viscosity than salts and ice. An uneven pressure and/or topography at the ice-salt boundary could have led to preferential oil (and salt?) accumulation below the south polar region. Throughout history (and today), the uneven oil-rich layer could have favored tidal motions and heat generation at the bottom of the ice shell.
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