[meteorite-list] Cosmochemist Discovers Potential Solution to Meteorite Mystery

[Ron Baalke]

http://news.uchicago.edu/article/2013/07/08/cosmochemist-discovers-potential-solution-meteorite-mystery

Cosmochemist discovers potential solution to meteorite mystery 

Chondrules may have formed from high-pressure collisions in early solar system

By Steve Koppes
University of Chicago
July 8, 2013

A normally staid University of Chicago scientist has stunned many of his 
colleagues with his radical solution to a 135-year-old mystery in cosmochemistry. 
"I'm a fairly sober guy. People didn't know what to think all of a sudden," 
said Lawrence Grossman, professor in geophysical sciences.

At issue is how numerous small, glassy spherules had become embedded within 
specimens of the largest class of meteorites - the chondrites. British 
mineralogist Henry Sorby first described these spherules, called chondrules, 
in 1877. Sorby suggested that they might be "droplets of fiery rain" which 
somehow condensed out of the cloud of gas and dust that formed the solar 
system 4.5 billion years ago.

Researchers have continued to regard chondrules as liquid droplets that 
had been floating in space before becoming quickly cooled, but how did 
the liquid form? "There's a lot of data that have been puzzling to people," 
Grossman said.

Grossman's research reconstructs the sequence of minerals that condensed 
from the solar nebula, the primordial gas cloud that eventually formed 
the sun and planets. He has concluded that a condensation process cannot 
account for chondrules. His favorite theory involves collisions between 
planetesimals, bodies that gravitationally coalesced early in the history 
of the solar system. "That's what my colleagues found so shocking, because 
they had considered the idea so 'kooky,'" he said.

Cosmochemists know for sure that many types of chondrules, and probably 
all of them, had solid precursors. "The idea is that chondrules formed 
by melting these pre-existing solids," Grossman said.

One problem concerns the processes needed to obtain the high, post-condensation 
temperatures necessary to heat the previously condensed solid silicates 
into chondrule droplets. Various astonishing but unsubstantiated origin 
theories have emerged. Maybe collisions between dust particles in the 
evolving solar system heated and melted the grains into droplets. Or maybe 
they formed in strikes of cosmic lightning bolts, or condensed in the 
atmosphere of a newly forming Jupiter.

Another problem is that chondrules contain iron oxide. In the solar nebula, 
silicates like olivine condensed from gaseous magnesium and silicon at 
very high temperatures. Only when iron is oxidized can it enter the crystal 
structures of magnesium silicates. Oxidized iron forms at very low temperatures 
in the solar nebula, however, only after silicates like olivine had already 
condensed at temperatures 1,000 degrees higher.

At the temperature at which iron becomes oxidized in the solar nebula, 
though, it diffuses too slowly into the previously formed magnesium silicates, 
such as olivine, to give the iron concentrations seen in the olivine of 
chondrules. What process, then, could have produced chondrules that formed 
by melting pre-existing solids and contain iron oxide-bearing olivine?

"Impacts on icy planetesimals could have generated rapidly heated, relatively 
high-pressure, water-rich vapor plumes containing high concentrations 
of dust and droplets, environments favorable for formation of chondrules," 
Grossman said. Grossman and his UChicago co-author, research scientist 
Alexei Fedkin, published their findings in the July issue of Geochimica 
et Cosmochimica Acta.

Grossman and Fedkin worked out the mineralogical calculations, following 
up earlier work done in collaboration with Fred Ciesla, associate professor 
in geophysical sciences, and Steven Simon, senior scientist in geophysical 
sciences. To verify the physics, Grossman is collaborating with Jay Melosh, 
University Distinguished Professor of Earth & Atmospheric Sciences at 
Purdue University, who will run additional computer simulations to see 
if he can recreate chondrule-forming conditions in the aftermath of planetesimal 
collisions.

"I think we can do it," Melosh said.

Longstanding objections

Grossman and Melosh are well-versed in the longstanding objections to 
an impact origin for chondrules. "I've used many of those arguments myself," 
Melosh said.

Grossman re-evaluated the theory after Conel Alexander at the Carnegie 
Institution of Washington and three of his colleagues supplied a missing 
piece of the puzzle. They discovered a tiny pinch of sodium - a component 
of ordinary table salt - in the cores of the olivine crystals embedded 
within the chondrules.

When olivine crystallizes from a liquid of chondrule composition at temperatures 
of approximately 2,000 degrees Kelvin (3,140 degrees Fahrenheit), most 
sodium remains in the liquid if it doesn't evaporate entirely. But despite 
the extreme volatility of sodium, enough of it stayed in the liquid to 
be recorded in the olivine, a consequence of the evaporation suppression 
exerted by either high pressure or high dust concentration. According 
to Alexander and his colleagues, no more than 10 percent of the sodium 
ever evaporated from the solidifying chondrules.

