Biogeochemical Applications to Solar System Bodies

In this chapter, we examine biogeochemical applications of the FREZCHEM model to Earth, Mars, and Europa, where cold aqueous environments played and continue to play a critical role in defining surficial geochemistry. Interpretations include the potential for life in these environments. These simulations cover applications to seawaters, saline lakes, regoliths, aerosols, and ice cores and covers. These examples are the proverbial tip of the iceberg in terms of the potential of this model to describe cold aqueous geochemical processes. At the end of the chapter, we discuss application limitations, cases where the underlying thermodynamic assumptions are at variance with real-world situations. 

One of the great virtues of the FREZCHEM model is its ability to examine complex chemistries. The number of independent components for the systems examined in this chapter range from four to eight. Earth seawater consisting of Na+, K+, Mg2+, Ca2+, Cl-, SO4-, and alkalinity has seven independent components (six salts and water). The most complex system evaluated is the snowball Earth seawater (eight independent components), which in addition to the above seven components also includes Fe2+. This ability to cope with complexity makes models like FREZCHEM more realistic in describing natural systems than simpler binary and ternary diagrams we demonstrate this point with data from Don Juan Pond, the most saline body of water on Earth. 

Salts seem to be nearly as widespread as ice (though not as abundant) in the Solar System, being present on nearly every object where ice has also been observed and where indications of a watery past are in evidence (Enceladus is the key exception so far). There are very few instances (Venus is the most notable one) where salts occur on worlds where H2O is absent or nearly absent. This is a striking correlation, given the terrestrial situation of most evaporites, which are mostly associated with hot and arid areas of Earth. Even so, notable instances of ice-associated evaporites also occur on Earth. In detail, each world and the paragenesis of its salts tells a different story. The key point here is that in this Solar System, salts commonly are associated directly with ice. Hence, phase equilibria generally are described by FREZCHEM, even if on Earth some of the most important evaporite deposits were not formed in the presence of ice or icy-cold conditions. 

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