[meteorite-list] A New Type of Stardust

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:38 2004
Message-ID: <200308310451.VAA09091_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Aug03/stardust.html

A New Type of Stardust
Planetary Science Research Discoveries

     --- Interplanetary dust particles contain rare
     grains that formed in stars older than the Sun.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology
August 29, 2003

Many meteorites contain small quantities of microscopic particles with
unusual mixes of isotopes. The abundances of the isotopes, such as the
relative amounts of oxygen-16, 17, and 18, indicate an origin outside the
solar system. The dust particles are bits of stars, some of which no longer
exist. Since their discovery in the late 1980s, only grains made of carbon
(diamond and graphite), carbides, and oxides have been found. None were
silicates--compounds that contain silicon, oxygen, and assorted other ions
such as magnesium and calcium. This seemed peculiar to scientists because
meteorites and the rocky planets are made mostly of silicate minerals. Part
of the problem might have been a sampling bias introduced by the way grains
of stardust were extracted, which involves dissolution of meteorites by
strong acids. Silicates are more easily dissolved than carbides, oxides, or
carbon compounds. However, Scott Messenger and his colleagues at Washington
University in St. Louis and at the Johnson Space Center in Houston have
found silicate grains using advanced instrumentation in interplanetary dust
particles. Finding these particles, probably remnants of comets, shows that
presolar silicates exist, but it leaves open the question of why none have
been found in meteorites.

Reference:

     Messenger, S., Keller, L. P., Stadermann, F. J., Walker, R. M., and
     Zinner, E. (2003) Samples of stars beyond the solar system: silicate
     grains in interplanetary dust. Science, vol. 300, p. 105-108.

             --------------------------------------------------

Stardust in Meteorites

Astronomical observations and astrophysical theory tell us that the solar
system formed by the collapse of a vast molecular cloud. A portion of the
cloud collapsed into the primitive Sun surrounded by a disk of gas and dust.
Planets, asteroids, and comets formed in this disk. One of the most exciting
discoveries made by meteoriticists in NASA's Cosmochemistry Program is that
meteorites contain tiny grains that once inhabited the interstellar cloud.
These grains, called presolar grains or stardust, are typically only a few
micrometers in diameter. They survived cloud collapse and heating in the
accretion disk surrounding the nascent Sun. These relicts from stars give us
a close-up look at the grains that inhabit interstellar space and offer a
highly informative complement to astronomical observations of stars and
interstellar clouds. They are minuscule, frozen chips of stars available for
close study.

                       [Hubble mosaic of Orion nubula]
       This mosaic of images taken by the Hubble Space Telescope
       shows a portion 2.5 light years wide of the Orion nebula, a
       huge star factory. It reveals at least 153 glowing
       protoplanetary disks in which stars and perhaps planets are
       forming. Regions like this are natural laboratories for
       studying how stars form, so astronomers view them in as many
       ways as possible to determine what chemical compounds make up
       the vast clouds. Studies of presolar grains in meteorites
       allow scientists to examine interstellar dust grains up close
       in the laboratory. Meteoriticists do astronomy with
       microscopes instead of telescopes.

Evidence that the grains are presolar comes from the relative abundances of
the isotopes of common elements, such as silicon, oxygen, and carbon. The
abundances of the isotopes differ from all samples of typical solar system
material as found on the planets, asteroids (as sampled by meteorites), and
comets. The aberrant isotopic compositions are caused by nuclear reactions
in dying and exploding stars. All the isotopes of elements other than
hydrogen and helium are synthesized by nuclear reactions in the interiors of
stars. The isotopes are expelled into interstellar space by stellar winds or
monumental explosions. Many condense into dust grains. These products of the
life and death of stars mixed into the cloud from which the Sun developed,
forming the raw materials for the solar system. Thus, the solar system is a
mixture of materials from countless stars. During the formation of the solar
system, most of the material was homogenized, giving the normal solar system
isotopic compositions for the elements. A small percentage of grains escaped
homogenization, giving us a window into the nature of stellar evolution and
interstellar clouds.
             --------------------------------------------------

