Study Suggests Venus Could Have Been Wet Planet

The new evidence suggesting a wetter Venusian history comes from a series of experiments documenting the chemical stability of tremolite for several billion years at temperatures similar to that of Venus' surface, about 740 Kelvin or roughly 870 degrees Fahrenheit (F).

Tremolite is a mineral that forms in the presence of water. If tremolite or some other hydrous mineral can be detected on the surface of Venus, then it can be concluded that Earth's once-wet neighbor lost its water over time, putting to rest an enduring question in planetary science.

Graduate student Natasha M. Johnson and Professor Bruce Fegley, Jr., Ph.D., of the Planetary Chemistry Laboratory in Earth and Planetary Sciences at Washington University, reported their findings in the paper "Water on Venus: New Insights from Tremolite Decomposition," Icarus, 146, pp. 301-306, July, 2000.

"Ours is the first study that investigates hydrous mineral decomposition rates with applications to Venus," says Johnson. "We have shown that tremolite can withstand extreme temperatures and remain intact for billions of years. If we can go to Venus and find tremolite, or some other hydrous mineral, then we would have proof that Venus had water in its past."

Indirect evidence that Venus had water in the past is found in its high deuterium/hydrogen (D/H) ratios. If the high D/H ratios are the result of lighter hydrogen (deuterium is a heavier form of hydrogen) escaping Venus' atmosphere to space, then it is possible that Venus had water in the past. But the D/H ratio of Venus varies relative to that of Earth, and comets and meteorites can also have high D/H ratios, so other types of evidence of water are needed.

Johnson and Fegley's research on the decomposition rate of tremolite shows that the evidence is in the rocks. "We want to know if it is worth our time to go to Venus and look for minerals that have water in them," says Johnson. "When you go backpacking, you want to know where you are going and what you need to carry. These experiments are laying the foundation, and saying, "Hey, should we, or should we not, bring a parka?" Should we be looking for hydrous minerals on Venus or is it a waste of time?"

Johnson and Fegley conducted over 200 experiments, heating samples of tremolite in laboratory furnaces at temperatures of up to 1240 Kelvin (about 1770 degrees Fahrenheit) for as long as 20 months, periodically weighing them to document the amount and rate of decomposition.

Tremolite, an amphibole, and other hydrous minerals contain OH (hydroxyl groups as part of a lattice holding these minerals together. Amphiboles are formed when lava and magma interact with water. In the case of tremolite, it is a metamorphic mineral generally found in dolomitic-type limestone. Amphiboles are thermodynamically unstable and according to theory should decompose rather quickly at high temperatures.

But Johnson and Fegley's experiments indicate that tremolite is much more stable than previously thought, and would take about 4 billion years to decompose by half in conditions similar to Venus' surface. "Diamonds are a good analogy for what is happening with tremolite," says Johnson. "Diamonds are unstable at the surface of the Earth; graphite is the stable form. But you don't see diamonds popping into little chunks of graphite on people's fingers."

If tremolite and other amphiboles formed on Venus at some time in the past, they should be detectable using infrared reflectance spectroscopy and other current technology.

The researchers also are measuring decomposition properties of other hydrous minerals. Surprisingly little is known about these minerals with the exception of those with commercial purposes like asbestos and other insulators. "This research could give us some idea about the formation of our solar system, and has applications on Earth for investigating metamorphic regimes or subduction zones," says Johnson.