[meteorite-list] Organic Globules from the Cold Far Reaches of the Proto-Solar Disk (Tagish Lake Meteorite)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 25 Jan 2007 14:59:05 -0800 (PST)
Message-ID: <200701252259.OAA20778_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Jan07/organicGlobules.html

Organic Globules from the Cold Far Reaches of the Proto-Solar Disk
Planetary Science Research Discoveries
January 25, 2007

--- Hollow organic globules in the Tagish Lake meteorite probably formed
far from the proto-Sun, maybe even in interstellar space before our
Solar System formed.

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

Tagish Lake specimen in the ice

Keiko Nakamura-Messenger and colleagues at the NASA Johnson Space Center
in Houston, Texas, studied minute globules of organic material in the
Taglish Lake carbonaceous chondrite. This meteorite was collected soon
after it fell, so is fresh and likely to be uncontaminated with
terrestrial organic compounds. Using microanalytical techniques the
NASA team found that the globules had hydrogen and nitrogen isotopic
compositions consistent with chemical reactions at strikingly frigid
temperatures, only 10 to 20 K (-253 to -263 C. Temperatures that
low occur in cold, interstellar molecular clouds like the one that
collapsed to form the Solar System or in the outermost part
of the disk surrounding the Sun when it was forming. These organic
globules might represent the type of prebiotic carbon compounds that
were delivered to young Earth.

Reference:

    * Nakamura-Messenger, Keiko, Scott Messenger, Lindsay P. Keller,
      Simon J. Clemett, and Michael E. Zolensky (2006) Organic globules
      in the Tagish Lake Meteorite: Remnants of the proto-solar disk.
      Science, v. 314, p. 1439-1442.

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

An Uncontaminated Sample

One of the greatest arguments in meteoritics erupted in the early 1960s
with the announcement of the discovery of tiny fossils in carbonaceous
chondrites. The research had been done by microbiologist George Claus
(New York University Medical Center) and organic chemist Bartholomew
Nagy (Fordham University). They thought they had found spores in the
carbonaceous chondrites they studied. Botanists and biologists were
mystified by the structures, which Claus and Nagy called "organized
elements," but all agreed that they were probably biological in origin.
Edward Anders and colleagues at the University of Chicago were
instrumental in debunking the claim. They showed that the unusual
structures were probably ragweed pollen that had been altered by a dye
Nagy had used to make them easier to see. They were biological all
right, but not extraterrestrial.

Contamination issues have also popped up in the debate about fossils in
the ALH 84001 Martian meteorite (for examples, see PSRD articles:
Fossils Blowing in the Wind: More Contamination of Antarctic Meteorites
<http://www.psrd.hawaii.edu/July99/contamination.html>, Organic
Compounds in Martian Meteorites May be Terrestrial Contaminants
<http://www.psrd.hawaii.edu/Feb98/OrganicsALH84001.html>, Martian
Organic Matter in ALH 84001?
<http://www.psrd.hawaii.edu/June99/organicsBecker.html>.) ALH 84001
resided in Antarctica for 13,000 years before being found, ample time
for contamination, even in the cold, seemingly lifeless Antarctic. Earth
is so teeming with life that it is easy to contaminate a rock sitting in
the ice or ground for even a few days or weeks.

No such problems with the Taglish Lake meteorite. It was found
immediately after its fall (see PSRD article: Tagish Lake--A Meteorite
from the Far Reaches of the Asteroid Belt
<http://www.psrd.hawaii.edu/Dec02/TagishLake.html>), so is unlikely to
be contaminated. It is also loaded with carbon compounds (2.6 % carbon
by weight), giving us lots of sample to work with. Previous analyses
showed that much of the carbon was in organic compounds, and in 2002
Nakamura-Messenger had found sub-micrometer hollow globules. However,
she could not unambiguously show that the globules were
extraterrestrial. New techniques for analyzing materials much smaller
than a micrometer, in the nanometer range, are now available, and the
Johnson Space Center is well-equipped with them.

[Tagish Lake meteorite, M. Zolensky]
A 159-gram
piece of Tagish Lake meteorite recovered in Canada. The right side of
the broken stone shows the fusion crust formed by melting of the surface
by friction with the atmosphere as it fell. The cube is 1 cm across.
Image courtesy of Mike Zolensky, NASA JSC.(Click image for enlargement.)

