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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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