[meteorite-list] Mars Magmas Once Contained A Lot Of Water, Researchers Report
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:05 2004
Massachusetts Institute of Technology
Deborah Halber, MIT News Office
(617) 258-9276, dhalber_at_mit.edu
JANUARY 24, 2001
Mars magmas once contained a lot of water, researchers from MIT and U. of
Finding suggests that volcanos helped bring water to the planet's surface
millions of years ago
CAMBRIDGE, Mass. -- Evidence from a Martian volcanic rock indicates that
Mars magmas contained significant amounts of water before eruption on the
planet's surface, researchers from the Massachusetts Institute of Technology,
the University of Tennessee and other institutions report in the Jan. 25
issue of Nature.
Scientists say that channels on Mars's surface may have been carved by
flowing water and an ancient ocean may have existed there, but little is
known about the source of the water. One possible source is volcanic
degassing, in which water vapor is produced by magma spewing from volcanos,
but the Martian rocks that have reached Earth as meteorites have notoriously
low water content.
This study shows that before the molten rock that crystallized to form
Martian meteorites was erupted on the surface of the planet, it contained
as much as 2 percent dissolved water.
When magma reaches the planet's surface, the solubility of water in the
molten liquid decreases and the water forms vapor bubbles and escapes as gas.
The process is similar to the release of gas bubbles that occurs when you
open a can of soda.
Although this doesn't explain how water got into Mars in the first place, it
does show that water on the red planet once cycled through the deep interior
as well as existed on the surface, as similar processes have cycled water
through the Earth's interior throughout geologic history.
A VISITOR FROM MARS
Timothy L. Grove, professor of Earth, Atmospheric and Planetary Sciences at
MIT, and University of Tennessee geologist Harry Y. McSween Jr. analyzed the
Mars meteorite Shergotty to provide an estimate of the water that was present
in Mars magmas prior to their eruption on the surface.
Shergotty, a meteorite weighing around 5 kilograms was discovered in India
in 1865. It is one of a handful of proven Mars meteorites that landed on
Earth. It is relatively young -- around 175 million years old -- and may
have originated in the volcanic Tharsis region of the red planet.
Its measured water content is only around 130-350 parts per million. But by
exploring the amount of water that would be necessary for its pyroxenes --
its earliest crystallizing minerals -- to form, the researchers have
determined that at one time, Shergotty magma contained around 2 percent
water. They also have detected the presence of elements that indicate the
growth of the pyroxenes at high water contents.
This has important implications for the origin of the water that was present
on the surface of the planet during the past. This new information points to
erupting volcanos as a possible mechanism for getting water to Mars's surface.
SQUEEZING HYDROGEN INTO ROCKS
In the interior of Mars, hot magma is generated at great depth. It then
ascends into the shallower, colder outer portions of the Martian interior,
where it encounters cooler rock that contains hydrogen-bearing minerals.
These minerals decompose when heated by the magma and the hydrogen is released
and dissolves in the magma.
The magma continues its ascent to the surface of the planet. When it reaches
very shallow, near-surface conditions in the crust, the magma erupts and its
water is released in the form of vapor.
The magma holds the water-creating hydrogen as the rock circulates underneath
the crust. It undergoes changes as it moves from areas of enormous heat and
pressure to cooler areas nearer the surface. When it finally erupts through
a volcano, the magma releases its water in the form of vapor.
Grove recreates Mars and moon rocks in his laboratory for these studies. By
subjecting synthetic rocks to conditions of high temperature and pressure,
he can tell how much water was contained in magma at the time that its
crystals were formed. "What my experiment can do is estimate how much water
was involved in the process that led to the formation of Mars meteorites.
The only way you can reproduce the unique chemical composition of these
minerals is to have water present," he said.
Other authors on the Nature paper include McSween's graduate student,
Rachel C. F. Lentz; Lee R. Riciputi of the chemical and analytical sciences
division of Oak Ridge National Laboratory; Jeffrey G. Ryan, a geologist at
the University of South Florida; and Jesse C. Dann and Astrid H. Holzheid
of MIT's Department of Earth, Atmospheric and Planetary Sciences.
This work was partly supported by NASA.
Received on Thu 25 Jan 2001 02:44:11 PM PST