Evidence from a Martian volcanic rock indicates that Mars magmas contained significant amounts of water before eruption on the planet's surface, researchers from MIT, the University of Tennessee and other institutions report in the January 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 onto 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
MIT Professor of Earth, Atmospheric and Planetary Sciences Timothy L. Grove 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.
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.
Professor 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 Tennessee 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 Visiting Scientist Jesse C. Dann and former postdoctoral associate Astrid H. Holzheid of MIT.
This work was partly supported by NASA.
A version of this article appeared in MIT Tech Talk on January 31, 2001.
