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

Kaye, Jonathan Z.; Marion, Giles M.; Sassen, Roger; Head, James W.; Iii,; Crowley, James K.; Ballesteros, Olga Prieto; Grant, Steven A.; Kargel, Jeffrey S.; Hogenboom, David L.

We have considered a wide array of scenarios for Europa's chem-ical evolution in an attempt to explain the presence of ice and hy-drated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below. We postulate that, following formation of the jovian system, 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 and in-tracrustal differentiation, (d) degassing of Europa's mantle and gas venting, (e) hydrothermal processes, and (f) chemical surface al-teration. Our models were developed in the context of constraints provided by Galileo imaging, near infrared reflectance spectroscopy, and gravity and magnetometer data. Low-temperature aqueous dif-ferentiation 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 range of possible layered structures may result; the final state depends on the details of intracrustal differentiation. Devolatiliza-tion 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 sulfuric acid, or a crust con-taining abundant clathrate hydrates. Realistic chemical–physical 226 0019-1035/00 $35.00 All rights of reproduction in any form reserved. EUROPA'S OCEAN 227 evolution scenarios differ greatly in detailed predictions, but they generally call for a highly impure and chemically layered crust. Some of these models could lead also to lateral chemical hetero-geneities by diapiric upwellings and/or cryovolcanism. We describe some plausible geological consequences of the physical–chemical structures predicted from these scenarios. These predicted conse-quences and observed aspects of Europa's geology may serve as a basis for further analysis and discrimination among several al-ternative scenarios. Most chemical pathways could support viable ecosystems based on analogy with the metabolic and physiological versatility of terrestrial microorganisms.

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