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Analysis of the Europan surface composition based on Galileo NIMS spectra.

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Abstract

Reflectance spectra of the surface of the Jovian satellite Europa exhibit absorption bands in the 1.0 to 2.5 micron range which have been attributed to water ice (Pilcher et al., 1972; Calvin et al., 1995). However, significant departures from the spectral behavior of ordinary water ice have been observed. The Galileo NIMS spectral data vary in absorption band strength, shape, and position over the surface of the satellite. Several mechanisms have been proposed to explain this departure, including the presence of additional surface components, scattering effects due to small bubbles or pits in the ice, grain size effects, thermal and mechanical stresses, radiation damage to the ice, and the presence of water in a bound state such as water of hydration. The postulation of additional surface components suggest minerals and salts of varying hydration states, which could contribute to the observed spectral effects. Hydrated salts, especially the magnesium sulfates epsomite, hexahydrite, and bloedite have received a great deal of attention. Besides salts, hydrates such as zeolites and clays are under consideration, along with simple organics and acids. Previous work (Dalton and Clark, 1999; McCord et al., 1998, 1999) has relied on linear mixture analysis to model the Europa spectrum with mixtures of ices and hydrates, yielding relatively high hydrate abundances ranging from tens of percent to 100%. In the case where the albedo of the additional component closely matches that of the ice, this is a reasonable first approximation. Given that the surface is likely to be very fine-grained, intimate mixtures need to be considered. Nonlinear mixing and spectral effects can provide an additional bound on estimates of surface composition. Laboratory and numerical simulations permit further evaluation of the various hypotheses, placing upper limits on a number of materials. A synthesis of these results indicates that high abundances of the materials proposed thus far are not consistent with the observed Europa spectra. Specifically, at concentrations sufficient to produce the observed asymmetry in the 2.0 micron band, all of the hydrated salts and zeolites exhibit other bands not observed in the Europa spectrum. Although a few percent, and in some cases up to a few tens of percent, could still be present at or near the surface, these materials cannot satisfactorily explain the Europa spectrum from 1.0 to 3.0 microns. Other materials, such as simple organics (Delitsky and Lane, 1998) and acids such as sulfuric (Carlson et al., 1999) should not be overlooked at this stage; and other spectral effects such as radiation damage, thermal and mechanical effects, and scattering have not been ruled out and must still be examined in order to fully account for the observations.
1999BAAS...31.1171D
... The presence of hydrated salt minerals, perhaps from brine formed from ocean salts, on a terrain fractured by tidal stress has been suggested to account for two bands in the Galileo Near Infrared Mapping Spectrometer (NIMS) spectra (McCord et al. 1998a(McCord et al. , 1999(McCord et al. , 2001a and is consistent with satellite models that include upwelling of ocean material (e.g., Kargel et al. 2000, Zolotov andShock 2001). However, alternate geochemical (McKinnon 2002) and spectral (Carlson et al. 1999b, Dalton and Clark 1999, Dalton 2000, Carlson et al. 2003b interpretations have been proposed. In this chapter we focus on the alteration of proposed surface materials by the incident radiation, with emphasis on the icy satellites, particularly Europa. ...
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Early observations and results; Charged particle bombardment, variability; Dose vs.depth: radiation and regolith formation, trapping and escape; Radiation effects: irradiation of ice, irradiation of SO2 and sulfur - pure and in ice, irradiation of CO2 and carbon species in ice, irradiation of salts and acids, adsorption; Summary of satellite irradiation effects: Metis, Amalthea and Thebe, Io, Europa, Ganymede, Callisto.
... Two classes of related materials have been suggested to explain observations of Europa's prevalent nonice material: (1) hydrated magnesium and sodium sulfates and/or sodium carbonates (the "salt hypothesis," McCord et al. 1998aMcCord et al. , 1999aPrieto et al. 1999), or (2) hydrated sulfuric acid with traces of radiolytic sulfur impurities (the "acid hypothesis," Carlson et al. 1999). Dalton and Clark (1999) have emphasized ambiguities in attributing the observed distorted water bands to specific hydrates, and they have pointed out slight spectral mismatches between the Europa nonice material and proposed substances. These mismatches likely are due to differences between the conditions or grain size pertaining to lab data and those of Europa. ...
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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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