How Did Water Get to Earth?
23 Jan 2003
(Source: University of Arizona)
SHOCK WAVES THROUGH THE SOLAR NEBULA COULD EXPLAIN WATER-RICH SPACE ROCKS
From Lori Stiles, UA News Services, 520-621-1877
January 23, 2003
Shock waves through icy parts of the solar nebula could well be the mechanism that enriched meteorites with water -- water that some believe provided an otherwise dry Earth with oceans, according to a new study published in the current issue of Science (Jan. 24).
Scientists have long debated how "chondrules" might have formed. Chondrules are millimeter-sized blobs of once-melted minerals found within chondritic meteorites, which are thought to be the oldest objects in the solar system. In some of these meteorites, chondrules are rimmed by fine silicate dust particles that have reacted with water.
Researchers at first speculated that chondrules and their water-rich rims formed when water molecules in the solar nebula collided with dust. But a 1987 study dispelled that idea, because the time it would take for the minerals to form in this manner would be longer than the lifetime of the solar nebula.
Planetary scientists at the University of Arizona and University of Hawaii now report that chondrule-forming shock waves in icy regions of the nebula could have produced conditions that allowed rapid mineral hydration. Fred J. Ciesla, Dante S. Lauretta and Lon L. Hood of the UA and Barbara Cohen of the UH collaborated in the study.
Fred J. Ciesla
Lauretta and Cohen speculated years ago that a big energetic event, like a shock wave, might produce enough energy to vaporize ice particles and briefly create conditions that made such quick hydration reactions possible.
Ciesla modeled the scenario of what happened to particles of silicate and ice during a shock wave event.
"And what happens is, the ice particles vaporize in this very energetic event, producing high water vapor pressure. During this brief period of increased water pressure, the hydration reaction occurs much faster than previously predicted," Ciesla said. "During this brief period, the chondrules melt and the rims form in the same event."
Gas slows as it passes through a shock front, increasing in temperature and density. But solid particles entrained in the gas continue through the shock wave at high velocity. "The solid particles heat up because they are speeding through the slower-moving gas. And just as a meteor is heated up and burns when it enters Earth's atmosphere, particles are heated when they collide with the gas molecules. Gas both heats and slows the chondrules, so they melt and begin to cool. The water vapor then reacts with the dust to form these hydrated silicates, and the chondrules accrete these silicates to form their rims."
"An interesting characteristic of these particular meteorites is that they contain a lot of water, and the deuterium-to-hydrogen ratios in that water matches the ratios we find in Earth's water," Ciesla noted.
Why Earth has water is a mystery, for "especially early on in the solar nebula, the area where the Earth formed was too hot for water to incorporate into a solid body," Ciesla said. Meteorites may have delivered at least part of Earth's water, although that remains open to debate, he added.
The scenario also suggests how so much organic material has survived in the carbonaceous chondrite meteorites. If water reacted with the fine dust in the solar nebula as the new research suggests, temperatures in the meteorites would have remained low enough for organic molecules to survive and be delivered, along with water, to Earth.
Although the idea that shock waves formed the hydrated rock and chondrules found in the most primitive meteorites stands up to quantitative analysis, scientists are still speculating about where the shock waves come from, and it's a topic Ciesla hopes to address in this doctoral thesis.
UA planetary scientist Lon Hood, one of the authors on the Science paper, originally theorized that as Jupiter was forming, it excited the orbits of the many "planetismals," or planet building-blocks, in the region that became the present day asteroid belt so that they were propelled through the gas in the solar nebula at speeds greater than the speed of sound, creating shock waves. Ciesla has begun testing that idea.
Other ideas on the origin of shock waves also involve Jupiter is some way, he said.
IMAGE: A high-resolution electron image of a chondrule surrounded by a rim of fine-grained dust that has reacted with water can be downloaded at http://www.lpl.arizona.edu/~fciesla/figs/fig1.tif The image, courtesy of Dante Lauretta, appears in the Science article.