Spectral comparison of heavily hydrated salts with disrupted terrains on Europa

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• 作者：J.B. Dalton ; O. Prieto-Ballesteros ; J.S. Kargel ; C.S. Jamieson ; J. Jolivet and R. Quinn
• 关键词：Europa ; Experimental techniques ; Surfaces ; Satellite ; Spectroscopy
• 刊名：Icarus
• 出版年：2005
• 出版时间：2005
• 年：2005
• 卷：177
• 期：2
• 页码：472-490
• 全文大小：470 K

文摘

Hydrated magnesium sulfate salts have been proposed as major components of the disrupted, reddish terrains on the surface of Europa. This is based on near-infrared reflectance spectra which contain distorted and asymmetric water absorption features typical of moderately hydrated materials such as hexahydrite (MgSO₄6H₂O) and epsomite (MgSO₄7H₂O). Hydrated magnesium sulfates having many waters of hydration could produce improved spectral matches. Here we present cryogenic laboratory spectra of highly hydrated sulfur-bearing salts, including hexahydrite, epsomite, bloedite (Na₂Mg(SO₄)₂4H₂O), mirabilite (Na₂SO₄10H₂O), sodium sulfide nonahydrate (Na₂S9H₂O), supersaturated MgSO₄, NaHCO₃, and Na₂SO₄ brines, and magnesium sulfate dodecahydrate (MgSO₄12H₂O). All have been measured under conditions of pressure and temperature appropriate to the surface environment of Europa. Novel methods for preparation, verification and analysis of MgSO₄12H₂O, which is not stable at standard temperature and pressure (STP), are described. At 100 K, all of these materials exhibit distorted and asymmetric absorption features similar to those in the Europa observations, as well as several weaker, narrow absorptions having widths ranging from 15 to 80 nm. While the agreement with Galileo NIMS observations of dark terrains on Europa is indeed better for highly hydrated salts than for salts of lower hydration states, we conclude that none of these materials alone can account for all of the observed spectral character. As previously suggested, Europa's reddish material appears to be a complex mixture of sulfate hydrates and other materials.