FREZCHEM: An Aqueous Geochemical Model for Mars

Affiliation(s) PI Project period Funded by
DEES Marion, Giles M 01/01/2011 - 12/31/2012 National Aeronautics & Space Administration

Project Description

The FREZCHEM model is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions using the Pitzer approach for the temperature range from < -70 to 25?C and the total pressure range from 1 to 1000 bars. The model is currently parameterized for the Na-K-Mg-Ca-Fe(II)-Fe(III)-Al-H-Cl-ClO4-Br-SO4-NO3-OH-HCO3-CO3-CO2-O2-CH4-Si-H2O system and includes 101 solid phases including ice, 15 chloride minerals, six perchlorate salts, 30 sulfate minerals, 15 bicarbonate/carbonate minerals, five solid-phase acids, three nitrate minerals, six acid-salts, two gas hydrates, 13 iron/aluminum oxide/hydroxides, three silicon minerals, and two bromide sinks. Physicochemical properties calculated by the model include: activity coefficients and activities of solutes and water, osmotic coefficient, ionic strength, density of aqueous solutions and ice, quantity of water, pH, equilibrium with respect to solid phases and atmospheric gases, temperature and pressure dependencies of chemical reactions, and the eutectic temperature of aqueous solutions. All of these properties are important in quantifying geochemical equilibria. In addition, some properties have a direct bearing on potential limiting factors for life such as (1) the activity of water and the osmotic coefficient, which are measures of salinity, (2) pH, which is a measure of acidity, (3) the presence or absence of water, (4) temperature, and (5) pressure. This model has been widely used to examine geochemical processes on Earth, Mars, and Europa. The model is uniquely structured because it can deal with multiple chemistries at subzero temperatures, which are highly relevant for Mars. I have made the FORTRAN code for this model freely available to interested scientists. Nevertheless, the continual updating, dissemination, and archiving of the model is challenging. Furthermore, early versions of the model had convergence problems. To solve these problems, we were NASA-MFR funded to (1) develop a more user-friendly FORTRAN code, (2) implement an Internet-accessible version of FREZCHEM, and (3) develop an archive for various model versions. This project has been completed (peruse http://frezchem.dri.edu), except for a problem with gas hydrate chemistry. The objectives of this new proposal are to (1) update the Internet version from 56 to 101 solid phases, which will involve adding Fe(III), Al, Si, ClO4, Br, and gas hydrate chemistries, and (2) update model (FORTRAN code) archives. This effort will make the model readily and easily available, even to casual users, now and in the future. As a tool for Martian applications, it is important to keep both FORTRAN codes and the Internet model up to date and available for all world-wide potential users of FREZCHEM. Today, such a user-friendly model would be especially helpful in interpreting the exciting new data from the Mars Phoenix mission, where perchlorate/carbonate chemistries are especially important.