Europa’s ridged plains and smooth low albedo plains: Distinctive compositions and compositional gradients at the leading side–trailing side boundary

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• 作者：James H. Shirley ; James B. Dalton III ; Louise M. Prockter ; Lucas W. Kamp
• 关键词：Europa ; Satellites ; Composition ; Spectroscopy ; Jupiter ; Satellites
• 刊名：Icarus
• 出版年：2010
• 出版时间：November 2010
• 年：2010
• 卷：210
• 期：1
• 页码：358-384
• 全文大小：2697 K

文摘

This investigation uses linear mixture modeling employing cryogenic laboratory reference spectra to estimate surface compositions and water ice grain sizes of Europa’s ridged plains and smooth low albedo plains. Near-infrared spectra for 23 exposures of ridged plains materials are analyzed along with 11 spectra representing low albedo plains. Modeling indicates that these geologic units differ both in the relative abundance of non-ice hydrated species and in the abundance and grain sizes of water ice. The background ridged plains in our study area appear to consist predominantly of water ice (46 % ) with approximately equal amounts (on average) of hydrated sulfuric acid (27 % ) and hydrated salts (27 % ). The solutions for the smooth low albedo plains are dominated by hydrated salts (62 % ), with a relatively low mean abundance of water ice (10 % ), and an abundance of hydrated sulfuric acid similar to that found in ridged plains (27 % ). The model yields larger water ice grain sizes (100 μm versus 50–75 μm) in the ridged plains. The 1.5-μm water ice absorption band minimum is found at shorter wavelengths in the low albedo plains deposits than in the ridged plains (1.498 ± .003 μm versus 1.504 ± .001 μm). The 2.0-μm band minimum in the low albedo plains exhibits a somewhat larger blueshift (1.964 ± .006 μm versus 1.983 ± .006 μm for the ridged plains).

The study area spans longitudes from 168° to 185°W, which includes Europa’s leading side–trailing side boundary. A well-defined spatial gradient of sulfuric acid hydrate abundance is found for both geologic units, with concentrations increasing in the direction of the trailing side apex. We associate this distribution with the exogenic effects of magnetospheric charged particle bombardment and associated chemical processing of surface materials (the radiolytic sulfur cycle). However, one family of low albedo plains exposures exhibits sulfuric acid hydrate abundances up to 33 % lower than found for adjacent exposures, suggesting that these materials have undergone less processing, thus implying that these deposits may have been emplaced more recently.

Modeling identifies high abundances (to 30 % ) of magnesium sulfate brines in the low albedo plains exposures. Our investigation marks the first spectroscopic identification of MgSO₄ brine on Europa. We also find significantly higher abundances of sodium-bearing species (bloedite and mirabilite) in the low albedo plains. The results illuminate the role of radiolytic processes in modifying the surface composition of Europa, and may provide new constraints for models of the composition of Europa’s putative subsurface ocean.