Abstract
Tirez Lake (La Mancha, central Spain) is proposed as a terrestrial
analogue of Europa's ocean. The proposal is based on the comparison
of the hydrogeochemistry of Tirez Lake with the geochemical features
of the alteration mineralogy of meteoritic precursors and with Galileo's
Near Infrared Mapping Spectrometer data on Europa's surface. To validate
the astrobiological potential of Tirez Lake as an analog of Europa,
different hydrogeochemical, mineral, and microbial analyses were
performed. Experimental and theoretical modeling helped to understand
the crystallization pathways that may occur in Europa's crust. Calculations
about the oxidation state of the hypothetical Europan ocean were
estimated to support the sulfate-rich neutral liquid model as the
origin of Europa's observed hydrated minerals and to facilitate their
comparison with Tirez's hydrogeochemistry. Hydrogeochemical and mineralogical
analyses showed that Tirez waters corresponded to Mg-Na-SO4-Cl brines
with epsomite, hexahydrite, and halite as end members. A preliminary
microbial ecology characterization identified two different microbial
domains: a photosynthetically sustained community represented by
planktonic/benthonic forms and microbial mat communities, and a subsurficial
anaerobic realm in which chemolithotrophy predominates. Fluorescence
in situ hybridization has been used to characterize the prokaryotic
diversity of the system. The subsurficial community seemed to be
dominated by sulfate-reducing bacteria and methanogens. Frozen Tirez
brines were analyzed by Fourier-transform infrared techniques providing
spectra similar to those reported previously using pure components
and to the Galileo spectral data. Calorimetric measurements of Tirez
brines showed pathways and phase metastability for magnesium sulfate
and sodium chloride crystallization that may aid in understanding
the processes involved in the formation of Europa's icy crust. The
use of fluorescence hybridization techniques for microbial detection
and characterization in hypersaline environments makes this methodology
strongly advisable for future Europa astrobiological missions.
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