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Washington University in St. Louis
Researchers
at Washington University in St. Louis, studying hydrous mineral
decomposition rates at extreme temperatures, have concluded that
hot and dry Venus may have been a wet planet in the past, like
Earth and ancient Mars.
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.
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