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SHOCK WAVES THROUGH THE SOLAR NEBULA 
COULD EXPLAIN WATER-RICH SPACE ROCKS
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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.

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Contact information
Fred J. Ciesla
520-621-1611  fciesla{at}lpl.arizona.edu
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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.

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