[meteorite-list] New View of Gas and Dust in the Solar Nebula (Genesis)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 26 Aug 2010 12:46:49 -0700 (PDT)
Message-ID: <201008261946.o7QJknMW005281_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Aug10/gas-dust-Oisotopes.html

New View of Gas and Dust in the Solar Nebula
Planetary Science Research Discoveries
August 25, 2010


--- The current view holds that gas and dust in the solar nebula began
with the same oxygen isotopic composition, then changed by processes in
the nebula. A new view suggests that dust and gas had vastly different
mixtures of oxygen isotopes in the first place.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

The recognizable components in meteorites differ in their relative
abundances of the three oxygen isotopes (^16 O, ^17 O, and ^18 O). In
particular, the amount of ^16 O varies from being like that of the Earth
to substantially enriched compared to the other two isotopes. The
current explanation for this interesting range in isotopic composition
is that dust and gas in the solar nebula (the cloud of gas and dust
surrounding the primitive Sun) began with the same ^16 O-rich
composition, but the solids evolved towards the terrestrial value. A new
analysis of the problem by Alexander Krot (University of Hawai'i) and
colleagues at the University of Hawai'i, the University of Chicago,
Clemson University, and Lawrence Livermore National Laboratory leads to
the bold assertion that primordial dust and gas differed in isotopic
composition. The gas was rich in ^16 O as previously thought (possibly
slightly richer in ^16 O than the measurements of the solar wind
returned by the Genesis Mission, but that the dust had a composition
close to the ^16 O-depleted terrestrial average. In this new view, the
dust had a different history than did the gas before being incorporated
into the Solar System. Solids with compositions near the terrestrial line
may have formed in regions of the solar nebula where dust had
concentrated compared to the mean solar dust/gas ratio (1 : ~100). The
idea has great implications for understanding the oxygen-isotope composition
of the inner Solar System and the origin of materials in the molecular cloud
from which the Solar System formed.


Reference:

    * Krot, A. N., Nagashima, K., Ciesla, F. J., Meyer, B. S., Hutcheon,
      I. D., Davis, A. M., Huss, G. R., and Scott, E. R. D. (2010)
      Oxygen Isotopic Composition of the Sun and Mean Oxygen Isotopic
      Composition of the Protosolar Silicate Dust: Evidence from
      Refractory Inclusions. /The Astrophysical Journal,/ v. 713, p.
      1159-1166.
    * *PSRDpresents:* New View of Gas and Dust in the Solar Nebula
      --Short Slide Summary <PSRD-gas-dust-Oisotopes.ppt> (with
      accompanying notes).


Two Reservoirs of Oxygen Isotopes

The oxygen we breathe is composed of three isotopes with atomic weights
of 16, 17, and 18. ^16 O is the most abundant (99.76% of all the
oxygen), followed by ^18 O, with ^17 O bringing up the rear (only about
4 ten-thousandths the abundance of ^16 O). In spite of ^16 O being so
abundant compared to the others, the set of three isotopes provides
exceedingly important information about how the Solar System formed and
about geochemical processing on the planets.

One informative way to plot oxygen isotopic data is to use all three
isotopes by plotting the ^17 O/^16 O ratio against the ^18 O/^16 O
ratio, as shown in the diagram below. In general, rocks in and on a
given planet fall along a well-defined line with a slope of about ??; the
line for terrestrial rocks is labeled "TF" in the graphs below. A
striking discovery made more than three decades ago by Robert Clayton
(University of Chicago) and coworkers was that primitive materials in
chondrites plot along a line that suggests addition or subtraction of
^16 O.

[Plot of oxygen isotope ratios in chondrules and CAIs in meteorites.]
Plot showing the ^18 O/^16 O and ^17 O/^16 O ratios in chondrules and
CAIs in meteorites in parts per thousand. Data have been standardized to
standard mean ocean water (SMOW) and plotted as deviations from that
value. The meteorite particles define a line with much steeper slope
than the fractionation line (TF) line, which is consistent with loss or
addition of ^16 O. A shorthand way to show the deviation from the TF
line is to plot the vertical displacement of any point from it, as
indicated graphically in purple. This parameter (??^17 O) is called "big
delta O-17" by cosmochemists. We use it in subsequent diagrams.