The Presolar Dust Grain Menagerie

As far back as the 1960s, there were hints that meteorites might house
presolar grains. The clues came from analyses of bulk meteorites, but since
the dust grains are only microns across, bulk analyses were too blunt a
tool. It required development of high-tech instruments capable of analyzing
the abundances of isotopes in micrometer-sized grains. The star analytical
tool is secondary ion mass spectrometry, nicknamed "SIMS" or "ion
microprobe". In SIMS analysis, a primary beam of high-energy ions is aimed
at a small area of a sample, such as a mineral grain. The ion beam sputters
atoms and molecules from the sample, creating charged ions (called secondary
ions). The secondary ions are extracted with a strong electrostatic field
and sorted by mass with a large magnet. A series of ion detectors counts the
ions in different mass categories, giving the abundance in the original
sample. (The raw data is corrected for assorted effects and normalized to
well-analyzed standards.) The technique allows measurement of both elements
and isotopes. A recent advance in instrumentation is the development of the
NanoSIMS, which allows analyses of astonishingly small grains--only a few
tens of nanometers (billionths of a meter) in diameter.

                    [NanoSIMS at Washington University]
       This is a photograph of the NanoSIMS at Washington University
       in St. Louis, the first such instrument in the world. The
       major parts are labeled. This state-of-the-art instrument
       enabled the work by Scott Messenger and his colleagues, and
       opens the door to numerous other discoveries in
       cosmochemistry. Photo courtesy of Frank Stadermann, Washington
       University.

The standard way of finding presolar grains is to dissolve away everything
else in a meteorite. Stardust seekers crush chips of meteorites to a powder
and pour various acid concoctions onto the powdered rock. Fortunately, the
minerals that condense in stars are resistant to the strong solutions and
remain in an undissolved residue. Of course, this might bias our sampling of
presolar grains. Perhaps some of the easily dissolved grains are also
stardust. Nevertheless, many of the residue grains are presolar. In other
words, you burn down the haystack to find needles the size of pinpoints and
analyze them with state-of-the-art analytical equipment.

                        A presolar (stardust) grain of silicon carbide, SiC.
[presolar grain of SiC] The grain is only 3 micrometers across. Photo by
                        Rhonda Stroud, Naval Research Laboratory, and
                        displayed in Nittler (2003).

Larry Nittler (Carnegie Institution of Washington) summarizes recent
advances in research on stardust. The table below, from Nittler's paper,
shows the types of presolar grains that have been discovered, their
abundances, and sizes. There is some question about the amazingly small
nanodiamonds being presolar, as they are too small to analyze individually,
even with our advanced tools. A quick look at the table reveals that
something is missing. There are no silicate minerals. Only carbides,
silicides, nitrides, and oxides are in the certified list. This is
surprising because silicon and oxygen, the key ingredients in silicates, are
abundant elements in the solar system. Pick up a rock from the Earth,
another planet, or an asteroid and you are almost certain to be hefting a
bunch of silicate minerals.

Types of presolar grains extracted from meteorites by acid dissolution (from
                               Nittler, 2003)

             Type Abundance (parts Size
                            per million)
       Nanodiamond (C) 1400 2 nanometers
       Silicon carbide
            (SiC) 14 0.1-20 micrometers
         Graphite (C) 10 1-20 micrometers
         Carbides of
          titanium,
          zirconium, Small grains
         molybdenum, inside presolar 5-220 nanometers
        ruthenium, and graphite
          iron, and
      iron-nickel metal
       Silicon nitride
           (Si3N4) >0.002 About 1 micrometer

       Corundum (Al2O3) About 0.05 0.5-3 micrometers

       Spinel (MgAl2O4) <0.05 0.1-3 micrometers

           Hibonite
         (CaAl12O19) 0.002 2 micrometers

       Titanium dioxide
            (TiO2) One grain About 1 micrometer

One reason for the absence of silicates in our collection of presolar grains
in meteorites may be that we destroy them during the extraction process.
Perhaps there are presolar silicates in meteorites, but they are dissolved
away during the dissolution process. This is one of the reasons why Scott
Messenger and his colleagues decided to search for presolar silicates in
interplanetary dust particles.
             --------------------------------------------------

Collecting Interplanetary Dust

An important source of information about the early solar system comes from
the tiniest meteorites of all--interplanetary dust particles. As small, fast
moving particles enter the atmosphere, they are gradually slowed down by
atmospheric drag. Depending on their velocity and angle of entry, particles
will be heated to greater or lesser extent. Some burn up entirely.
Nevertheless, countless particles survive atmospheric entry and slowly drift
around at high altitudes.

