[meteorite-list] Melted Crumbs from Asteroid Vesta

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 18 Sep 2007 12:28:22 -0700 (PDT)
Message-ID: <200709181928.MAA05730_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Sept07/cosmicSpherules.html

Melted Crumbs from Asteroid Vesta
Planetary Science Research Discoveries
September 17, 2007

--- Researchers studying some of the rarest of the smallest meteorites
call them melted crumbs from asteroid Vesta.

Written by Linda M. V. Martel
Hawai'i Institute of Geophysics and Planetology

Micrometeorite bombardment accounts for almost 30,000 tons of material
entering Earth's atmosphere each year. Though most of the material
evaporates during entry or is lost to sea or falls on the land
unnoticed, thousands of micrometeorites have been collected successfully
from deep-sea sediments and from the snow and ice of the polar caps.
Susan Taylor (Cold Regions Research and Engineering Laboratory) and
colleagues collected micrometeorites with an ingeniously designed robot
from a decidedly out-of-the-way place: Amundsen-Scott South Pole Station
water well. She and Greg Herzog and Jeremy Delaney (Rutgers University)
selected 10 out of thousands of these extraterrestrial particles, 75 to
700 micrometers in size, because of their unusual shapes and mineralogy,
and measured the Fe/Mn and Fe/Mg elemental ratios, which are known to
help constrain the type and source of meteorites. The results show that
nine of the cosmic spherules are broadly chondritic in composition as
expected. However, one, along with six others reexamined from a previous
study, are atypical with nonchondritic compositions. Taylor and
coauthors propose an origin from an achondrite,
Howardite-Eucrite-Diogenite (HED)-like parent body such as asteroid
Vesta. HED-like objects account for about 6% of all meteorites, and only
about 0.5% of all micrometeorites perhaps because of a natural
mechanical toughness that would resist breakup and fragmentation.

Reference:

    * Taylor, S., Herzog, G. F., and Delaney, J. S., 2007, Crumbs from
      the Crust of Vesta: Achondritic Cosmic Spherules from the South
      Pole Water Well, Meteoritics and Planetary Science, v. 42, p. 223-233.

PSRDpresents: Melted Micrometeorites --Short Slide Summary
<http://www.psrd.hawaii.edu/Sept07/PSRD-cosmicSpherules.ppt> (with accompanying notes).

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

Concentrations of Cosmic Debris

What better way to handle the summer heat than with a cool little story
about ice ...Antarctic ice, that is, studded with the most minuscule
grains from the cosmos. Researchers are studying extraterrestrial
materials that are literally particles and spherules less than a
millimeter in size but whose combined mass accounts for about 1,000 tons
of new stuff added to Earth yearly. A micrometeorite is generally
defined as a tiny meteorite larger than 50 micrometers but smaller than
a millimeter. Micrometeorites that have melted, either partially or
completely when plunging through Earth's atmosphere, are called cosmic
spherules. RADARSAT-1 satellite mosaic of Antarctica, Canadian Space
Agency <http://apod.nasa.gov/apod/ap991116.html> Just as the Antarctic
blue ice serves as an ideal collector of meteorites (see PSRD articles:
Meteorites on Ice <http://www.psrd.hawaii.edu/Nov01/metsOnIce.html> and
Searching Antarctic Ice for Meteorites
<http://www.psrd.hawaii.edu/Feb02/meteoriteSearch.html>) it also preserves
micrometeorites and cosmic spherules that land on the surface and are
subsequently incorporated into ice layers. In his book, Meteorites, Ice,
and Antarctica: A Personal Account, William Cassidy (University of
Pittsburgh, and founder of the U. S. Antarctic Search for Meteorites
project, ANSMET) retells a detail from the thrilling story of Paul
Siple, the Boy Scout on Admiral Richard Byrd's expeditions to Antarctica
in 1928-1930 and 1933-1935. One day Siple collected a jar of grains from
the bottom of an icy cavity where the crew had melted ice beneath the
surface at one of the base camps for drinking water. Though no one knows
where his jar is now, Siple's collection no doubt contained
micrometeorites mixed with rock bits entrained in the glacier and
volcanic debris from past eruptions of nearby volcanoes. Today's
researchers are using modern systematic collections of micrometeorites
and cosmic spherules from the Amundsen-Scott South Pole water well to
make new discoveries in cosmochemistry and gain insights into the origin
of the Solar System.

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

Collecting Cosmic Debris

The South Pole Water Well is a 4000-cubic-meter subsurface water pool,
100 meters below the surface. It has supplied the drinking water to the
U. S. Amundsen-Scott South Pole Station in Antarctica since January
1995. This well is one of the largest sources of micrometeorites --
cosmic particles less than one millimeter -- exploited to date. The well
itself acts to concentrate the micrometeorites because the large pool
volume and low circulation rates allow the particles to form a lag
deposit on the bottom of the well. There is a small well house on the
surface that houses the drilling and operating equipment. An adjacent
hole and shelter were constructed in 1997 next to the well house
expressly for the collection of the micrometeorites. The
remote-controlled robotic collector (designed and built at the Cold
Regions Research and Engineering Laboratory in Hanover, New Hampshire)
is lowered down a 30-centimeter-diameter well neck to the icy bottom of
the well pool where it maneuvers to suction, filter, and collect the
micrometeorites and cosmic spherules without contaminating the well or
water.