The explanation for the difference between primitive materials and the
terrestrial line is that the dust and gas that made up the primitive
Solar System were both rich in ^16 O, but that some process produced
substantial amounts of dust depleted in it. Several imaginative ideas
were invented by cosmochemists to explain the existence of two
isotopically distinct reservoirs. One class of models depicts formation
of the ^16 O-poor reservoir (the one near the terrestrial fractionation
line in the diagrams) by a chemical effect produced by irradiation of
carbon monoxide (CO) by ultraviolet light. Observations of molecular
clouds indicate that ultraviolet radiation
inside the cloud can preferentially dissociate CO made with ^17 O or ^18 O.
Ultraviolet light that can dissociate CO made with ^16 O cannot
penetrate beyond the surface of the cloud. The oxygen released from
dissociated CO can combine with hydrogen to produce water ice that is
rich in ^17 O and ^18 O. Evaporation of this ice in the inner Solar
System creates a gas rich in water and depleted in ^16 O. This dynamic
process, called "self-shielding," can produce large variations in the
proportions of ^16 O relative to the other two isotopes. The theory
predicts that the planets were made of material that contained excess
water ice that was preferentially enriched in ^17 O and ^18 O, and that
the Sun should have an oxygen isotopic composition like those meteoritic
grains richest in ^16 O in primitive materials in chondrite meteorites,
such as calcium-aluminum-rich inclusions (CAIs).

Sasha Krot and his coauthors note that the self-shielding model explains
the oxygen isotopic compositions of asteroids and the inner planets, but
raise several warning flags. A key one is that self-shielding assumes
that dust in the solar nebula started out rich in ^16 O (large value of
??^17 O), but no such samples of primitive dust have been found. Another
problem is that self-shielding predicts that primitive materials should
have some correlation of ^16 O abundance and their ages (as measured by
short-lived ^26 Al decay), but except for CAIs being the oldest and
richest in ^16 O, there is no systematic variation in the age of other
primitive objects such as chondrules and their oxygen isotopic
compositions.

To reassess the problem, Krot and coworkers focused on trying to figure
out the average ??^17 O of the dust in the solar nebula. To do so they
used recently-measured values of the Sun's oxygen isotopic composition
refractory inclusions, using a secondary ion mass spectrometer at the
University of Hawai'i. Before looking at those new data, we'll look at
measurements of oxygen isotopes in the Sun.
    
Measuring Oxygen Isotopes in the Sun

A primary goal of the Genesis mission was to determine the composition
of the Sun by measuring the composition of the solar wind. It did so by
traveling to a region of space where the gravitational pull of the Sun
and Earth are balanced, a special location called the Lagrange 1 point.
Located about 1.5 million kilometers from Earth, the spacecraft followed
a looping path as it collected solar wind. The location is out of
Earth's atmosphere, of course, but also far from its magnetic field,
allowing an array of collectors to sample the solar wind.

Genesis carried several types of collectors. One was the Solar Wind
Concentrator, which magnified the solar wind by a factor of about 20 by
using grids held at high voltage. The high voltage caused 90% of the
protons (hydrogen nuclei) to be deflected, thereby decreasing the
background for the other elements and their isotopes. The solar wind
ions slammed into high-purity collectors. Those for oxygen isotopic
measurements were made of silicon carbide. The collection system is a
triumph in instrument engineering and materials science.

Even with ultra-pure material and concentration of the solar wind, it is
still difficult to measure the small concentrations of oxygen collected
during the mission. This is particularly true for the tiny amounts of
^17 O and ^18 O. To make the measurements, Kevin McKeegan and his
colleagues at the University of California, Los Angeles and the
University of Bristol, United Kingdom built a hybrid contraption
composed of a secondary ion mass spectrometer (SIMS) and an accelerator
mass spectrometer, which they dubbed the MegaSIMS. The SIMS front-end is
a standard ion microprobe (specifically, a Cameca ims 6f), which
sputters ions off a sample and sends them through the mass spectrometer
system (see *PSRD* article, Ion Microprobe).
The accelerator mass spectrometer
part of the MegaSIMS accelerates the ions to extraordinary energies
before sending them through the mass spectrometer and onto detectors.
The high energy allows complete destruction of ions of OH made from ^16
O, which have masses of 17, the same as ^17 O. This allows an accurate
measurement of ^17 O by removing the troublesome interfering molecule.

