Europa's crust and ocean: Origin, composition, and the prospects for life

We have considered a wide array of scenarios for Europa's chemical evolutio n in an attempt to explain the presence of ice and hydrated materials on it s surface and to understand the physical and chemical nature of any ocean t hat may lie below, We postulate that, following formation of the jovian sys tem, the europan evolutionary sequence has as its major links: (a) initial carbonaceous chondrite rock, (b) global primordial aqueous differentiation and formation of an impure primordial hydrous crust, (c) brine evolution an d intracrustal differentiation, (d) degassing of Europa's mantle and gas ve nting, (e) hydrothermal processes, and (f) chemical surface alteration, Our models were developed in the context of constraints provided by Galileo im aging, near infrared reflectance spectroscopy, and gravity and magnetometer data. Low-temperature aqueous differentiation from a carbonaceous CI or CM chondrite precursor, without further chemical processing, would result in a crust/ocean enriched in magnesium sulfate and sodium sulfate, consistent with Galileo spectroscopy. Within the bounds of this simple model, a wide r ange of possible layered structures may result; the final state depends on the details of intracrustal differentiation. Devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean/crust compositions, e,g,, an ocean/crust of sodium carbonate or sulfu ric acid, or a crust containing abundant clathrate hydrates, Realistic chem ical-physical evolution scenarios differ greatly in detailed predictions, b ut they generally call for a highly impure and chemically layered crust.. S ome of these models could lead also to lateral chemical hetero geneities by diapiric upwellings and/or cryovolcanism. We describe some plausible geolo gical consequences of the physical-chemical structures predicted from these scenarios. These predicted consequences and observed aspects of Europa's g eology may serve as a basis for further analysis and discrimination among s everal alternative scenarios. Most chemical pathways could support viable e cosystems based on analogy with the metabolic and physiological versatility of terrestrial microorganisms.