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

Nano Nano

Cosmochemists can analyze incredibly tiny objects, as discussed in PSRD
article: Analyzing Next to Nothing
<http://www.psrd.hawaii.edu/April00/analyzingSmall.html>. This is
clearly essential when looking for objects that are smaller than a
micrometer (1/1000 of a millimeter). Cosmochemists have developed some
slick techniques to extract small samples and prepare them for study by
electron microscopy and mass spectrometry. In the case of the Tagish
Lake study, Nakamura-Messenger and her colleagues selected small samples
about 100 micrometers in diameter from regions of the meteorite that did
not contain carbonate veins (a non-organic distraction to their study).
These samples were then mounted in molten elemental sulfur for further
processing. Sulfur was used because the usual glue is epoxy, an organic
compound--not good when studying organic compounds. Also, the sulfur
vaporizes in vacuum, so is easy to get rid of.

The mounted sample was then cut with a specialized diamond knife (called
a microtome) into ultra-thin slices only 50 to 70 nanometers across.
These were placed on a conductive grid for study by transmission
electron microscopy (TEM). When placed in a vacuum the sulfur vaporizes,
leaving the nano-slices on the TEM grid.

[How samples were prepared for analysis]
Schematic of the sample-preparation procedure used by Keiko
Nakamura-Messenger and her colleagues.

Examination in the TEM allowed the team to find the organic globules and
then to study their internal structures. Once TEM work was done, the
samples were transferred to a nano-SIMS, a specialized ion microprobe
used to obtain isotopic compositions at the nanometer scale. [See PSRD
article: Ion Microprobe
<http://www.psrd.hawaii.edu/Feb06/PSRD-ion_microprobe.html>.]

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

Globules and Their Anomalous Isotopic Composition

Using a TEM the team observed numerous hollow organic globules. The
average abundance was about one per 100 square micrometers, though the
globules often occur in groups of 2 to 5. The globules average 140 to
1700 nanometers in diameter, but the walls of the globules are only 100
to 200 nanometers thick. Nakamura-Messenger viewed the globules at high
magnification in the TEM and used a technique called electron loss
spectroscopy. These observations showed that the globule walls consist
of non-crystalline carbon (hence not graphite), with nitrogen, oxygen,
and hydrogen, consistent with them being organic compounds. The globule
walls do not contain any rocky materials. They are gooey organic stuff.

[globules]
TEM image (left) and carbon x-ray image (right) of a cluster of three
organic globules in the Tagish Lake meteorite. The globule at the left
in these photos labeled "G15-1" seems to be composed of smaller grains
within its wall (the arc of material surrounding a void). Images of the
isotopic composition (nitrogen and hydrogen) are shown below. The large
grain in the middle of globule G15-1 is forsterite (Mg2SiO4, the
magnesium end-member of the olivine solid solution series).

The nitrogen and hydrogen isotopic compositions of the globules are
significantly different from organic materials on Earth, in other
carbonaceous chondrites, and even in the bulk Tagish Lake meteorite. The
isotopic compositions are expressed as deviations from terrestrial
values of nitrogen-15/nitrogen-14 and deuterium/hydrogen. (Deuterium is
heavy hydrogen--hydrogen with an extra neutron, so it has an atomic
weight of 2, rather than 1 for hydrogen.) This is solid evidence that
the organic globules in Tagish Lake were not altered by chemical
reactions on the asteroid the meteorite came from. Equally important, it
is evidence that the globules are extraterrestrial. No contamination here.

[isotopic details of globules]
Maps of nitrogen and hydrogen isotopic compositions in globule G15-1
(see TEM image above). The scales indicate the deviation from
terrestrial values in parts per thousands. The globule shows significant
enrichments in nitrogen-15 relative to nitrogen-14 and in deuterium
(heavy hydrogen) relative to hydrogen. This is proof of extraterrestrial
origin of the globules and that they were not modified by chemical
reactions on the asteroid in which the Tagish Lake meteorite was made.

The nitrogen isotopic compositions of the globules are much larger than
most of the organic materials found in the Taglish Lake meteorite or in
other carbonaceous chondrites. This indicates that the globules were not
affected by processing on the Taglish Lake parent asteroid. In other
words, they were not contaminated or altered while sitting in an
asteroid. This important conclusion is strengthened by the observation
that globules located only a few micrometers from each other vary in
isotopic composition. Even mild aqueous processing of these globules
should have smoothed out those variations. On the other hand, globules
in direct contact have similar hydrogen and nitrogen isotopic
compositions, perhaps indicating that they became attached before being
incorporated into the Tagish Lake parent asteroid.