Kevin McKeegan and his colleagues are still working on getting final
values for the solar wind oxygen isotopic composition, but preliminary
results give a value for ??^17 O of ???26 ?? 5.6 parts per thousand. That
value is used in the diagrams that follow. It is similar to the most
refractory materials in chondrites, the CAIs.

Oxygen Isotopes in Meteorites

Chondrites contain two refractory components, calcium-aluminum-rich
inclusions (CAIs) and amoeboid olivine aggregates (AOAs). CAIs are the
oldest solids to form in the Solar System and are composed of (not
surprisingly) minerals that contain calcium, aluminum, and titanium.
(See *PSRD* article, Dating the Earliest Solids in our Solar System.)
Some CAIs have irregular shapes and
porous textures, indicative of materials condensed from a gas. Others
have igneous textures, indicative of remelting of previous condensates.
AOAs have irregular shapes and tiny grain sizes (mineral grains are
smaller than 20 micrometers). Some AOAs contain small CAIs inside them.

On the basis of detailed studies of CAIs and AOAs, theroretical
calculations, and condensation, vaporization, and melting experiments,
cosmochemists have concluded that two types formed by condensation in a
gas of solar composition. The condensation happened in a region where
the dust/gas ratio was solar (1 : ~100), so oxygen isotopic compositions
in AOAs and CAIs ought to reflect the composition of the solar gas,
hence of the Sun. Sasha Krot and his colleagues show that the big delta
oxygen-17 for minerals in CAIs and AOAs (as measured by regular,
non-megafied ion microprobes) cluster around ???23 parts per thousand, not
much different from the solar value measured by Kevin McKeegan's team on
the Genesis samples.


[graphic]
Oxygen isotopic compositions of refractory inclusions (CAIs and AOAs)
expressed as the deviation from the terrestrial fractional line (TF).
Symbols are color-coded to identify the specific mineral measured. A
small group of minerals that responded to metamorphism on the CV
carbonaceous chondrite body have values close to the terrestrial line.


Krot and coworkers conclude that gas of solar composition and the Sun
had a big delta O-17 of around -- 23 (plus or minus 1.9) parts per
thousand. But what was the composition of the dust? Was it the same as
the gas and the Sun, as the self-shielding model assumes, or different?
Clues to answer those questions come from study of an unusual class of
CAI, the F and FUN inclusions. (You have to like acronyms in
cosmochemistry.) The F stands for fractionation, which means that the
minerals in one inclusion string out along a line parallel to the
terrestrial fraction line. UN stands for unidentified nuclear effects.
FUN inclusions have large isotopic anomalies in many elements. The
preservation of these anomalies suggests that the dusty precursors to
FUN inclusions escaped complete evaporation in the solar nebula, so they
retain a record of the oxygen isotopic composition of the primordial dust.

Results from published and new measurements show that, in contrast to
most CAIs and AOAs, FUN CAIs show a large range in big delta O-17, from
the terrestrial value all the way down to the solar value. Because they
all show strong evidence for chemical processing (they lie along lines
parallel to the terrestrial fractionation line on plots of ^18 O/^16 O
and ^17 O/^16 O), this range in oxygen isotopic composition indicates
that the FUN inclusions or their precursor dust grains have exchanged
oxygen with gas in the solar nebula. Sasha Krot and coauthors conclude
that the range in big delta O-17 points to different extents of
equilibration between ^16 O-poor dust and ^16 O-rich nebula gas.
Chondrules and fine-grained matrix materials in chondrites, which lie
not far from the terrestrial fractionation line, probably formed in
regions of the nebula where dust was substantially concentrated (see
*PSRD* article, Tiny Molten Droplets, Dusty Clouds, and Planet Formation).
Those objects, including many
asteroids and the terrestrial planets, may have formed in these regions,
hence reflect the composition of the nebular dust, not the gas.

[graphic]
FUN refractory inclusions record a range in oxygen isotopic
compositions, from values similar to the Earth (TF line) to those
similar to the Sun. Sasha Krot and his colleagues suggest that this was
caused by varying amounts of isotopic exchange between a gas rich in ^16
O (large negative ??^17 O) and dust with much less ^16 O (about zero ??^17
O, like the Earth).
    
