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Recent observations from the Galileo satellite indicate that three of the Jupiter moons, Europa, Gandymede, and Callisto, may have subsurface oceans. Because water is an essential ingredient of life, the possible existence of the subsurface ocean and the nature of its composition is of great interest to planetary scientists and astrobiologists. Of the three moons, Europa has attracted the most attention. Data from the Galileo near-infrared mapping spectrometer (NIMS) indicate the possibility of hydrated salts on Europa's surface. The combination of a subterranean ocean, internal heat from tidal flexing, salt, and possible prebiotic materials from comet and meteorite impact may provide a possible condition for the formation and evolution of life. So far, the existence of both a subsurface ocean and hydrated salts is based on indirect evidence. The analysis of the NIMS spectra, for example, is based on the change of the water absorption spectra in hydrated salts in the 1.0 to 2.5-micron (um) region. The assignment is by no means unique.
To search for a frequency range that can pinpoint the presence of salt more defini-tively, large-scale quantum chemistry calculations of hydrated magnesium sulfate, MgSO4-nH2O, n = 1-3, were made using second-order many-body perturbation theory and with a basis set of Gaussian functions at the triple zeta + polarization level. The configuration of the lowest electronic states of MgSO4-2H2O and MgSO4-3H2O are shown in figures 1 and 2. The wavelengths and intensities of infrared absorption for the SO2 stretch modes are tabulated in Table 1.
Table 1. Wavelength and intensity of the SO2 stretch modes for MgSO4-nH2O, n = 1-3.
| Molecule | Vibrational Mode | Absorption wavelength,
um
¥ | Intensity,
Db2/A2 amu
¥ |
| MgSO4 | Asymmetric SO2 stretch | 7.424 | 7.01 |
| Symmetric SO2 stretch | 8.569 | 6.33
|
MgSO4
H2O | Free SO2 stretch | 7.502 | 8.90 |
| Bonded SO2 stretch | 9.911 | 10.95
|
MgSO4
2H2O | Free SO2 stretch | 7.576 | 9.00 |
| Bonded SO2 stretch | 9.804 | 10.80
|
MgSO4
3H2O | Free SO2 stretch | 7.593 | 8.48 |
| Bonded SO2 stretch | 9.372 | 9.00 |
The SO2 stretch modes occur at longer wavelengths than the infrared absorption modes of water. They can serve as a unique identifier for the presence of magnesium sulfate salts. Interesting results were obtained on the infrared spectra of water bonded to magnesium sulfate. These results can be used to design a better detector for possible salts on the surface of Europa.
Point of Contact: W. M. Huo
(650) 604-6161
whuo@mail.arc.nasa.gov
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Fig. 1. Configuration of the lowest electronic state of MgSO4-2H2O.
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Fig. 2. Configuration of the lowest electronic state of MgSO4-3H2O.
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