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

Cold Chemistry

Nakamura-Messenger and coauthors discuss a plethora of chemical
processes in interstellar space (specifically in cold molecular clouds)
and in the outer parts of the protosolar disk that could have led to the
formation of organic molecules and anomalies in the proportions of
nitrogen and hydrogen isotopes. They rule out nuclear reactions inside
stars because there are only modest anomalies in carbon isotopes. This
leaves chemical interactions at very low temperatures as the likely causes.

One of the interesting ideas is formation of small grains of water
containing dissolved carbon dioxide (CO2), ammonia (NH3), methyl alcohol
(CH3OH), and formaldehyde (H2CO). When subjected to ultraviolet
radiation (which is everywhere in space), these compounds react and form
complex organic compounds. It is intriguing that the penetration depth
of ultraviolet light is 100 to 200 nanometers, about the thickness of
the globule walls. Thus, exposure to ultraviolet light could form
organic compounds, leaving a core of impure ice left over. The particles
could have accreted into the Taglish Lake asteroid, and the ice would
have been lost when the asteroid heated up, causing the formation of
hydrated silicate minerals, carbonates, and other mineralogical features
seen in the meteorite, and leaving a void in the center of the globules.

Comet Halley, Giotto Mission However the globules formed, they probably
originated in the coldest part of the protosolar disk, where the Sun's
rays were dimmed by dust and distance, or even in the cold cloud in
which the Sun formed. Nakamura-Messenger and her colleagues point out
that similar organic globules should have been incorporated into comets.
In fact, chemically similar particles containing carbon, hydrogen,
oxygen, and nitrogen were detected in abundance by the European Space
Agency's Giotto Mission and the international Vega project (managed by
the Russian Space Research Institute) spacecraft that whizzed past comet
Halley in 1986. The particles detected had sizes in the range 40 to 2000
nanometers, similar to the Taglish Lake globules.

[Image of comet Halley taken by the European Space Agency's Giotto
spacecraft in 1986.]

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

Seeding the Earth?

The presence of grains in comet Halley with the same composition and
sizes as the organic globules in Taglish Lake lead Nakamura-Messenger
and her colleagues to suggest that organic globules might have been
common in prebiotic organic materials delivered to Earth very early in
its history. Perhaps the membrane-like structures were important
building blocks for life on Earth. Other research, such as that done by
Jennifer Blank (University of California, Berkeley) and her colleagues
show that organic materials will survive a high-velocity collision such
as those that take place when comets slam into planets. Not mentioned
explicitly by Nakamura-Messenger and coworkers is the possibility that
such organic structures were also incorporated into asteroids that never
heated up much, such as the main belt comets discovered by Henry Hsieh
and David Jewitt (University of Hawaii, Institute for Astronomy).

Maybe our ultimate ancestors were non-living globules in carbon-rich
asteroids and comets!

[images of comets]

Henry Hsieh and David Jewitt (University of Hawaii) discovered a new
class of comets, which they have dubbed "main-belt comets." The first
three they found are shown here, in the center of each image. The long
streaks are stars smeared by the motion of the Earth as the telescope,
located on Mauna Kea, Hawaii, tracked each comet. [Click on image for
additional information.]

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

ADDITIONAL RESOURCES

    * Blank, J. G., G. H. Miller, M. J. Ahrens, and R. E. Winans (2001)
      Experimental Shock Chemistry of Aqueous Amino Acid Solutions and
      the Cometary Delivery of Prebiotic Compounds. Origins of Life and
      Evolution of the Biosphere, v. 31, p. 15-51.
    * Hsieh, H. H. and D. Jewitt (2006) A Population of Comets in the
      Main Asteroid Belt. Science, v. 312, p. 561-563. (abstract
      <http://www.sciencemag.org/cgi/content/abstract/1125150>)
    * Mittlefehldt, D. W. (Dec. 2002) Tagish Lake -- A Meteorite from
      the Far Reaches of the Asteroid Belt. Planetary Science Research
      Discoveries. http://www.psrd.hawaii.edu/Dec02/TagishLake.html
    * Nakamura-Messenger, Keiko, Scott Messenger, Lindsay P. Keller,
      Simon J. Clemett, and Michael E. Zolensky (2006) Organic globules
      in the Tagish Lake Meteorite: Remnants of the proto-solar disk.
      Science, v. 314, p. 1439-1442.
    * Taylor, G. J. (April, 2000) Analyzing Next to Nothing. Planetary
      Science Research Discoveries.
      http://www.psrd.hawaii.edu/April00/analyzingSmall.html
Received on Thu 25 Jan 2007 05:59:05 PM PST