Why Were the Dust and Gas Different?

If Sasha Krot and coauthors are right, the Solar System formed from a
cloud in which gas and dust differed in the relative abundances of the
three oxygen isotopes. What caused the difference? It might have been a
natural product of galactic chemical evolution. Standard theory of
element formation depicts ^16 O being produced linearly with time, while
the other two oxygen isotopes increase in abundance with the square of
the time. This translates to ^17 O / ^16 O and ^18 O / ^16 O increasing
steadily with time. Thus, if the dust in interstellar space is on
average older than the gas, and the two do not exchange isotopes, then
formation of the Solar System from that volume of the molecular cloud
would result in the dust being more enriched in ^16 O than is the gas,
opposite to what is observed. On the other hand, if the molecular cloud
was dominated by dust from highly active or exploding stars that added
material shortly before the Solar System formed, then solar nebula dust
would have been enriched in ^17 O and ^18 O, hence poorer in ^16 O, as
observed.

Team Krot suggest three tests of their hypothesis. One is that the least
thermally processed dust in extraterrestrial samples, such as amorphous
(noncrystalline) dust in the interplanetary dust collection and probably
present in Kuiper Belt Objects ought to have oxygen that is poor in ^16
O (??^17 O much closer to the Earth value than to CAIs). Another test is
that ^16 O-rich crystalline objects should be very rare and related to
CAIs and AOAs. A third test involves the oxygen isotopic composition of
chondrules. These objects formed in dust-rich environments, so ought to
reflect the ^16 O-poor nature of the dust. Thus, ^16 O-rich chondrules
should be extremely rare (perhaps absent).

Meteorites, the Genesis Mission, astrophysical theory--this type of
cosmochemical research is at the interface with astrophysics.

Additional Resources Links open in a new window.

    * *PSRDpresents:* New View of Gas and Dust in the Solar Nebula
      --Short Slide Summary <PSRD-gas-dust-Oisotopes.ppt> (with
      accompanying notes).

    * Krot, A. N., Nagashima, K., Ciesla, F. J., Meyer, B. S., Hutcheon,
      I. D., Davis, A. M., Huss, G. R., and Scott, E. R. D. (2010)
      Oxygen Isotopic Composition of the Sun and Mean Oxygen Isotopic
      Composition of the Protosolar Silicate Dust: Evidence from
      Refractory Inclusions. /The Astrophysical Journal,/ v. 713, p.
      1159-1166. [NASA ADS entry
      <http://adsabs.harvard.edu/abs/2010ApJ...713.1159K>]
    * Krot, A. N. (2002) Dating the Earliest Solids in our Solar System.
      /Planetary Science Research Discoveries./
      http://www.psrd.hawaii.edu/Sept02/isotopicAges.html
      <../Sept02/isotopicAges.html>
    * Martel, L. M. V. and Taylor, G. J. (2006) Ion Microprobe.
      /Planetary Science Research Discoveries./
      http://www.psrd.hawaii.edu/Feb06/PSRD-ion_microprobe.html
      <../Feb06/PSRD-ion_microprobe.html>
    * McKeegan, K. D. and nine others (2009) Oxygen Isotopes in a
      Genesis Concentrator Sample. /40th Lunar and Planetary Science
      Conference,/ abstract #2494. [pdf
      <http://www.lpi.usra.edu/meetings/lpsc2009/pdf/2494.pdf>]
    * McKeegan, K. D. and nine others (2010) Genesis SiC Concentrator
      Sample Traverse: Confirmation of ^16 O-depletion of terrestrial
      oxygen. /41st Lunar and Planetary Science Conference/, abstract
      #2589. [pdf <http://www.lpi.usra.edu/meetings/lpsc2010/pdf/2589.pdf>]
    * Taylor, G. J. (2008) Tiny Molten Droplets, Dusty Clouds, and
      Planet Formation. /Planetary Science Research Discoveries./
      http://www.psrd.hawaii.edu/Nov08/chondrule_sodium.html
      <../Nov08/chondrule_sodium.html>
Received on Thu 26 Aug 2010 03:46:49 PM PDT