[South Pole Water Well surface housings]

These shelters (winch room and laboratory) at the South Pole Water Well
house the tower assembly, winch and cable system, video equipment, and
control equipment needed to support the remote-controlled collector.

After initial electron microprobe analyses of the micrometeorites and
cosmic spherules collected from the well, the research team chose ten
for further study because of their unusual shapes or mineralogy.
Scanning electron microscope images of the ten cosmic spherules are
shown below.

[SEM images of spherules]
Scanning electron microscope images of cosmic spherules analyzed by
Taylor, Herzog, and Delaney.

SP37-1 is a tiny glass particle with an elongated shape. SP37-2 and
SP37-5 are glass spherules with vesicles that were filled with
terrestrial oxide grains after landing on Earth. SP37-3 is another glass
spherule with vesicles (black areas) with an unusual plagioclase
feldspar (An90) grain, which appears as a non-circular dark grey shape
in the lower part of the spherule. Anorthite has not previously been
reported in cosmic spherules. SP37-4 is a glass spherule with no
vesicles but a fractured edge. SP37-6 is a glass spherule with two
(bright) metal regions and (darker grey) olivine grains. SP37-7 is a
glass spherule with a (bright) bead of iron sulfide. SP37-9 is a
vesicular fragment of a larger spherule. It contains some iron sulfide
grains and regions that did not completely melt. SP37-10 and SP37-11 are
glass spherules with parts dissolved away, leaving etched surfaces.
------------------------------------------------------------------------

Analyses

Susan Taylor and colleagues, Greg Herzog and Jeremy Delaney, measured
the concentrations of major and minor elements, including iron (Fe),
manganese (Mn), and magnesium (Mg) of 10 cosmic spherules collected from
the South Pole water well. Theirs is one of the first studies to use
Fe/Mn and Fe/Mg ratios on micrometeorites. These particular ratios have
been shown by previous researchers to be diagnostic tools for
distinguishing achondrites from chondrites and have been used to help
determine where the meteorites came from.

[graph of Fe/Mn and Fe/Mg ratios for cosmic spherules]

This plot of Fe/Mn and Fe/Mg ratios shows the results for the ten cosmic
spherules analyzed by Taylor and coauthors and for numerous cosmic
spherules from previous analyses. About two-thirds of all the data
points plot in the range typical of chondrites (shaded, triangular
region) as expected. Significantly, a group of seven cosmic spherules
plot below the chondritic region (these data points are indicated in
blue). The horizontal dashed line labeled "Lunar" represents ratios of
lunar samples. The horizontal dahed line labeled "HED" represents ratios
of HED (Howardite, Eucrite, Diogenite) achondrite meteorites.

Seven cosmic spherules, one from the current study (SP37-3) and six
others reexamined from a previous study, have Fe/Mn and Fe/Mg ratios
(plotted in blue) that lie well below the shaded chondritic region in
the figure (shown above). These data points plot near the values typical
of HED (basaltic) meteorites (see the dashed line) but clearly not of
lunar samples (see the upper dashed line).

Taylor and colleagues also considered whether or not the cosmic
spherules are like basaltic Martian meteorites. They found that the
mineralogies did not match, yet were not so far off as to rule out a
Martian origin. Given that HEDs outnumber Martian meteorites by about 10
to 1, the authors argue the seven noncondritic cosmic spherules are more
likely to have come from a HED-like parent body. Trace element and
oxygen isotope analyses of the seven cosmic spherules would give
researchers even more information to better determine the sources of the
spherules.

Another nod to an HED-like parent body is the plagioclase feldspar grain
found in cosmic spherule SP37-7. Plagioclase feldspar ranges in
composition from calcium aluminum silicate (anorthite) to sodium
aluminum silicate (albite). Taylor and colleagues found the anorthite
relic grain in SP37-7 is much more calcium-rich (An90, which means 90%
calcium in the sodium/calcium position in the crystal structure) than
found in shergottite Martian meteorites, but is comparable to anorthite
(An72-90) commonly found in HED meteorites. cosmic spherule SP37-3 In
the scanning electron microscope image shown here, the plagioclase
feldspar grain is labeled "Pg".