Grossman and his colleagues have calculated the conditions required to 
prevent any greater degree of evaporation. They plotted their calculation 
in terms of total pressure and dust enrichment in the solar nebula of 
gas and dust from which some components of the chondrites formed. "You 
can't do it in the solar nebula," Grossman explained. That's what led 
him to planetesimal impacts. "That's where you get high dust enrichments. 
That's where you can generate high pressures."

When the temperature of the solar nebula reached 1,800 degrees Kelvin 
(2,780 degrees Fahrenheit), it was too hot for any solid material to condense. 
By the time the cloud had cooled to 400 degrees Kelvin (260 degrees Fahrenheit), 
however, most of it had condensed into solid particles. Grossman has devoted 
most of his career to identifying the small percentage of substances that 
materialized during the first 200 degrees of cooling: oxides of calcium, 
aluminum and titanium, along with the silicates. His calculations predict 
condensation of the same minerals that are found in meteorites.

Over the last decade, Grossman and his colleagues have written a slew 
of papers exploring various scenarios for stabilizing iron oxide enough 
that it would enter the silicates as they condensed at high temperatures, 
none of which proved feasible as an explanation for chondrules. "We've 
done everything that you can do," Grossman said.

This included adding hundreds or even thousands of times the concentrations 
of water and dust that they had any reason to believe ever existed in 
the early solar system. "This is cheating," Grossman admitted. It didn't 
work anyway.

Instead, they added extra water and dust to the system and increased its 
pressure to test a new idea that shock waves might form chondrules. If 
shock waves of some unknown source had passed through the solar nebula, 
they would have rapidly compressed and heated any solids in their path, 
forming chondrules after the melted particles cooled off. Ciesla's simulations 
showed that a shock wave can produce silicate liquid droplets if he increased 
the pressure and the quantities of dust and water by these abnormally 
if not impossibly high amounts, but the droplets would be different from 
the chondrules actually found in meteorites today.

Cosmic shoving match

They differ in that actual chondrules contain no isotopic anomalies, whereas 
the simulated shock-wave chondrules do. Isotopes are atoms of the same 
element that have different masses from one another. The evaporation of 
atoms of a given element from droplets drifting through the solar nebula 
causes the production of isotopic anomalies, which are deviations from 
the normal relative proportions of the element's isotopes. It's a cosmic 
shoving match between dense gas and hot liquid. If the number of a given 
type of atoms pushed out of the hot droplets equals the number of atoms 
getting pushed in from the surrounding gas, no evaporation will result. 
This prevents isotope anomalies from forming.

The olivine found in chondrules presents a problem. If a shock wave formed 
the chondrules, then the olivine's isotopic composition would be concentrically 
zoned, like tree rings. As the droplet cools, olivine crystallizes with 
whatever isotopic composition existed in the liquid, starting at the center, 
then moving out in concentric rings. But no one has yet found isotopically 
zoned olivine crystals in chondrules.

Realistic-looking chondrules would result only if evaporation were suppressed 
enough to eliminate the isotope anomalies. That, however, would require 
higher pressure and dust concentrations that go beyond the range of Ciesla's 
shock-wave simulations.

Providing some help was the discovery a few years ago that chondrules 
are one or two million years younger than calcium-aluminum-rich inclusions 
in meteorites. These inclusions are exactly the condensates that cosmochemical 
calculations dictate would condense in the solar nebular cloud. That age 
difference provides enough time after condensation for planetesimals to 
form and start colliding before chondrules form, which then became part 
of Fedkin and Grossman's radical scenario.

They now say that planetesimals consisting of metallic nickel-iron, magnesium 
silicates and water ice condensed from the solar nebula, well ahead of 
chondrule formation. Decaying radioactive elements inside the planetesimals 
provided enough heat to melt the ice.

The water percolated through the planetesimals, interacted with the metal 
and oxidized the iron. With further heating, either before or during planetesimal 
collisions, the magnesium silicates re-formed, incorporating iron oxide 
in the process. When the planetesimals then collided with each other, 
generating the abnormally high pressures, liquid droplets containing iron 
oxide sprayed out.

"That's where your first iron oxide comes from, not from what I've been 
studying my whole career," Grossman said. He and his associates have now 
reconstructed the recipe for producing chondrules. They come in two "flavors," 
depending on the pressures and dust compositions arising from the collision.

"I can retire now," he quipped.

------------
Citation: "Vapor saturation of sodium: Key to unlocking the origin of 
chondrules," by Alexei V. Fedkin and Lawrence Grossman, Geochimica et 
Cosmochimica Acta, Vol. 112, July 2013, pages 226-250.

Funding: National Aeronautics and Space Administration