A NASA program collects the particles in the stratosphere using a
high-flying ER-2 reconnaissance plane (basically the same as the U-2 spy
plane). During each collecting flight, the plane carries circular collectors
that are free of contaminants and smeared with silicone oil. The collectors
remain flat against a wing until the plane reaches its cruising altitude of
about 20 kilometers (about 65,000 feet). Once at cruising altitude, the
collectors are deployed and gently-drifting particles hit the collectors,
like bugs on a windshield. Most of the particles are extraterrestrial (bits
of comets and asteroids), but some are volcanic dust particles, exhaust from
solid rocket motors, and other types of earthly aerosols. The terrestrial
dust particles are of great interest to atmospheric scientists because they
cause chemical reactions in the atmosphere and reflect sunlight back into
space.

                               [ER-2 airplane]
       ER-2 airplane in flight. Ultra-clean dust collectors are
       mounted under the wings (not shown in this view).

The particles are typically only 5 to 50 micrometers in diameter. Because a
lot of terrestrial dust is in that size range, the samples are handled and
stored in a special clean room at the Johnson Space Center. The room, dubbed
the Cosmic Dust Laboratory, contains only 100 particles larger than 0.5
micrometers per square foot. Laboratory scientists wear special lint-free
garments and use micromanipulators to handle the particles.

                          [cosmic dust lab at JSC]
       The cosmic dust laboratory at the Johnson Space Center.
       Laboratory scientists wear special garments, affectionately
       known as "bunny suits," to prevent contamination. Laboratory
       air is filtered so that each cubic foot of air contains no
       more than 100 particles larger than 0.5 micrometers.
             --------------------------------------------------

Silicate Grain in Interplanetary Dust Particles

The development of the NanoSIMS makes it possible to search individual
particles of interplanetary dust for presolar grains. Conventional SIMS
instruments cannot analyze grains smaller than a few micrometers, but the
grains making up a typical interplanetary dust particle (IDP) are much
smaller. The higher spatial resolution of the NanoSIMS allows individual
grains to be analyzed.

                            Typical cosmic dust particle. Note that the
                            particle is composed of numerous small grains.
[typical cosmic dust grain] Using the NanoSIMS at Washington University in
                            St. Louis, Scott Messenger was able to determine
                            the oxygen isotopic composition of individual
                            grains in particles like this one.

Messenger measured the abundances of the three oxygen isotopes in 1031
grains in several interplanetary dust particles. Lindsay Keller of NASA
Johnson Space Center determined the mineralogy of 113 grains using a
transmission electron microscope. The electron microscope studies show that
there is an assortment of silicate grains present, including both mineral
grains and GEMS (glass with embedded metal and sulfide). All but six of the
1031 grains had oxygen isotopic compositions like typical solar system
stuff. Those six special grains are presumed to be presolar because of the
distinctive compositions of their oxygen isotopes. This does not mean that
some of the other 1025 grains are not also presolar. They might have been
modified in the solar nebula, erasing the evidence of their origin as
stardust.

               [ratios of oxygen isotopes in presolar grains]
     Ratios of oxygen-17 and oxygen-18 to oxygen-16 allow us to
     distinguish presolar grains from typical solar system materials.
     The grains labeled are the six grains analyzed by Scott Messenger
     (out of 1031) that have oxygen compositions clearly different from
     the solar system, hence are presolar. The red numbers refer to
     groups of presolar grains identified from analyses of oxides
     separated from meteorites. Group 1 grains form in red giant or
     asymptotic giant branch (AGB) stars, and group 3 in metal-rich AGB
     stars. The stellar origins of group 4 stars are not known with
     certainty; they might be formed in type II supernovae.

       --------------------------------------------------------------

                     [NanoSIMS image of presolar grain]
       The NanoSIMS at Washington University allows investigators to
       make maps of the distribution of the oxygen isotopes in tiny
       particles. In this image, taken from the paper by Messenger
       and his colleagues, one of the grains in an interplanetary
       dust particle is clearly richer in oxygen-17. Its anomalous
       nature marks it as a candidate for being a presolar grain.

