13 June 2000
Water-Bearing Salt Crystals From Dawn Of Solar System
by Kate Melville
Brine-pocketed salt crystals within the "Zag" meteorite may be among the oldest materials found in the solar system, a U.K. research team has found. This surprisingly old age could spur scientists to speed up the prevailing scenario of the solar system's evolution, and opens the possibility that hospitable conditions for life might have arisen earlier than previously thought. The researchers report their findings in the 9 June issue of Science.
Using radioisotope dating, scientists at the University of Manchester and the Natural History Museum in London determined that the salt crystals probably formed within about two million years of the solar system's birth. If this age is correct, it means that the dust, gas, and ice swirling around the newborn sun clumped together into rocky fragments far more quickly than researchers have assumed. These fragments were the parent bodies for primitive meteorites like Zag and the essential building blocks for asteroids and planets.
In the scenario proposed by first author James Whitby and his colleagues, Zag's parent body accreted rapidly into a rocky mass containing water and radioactive isotopes. The isotopes' decay generated enough heat to melt any ice within the rock matrix, and soon caused the liquid to evaporate altogether. The salt crystals, (mainly sodium chloride, or "halite,") precipitated during the evaporation process, similar to the way halite forms when sea water evaporates on Earth.
The Zag meteorite, which fell in Morocco in 1998, was the second meteorite found carrying halite crystals. Like those in the Monahans meteorite (see Science, 27 August, 1999, p. 1364 and 1377), these crystals contained microscopic inclusions of water, the key ingredient for life. Both finds have raised hopes of learning more about the possibility that life might have evolved elsewhere in the solar system. To do so, researchers would first have to determine when water existed in these parent bodies and for how long.
Previous studies of other meteorite minerals could only place the time of liquid water within 100 million years after the solar system's formation, explains Ulrich Ott, of the Max-Planck-Institut f�r Chemie in Mainz, Germany, in a Perspective article that accompanies the research paper. Rubidium dating of the halite in Monahans had suggested that the crystals might be some of the oldest materials in the solar system, but this dating method is less precise than the one used by Whitby and his colleagues.
"Those results showed that this was interesting halite. Immediately, our next question was, 'how old is it?'" Whitby said.
A second crucial task was to make sure that the halite hadn't precipitated from water on Earth that contaminated the meteorite after it landed.
To tackle both these questions, Whitby and co-authors Ray Burgess, Grenville Turner and Jamie Gilmour, of the University of Manchester, and John Bridges, of the Natural History Museum, analyzed xenon, iodine, and argon isotopes extracted from a minute sliver of a Zag halite crystal. They found a surprisingly large amount of xenon-129, which forms when iodine-129 decays. Iodine-129 was present in the early solar system but is not found on Earth.
"I popped the halite in the machine and got this amazing peak showing an abundance of xenon-129. That told us immediately it wasn't terrestrial material. I hadn't really expected that, so it was quite stunning, really," said Whitby.
Because scientists know the rate at which iodine-129 decays into xenon-129, Whitby and his co-authors could then calculate the age of the halite. They estimate that the crystals formed about two million years after the birth of the solar system 4.57 billion years ago, suggesting that liquid water departed from its parent body soon after it had appeared.
"The results from the I-Xe dating by Whitby et al. are particularly astonishing," writes Ott in the Perspective article.
Until the discovery of halite in Monahans and Zag, the oldest materials in the solar system were thought to be chondrules, glassy spheres that make up much of the mass in primitive meteorites. Although the origins of these particles are still under debate, many scientists believe that chrondrule formation continued for about 5 million years after the solar system's birth.
From their analysis of the crystals and the rocky matrix around them, the research team proposes that the halite grew very quickly on a newly formed asteroid that had just formed by the collision of smaller particles. About 300 million years later, another large impact smashed the loose fragments together into a more solid conglomerate, a piece of which became the Zag meteorite.
The presence of liquid water also has important implications for understanding the geology of moons and planets with large amounts of heat in their interiors. Volcanic activity is closely tied with the availability of water, which plays a major role in the formation of magma.