Micrometeoroids are susceptible to mass loss and occasionally perhaps to
mass gain when passing through Earth's atmosphere. How do we know that
the measured concentrations of Fe, Mn, and Mg represent true
concentrations unaltered by the heat during passage through Earth's
atmosphere? Fortunately, as the authors point out, Mg is fairly
refractory, which means it is not easily lost by evaporation. Also, the
relatively constant ratios of Mn to other refractory elements (such as
Ca and Al) measured by Taylor and coauthors imply Mn is not lost either.
Iron, by contrast, is likely to be lost when cosmic spherules burn
through Earth's atmosphere. Loss of iron could occur by evaporation or,
more frequently, by physical separation of metal or sulfide. Loss of
iron would shift the data points toward the orign, along rather than off
the main trend (shaded region), and would not ruin the distinction we
see between chondritic and achondritic spherules. So, the authors are
confident that their Fe/Mg and Fe/Mn ratios represent the true
mineralogy of the spherules.

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

Crumbs from the Crust of Vesta

Taylor and colleagues show the Fe/Mg and Fe/Mn ratios of the cosmic
spherules are similar enough to HED meteorites that a HED-like parent
body is likely. The HED meteorites are a class of achondrites that are
igneous rocks formed from basaltic magmas. This class represents about
6% of meteorites that fall to Earth and only about 0.5% of all
micrometeorites from the South Pole water well. There are far fewer of
these HED-like micrometeorites and cosmic spherules than carbonaceous
chondrites. The authors infer that a natural mechanical toughness of
HED-like material would resist breakup and fragmentation, whereas the
comparative mechanical weakness of carbonaceous material would tend to
favor breakup and spherule formation.

The HEDs are the only class of meteorites whose spectral data have been
matched (but not without some ongoing debate) with spectra from a
potential parent body. That parent body is asteroid 4 Vesta. Hubble
Space Telescope data show the asteroid has a basaltic surface and a
giant impact crater near one pole. Isn't it remarkable that crumbs
measured in micrometers can tell us stories about 500-kilometer-diameter
asteroids?

[Vesta, click for more information]
<http://hubblesite.org/newscenter/archive/releases/1997/27/image/a/>

NASA's Hubble Space Telescope took images of asteroid Vesta in 1996.
This shows a computer model of Vesta created from Hubble
<http://hubblesite.org/newscenter/archive/releases/1997/27/image/a/>
topographic data. We'll know more about Vesta soon when NASA's Dawn
Mission <http://dawn.jpl.nasa.gov/> investigates Vesta and Ceres, two of
the largest protoplanets in the main asteroid belt.

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

ADDITIONAL RESOURCES

    * PSRDpresents: Melted Micrometeorites --Short Slide Summary
      <http://www.psrd.hawaii.edu/Sept07/PSRD-cosmicSpherules.ppt> (with accompanying notes).
    * Binzel, R. P. and Xu, S. (1993) Chips off of Asteroid 4 Vesta:
      Evidence for the parent body of basaltic achondrite meteorites.
      Science, v. 260, p. 186-191.
    * Cassidy, W. A., 2003, Meteorites, Ice, and Antarctica: A Personal
      Account. Cambridge University Press, 349 p.
    * Floss, C., 2003, QUE 93148: A Part of the Mantle of Asteroid 4
      Vesta? Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/Jan03/QUE93148.html
    * Love, S. G., and Brownlee, D. E., 1993, A Direct Measurement of
      the Terrestrial Mass Accretion Rate of Cosmic Dust, Science, v.
      262, p. 550-553.
    * Martel, L. M. V., 2001, Meteorites on Ice. Planetary Science
      Research Discoveries. http://www.psrd.hawaii.edu/Nov01/metsOnIce.html
    * Martel, L. M. V., 2002, Searching Antarctic Ice for Meteorites.
      Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/Feb02/meteoriteSearch.html
    * Maurette M., Olinger, C., Michel-Levy, M. C., Kurat, G., Pourchet,
      M., Brandstatter, F., and Bourot-Denise, M., 1991, A Collection of
      Diverse Micrometeorites Recovered from 100 Tonnes of Antarctic
      Blue Ice, Nature, v. 351, p. 44-47.
    * Taylor, S., Lever, J. H., and Harvey, R. P., 1998, Accretion Rate
      of Cosmic Spherules Measured at the South Pole, Nature, v. 392,
      p.899-903.
    * Taylor, S., Lever, J. H., and Harvey, R. P., 2000, Numbers, Types,
      and Composition of an Unbiased Collection of Cosmic Spherules,
      Meteoritics and Planetary Science, v. 35, p. 651-666.
    * Taylor, S., Herzog, G. F., and Delaney, J. S., 2007, Crumbs from
      the Crust of Vesta: Achondritic Cosmic Spherules from the South
      Pole Water Well, Meteoritics and Planetary Science, v. 42, p. 223-233.
    * Website Atlas of Micrometeorites
      <http://remf.dartmouth.edu/micrometeorites/>, from Dartmouth
      College. Scanning Electron Microscope images of 234
      micrometeorites collected from the bottom of the South Pole water
      well.
    * Website Catalog of Micrometeorites
      <http://dust.cc.gakushuin.ac.jp>, from the National Institute of
      Polar Research, Japan.
Received on Tue 18 Sep 2007 03:28:22 PM PDT