Spectral Behavior of Hydrated Sulfate Salts: Implications for Europa Mission Spectrometer Design

Abstract

Remote sensing of the surface of Europa with near-infrared instruments has suggested the presence of hydrated materials, including sulfate salts. Attention has been focused on these salts for the information they might yield regarding the evolution of a putative interior ocean, and the evaluation of its astrobiological potential. These materials exhibit distinct infrared absorption features due to bound water. The interactions of this water with the host molecules lead to fine structure that can be used to discriminate among these materials on the basis of their spectral behavior. This fine structure is even more pronounced at the low temperatures prevalent on icy satellites. Examination of hydrated sulfate salt spectra measured under cryogenic temperature conditions provides realistic constraints for future remote-sensing missions to Europa. In particular, it suggests that a spectrometer system capable of 2-5 nm spectral resolution or better, with a spatial resolution approaching 100 m, would be able to differentiate among proposed hydrated surface materials, if present, and constrain their distributions across the surface. Such information would provide valuable insights into the evolutionary history of Europa.

Chemical Composition of Icy Satellite Surfaces

“…This is because the absorptions arise from vibrations of the solvated water molecules, not from the host sulfate complex. However the electromagnetic influence of the host constrains the configuration of the whole molecule, affecting the vibrational frequencies and producing detailed spectral structure that can be highly diagnostic of the host compound and the level of hydration (Dalton 2003). Note that no single compound yet studied has perfectly matched the Europa observations when considered by itself.…”

Linear mixture modeling of Europa's non‐ice material based on cryogenic laboratory spectroscopy

“…Spectra of epsomite (MgSO 4 Á 7H 2 O), hexahydrite (MgSO 4 Á 6H 2 O) and bloedite (Na 2 Mg(SO 4 ) 2 Á 4H 2 O were measured at 100, 120, and 120 K, respectively [Dalton, 2000[Dalton, , 2003. The MgSO 4 , NaHCO 3 , and Na 2 SO 4 brines were measured at 100 K and reported by Dalton et al [2005].…”

“…The spectrum of water ice is highly temperaturedependent [Pauling, 1935;Ockman, 1957;Warren, 1984;Hudgins et al, 1993;Grundy and Schmitt, 1998] and so consequently are the spectra of many water-bearing solids [Hunt and Ashley, 1979]. This behavior is pronounced in many of the hydrated salts [Dalton, 2000[Dalton, , 2003McCord et al, 2001;Dalton et al, 2005]. At Europa surface temperatures of 80 -130 K [Spencer et al, 1999] spectra of these materials appear different than at room temperature.…”

Extraterrestrial hydrogeology

“…These models all have sul-fate as the most common anion. Remote sensing of the surface of Europa with near-infrared instruments revealed the presence of hydrated materials, including sulfate salts (Dalton 2003), which appears to support this notion. However, the sulfates could have originated from other sulfur compounds that were later oxidized by radiation.…”