Using astrophysical theory and previous results on presolar oxide grains
found in meteorites, Messenger and colleagues conclude that the six
confirmed presolar grains formed in three different stellar environments
(labeled in the diagram above). The groups reflect differences in the
masses, ages, and chemical compositions of the stars in which they formed.
Group 1 are rich in oxygen 17. They are thought to form in either red giant
stars or stars known as asymptotic giant branch (AGB) stars. Red giants form
when a star has used up its hydrogen by converting it to helium by nuclear
fusion. The star cools and expands, becoming a red giant. This phase is
followed by nuclear fusion of helium, forming heavier isotopes and elements.
Once the helium is used up, pressures and temperatures inside stars smaller
than 8 Suns are too low to allow further fusion to take place, so the star
becomes just a hot ball of gas. Its outer portions expand, however, and it
becomes a red giant again, known as an AGB star.

There was one group 3 grain, characterized by high amounts of oxygen-16,
pushing it towards the left in the diagram above. These are thought to have
formed in red giant and AGB stars particularly rich in what astronomers call
"metals," which is any element heavier than helium. The two group 4 grains
are low in oxygen-16, so plot on the right of the diagram. The environment
in which they formed is unknown. Some scientists have speculated that they
might form during supernova explosions. These happen in stars more massive
than about 8 Suns. Instead of fizzling out as do smaller stars, fusion
continues to occur in the deep interior, eventually producing heavy elements
such as iron. Once iron is produced fusion halts and the core of the star
collapses. When it rebounds, it sends out a strong shock wave that spews
most of the star into interstellar space in an explosion known as a type II
supernova.
             --------------------------------------------------

Checking with Telescopes

If the presolar grains found in interplanetary dust particles come mostly
from red giants or AGB stars, is there evidence that grains of silicates and
GEMS, the glassy objects that contain tiny grains of metallic iron and
sulfide, are present in such stars? Astronomers have used the European Space
Agency's Space Infrared Observatory [http://www.iso.vilspa.esa.es/] to make
spectroscopic observations of red giants and AGB stars. They find spectral
evidence for the presence of amorphous silicate (the glassy part of GEMS)
and silicates such as enstatite and forsterite. Thus, Messenger's
identification of presolar forsterite and GEMS is consistent with the
astronomical observations.

                                 The European Space Agency's Infrared Space
                                 Observatory was operational between
                                 November 1995 and May 1998. It allowed
                                 scientists to study distant stars,
[ESA Infrared Space Observatory] interstellar clouds, disks around stars,
                                 and galaxies at wavelengths between 2.5 and
                                 240 micrometers, observations difficult or
                                 impossible to do from the ground because of
                                 the Earth's atmosphere.

Future studies of interplanetary dust using the NanoSIMS will allow for
other isotopic measurements of other elements, a broader survey of the
abundances of certifiable presolar grains, and detailed tests of
astrophysical theories of how stars evolve and how grains are modified in
interstellar space. The distinction between cosmochemistry and astronomy is
blurring. They are complementary ways of looking at our origins, and both
reach for the stars and back in time. And there's more to come. Besides
continuing studies of interplanetary dust particles and meteorites, in 2006,
the Stardust mission will return samples of interstellar dust and comet Wild
2, which should provide a treasure of bits of stars beyond the solar system.

                      Artist's rendering of the STARDUST spacecraft flying
[STARDUST spacecraft] past comet Wild-2. The mission will also collect
                      particles of interstellar dust.

             --------------------------------------------------

ADDITIONAL RESOURCES

     Infrared Space Observatory, a European Space Agency mission.

     Messenger, S., Keller, L. P., Stadermann, F. J., Walker, R. M., and
     Zinner, E. (2003) Samples of stars beyond the solar system: silicate
     grains in interplanetary dust. Science vol. 300, p. 105-108.

     Nittler, L. R. (2003) Presolar stardust in meteorites: recent advances
     and scientific frontiers. Earth and Planetary Science Letters, vol.
     209, p. 259-273.

     Stardust, NASA's comet sample return mission.
Received on Sun 31 Aug 2003 12:51:05 AM PDT