VENUS, WATER AND TREMOLITE

We determined the decomposition rate of tremolite and fluorine-rich tremolite by isothermally heating powdered samples in dry flowing CO₂ or N₂ gas at ambient pressure. Tremolite decomposes into two pyroxene solid solutions, silica and water vapor. The decomposition rate was measured from the weight loss. Our latest results, obtained at lower temperatures for longer times, confirm our earlier experiments showing that tremolite will last for billions of years at current Venusian conditions. We also find that F-rich tremolite takes longer to decompose than F-free tremolite and that fluorine is depleted in the reacted samples. The kinetic stability of tremolite was surprising because thermodynamics predicts that it is unstable and should decompose at Venus' surface temperature and atmospheric water vapor pressure (740 Kelvin and 0.003 bar). An ongoing experiment contains a tremolite sample at 740 K that has not decomposed after three years.

Our results are important for understanding the history of water on Venus. Water may have been more abundant on Venus in the past, as indicated by the high D/H ratio in atmospheric water vapor. The current bone-dry conditions and blazing heat discouraged any hope of finding fossil hydrous minerals within or on the surface. However, our work suggests that if Venus was once wet amphiboles should be present today. The fossil hydrous minerals are potentially detectable by several methods (e.g., XRD, imaging IR spectroscopy) that can be used on landers, penetrators, and/or balloons.

Acknowledgments: This work was supported by the NASA Planetary Atmospheres Program.