[meteorite-list] Protoplanetary Disc - CAI and Chondrule formation
From: Galactic Stone & Ironworks <meteoritemike_at_meteoritecentral.com>
Date: Fri, 2 Nov 2012 21:00:07 -0400
I ran across something interesting on Facebook today. See the graphic
at this link - http://i268.photobucket.com/albums/jj24/Meteoritethrower/Meteorites/proto-disc.jpg
The text that accompanied the graphic is quoted below. I had not
heard about this theory that chondrules and CAI's formed at the same
time. It was always my understanding that chondrules formed after the
CAI's. Is there anyone on this List with expertise in this field that
can confirm or deny this "new theory"...?
New Theory on Formation of Solar System's First Stuff. A new view of
the solar system's early days proposes that the first two kinds of
solid materials ? the precursors of space rocks and ultimately planets
? both formed at the same time.
When the sun was born about 4.6 billion years ago, it was surrounded
by a cloud of gas and dust that eventually became the asteroids,
comets and planets. An initial step in that process had to be the
generation of clumps of solid material.
Previously, researchers believed the two known types of early solids
formed several million years apart from each other. But a new dating
technique from the University of Copenhagen's James Connelly and
fellow researchers shows different results.This means the early days
of the solar system look different than previously thought. Connelly
and his teammates propose their new model in a paper published in the
Nov. 2 issue of the journal Science.
Gas and dust compression
Connelly's team focused on two types of solids: calcium-aluminum?rich
inclusions (CAIs) and chondrules. Both of these solids are found in
meteorites, which are pieces of space rocks typically billions of
years old that find their way to Earth and are often discovered by
scientists and amateurs.
These materials "have records in them of events and processes in the
early part of the solar system," Connelly said, particularly of the
time that the sun and planets were forming from a spinning disk more
than 4.5 billion years ago. [Planetfall: Wonders of the Solar System
CAIs form from molten gas droplets under temperatures higher than
1,880 degrees Fahrenheit (1,030 degrees Celsius, or 1,300 degrees
Kelvin), while chondrules are dust collections that rapidly melt and
cool in a lower temperature region of less than 1,340 degrees
Fahrenheit (727 degrees Celsius, or 1,000 degrees Kelvin).
Under the new model of the solar system's formation, the spinning disk
that eventually formed the sun and the planets had a large amount of
energy within it. Particles were flattening into planes along the
disk. In the center, the sun formed as material lost momentum and
As the material collapsed on to the protoplanetary disk, huge
shockwaves formed that produced "flash" heating, or warmth that began
and then dissipated within a matter of hours. These surges of energy
affected the CAIs and the chondrules. The finding could be important,
because it illustrates a generic way that all protoplanetary disks in
the known universe may have formed solids.
Other dating methods said that the energy from our solar system's
protoplanets, as they whizzed around in their orbits, predicted the
chondrules formed some 2 million years after CAIs did. However, this
timing didn't jive with astronomical observations of other planetary
systems, which predicted a shorter formation period.
The other model, Connelly said, implied "there is something unique
about our solar system that allows these inclusions to be formed. It
looked as if these things were lasting a bit too long."
A new method of dating
The old dating method relies on measuring the amount of aluminum 26,
which is a radioactive form, or isotope, of aluminum, present in
meteorites to date the solar system. But there is one weakness with
this technique, Connelly said: Using this form of aluminum assumes it
was distributed evenly throughout the solar system.
If two objects formed at the same time in different locations of the
disk, they may not necessarily have the same amount of this aluminum
isotope inside of them because there may be different proportions of
aluminum in different locations. The old assumption was that if the
aluminum proportions were different, they formed at different times.To
come up with the new solid history, Connelly and his team adapted
techniques he learned at the Royal Ontario Museum years ago while
dating zircon minerals.
The researchers broke apart meteorite samples, washing and gradually
dissolving them to partition lead from the rest of the sample,
removing contaminants that can affect the dating process.
Connelly's team used mass spectrometers to measure the isotopic
composition of lead and uranium, and used the known rate of the decay
of uranium to determine absolute ages of CAIs and chondrules in the
Uranium and lead are commonly used to date geologic events on Earth
because isotopes of uranium have suitable half-lives (meaning, the
time in which half the particles decay to lead) for this work.
In meteorites, using the same process is more challenging because the
uranium and lead are in small quantities. However, uranium-lead dating
is the best method to learn about the early solar system because
dating with it is so precise, Connelly said. It can differentiate
events that are less than a million years apart.
The team's new "preferred estimate" for the age of CAIs is 4.56730
billion years, plus or minus 160,000 years.CREDIT: NASA/JPL
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