[meteorite-list] Getting to Know Vesta

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 28 Nov 2007 09:18:23 -0800 (PST)
Message-ID: <200711281718.JAA12027_at_zagami.jpl.nasa.gov>


Citation: Martel, L. M. V. (November, 2007). Getting to Know Vesta.
Planetary Science Research Discoveries.
http://www.psrd.hawaii.edu/Nov07/HEDs-Vesta.html (date accessed).

Getting to Know Vesta
Planetary Science Research Discoveries

--- Scientists are primed with geochemical data from HED meteorites for
Dawn's encounter with asteroid 4 Vesta.

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

The howardite-eucrite-diogenite class of meteorites (called the HEDs)
are rocks formed from basaltic magmas. What makes them special is that
the HEDs have reflectance spectra in the visible and near-infrared that
match spectra from asteroid 4 Vesta, implying Vesta is their parent
body. We will soon have new data from Vesta from NASA's Dawn orbiting
spacecraft, which carries a gamma ray and neutron detector, dubbed the
GRaND instrument. GRaND will orbit asteroid 4 Vesta and dwarf planet
Ceres and map the near-surface abundances of major and minor elements,
and volatiles found in ices (in the case of Ceres) such as hydrogen,
carbon, nitrogen, and oxygen. Tomohiro Usui and Harry Y. (Hap) McSween,
Jr. (University of Tennessee) have proposed a way to interpret the
upcoming GRaND data from Vesta based on well-analyzed samples of HED
meteorites and a mixing model they devised that uses element ratios of
the three expected rock types. In turn, the new data from Vesta may help
scientists better understand the geologic context for HED meteorites.


    * Usui, T. and McSween, Jr., H. Y. (2007) Geochemistry of 4 Vesta
      based on HED meteorites: Prospective study for interpretation of
      gamma ray and neutron spectra for the Dawn mission. Meteoritics
      and Planetary Science, v. 42, p. 255-269.

PSRDpresents: Getting to Know Vesta --Short Slide Summary
<http://www.psrd.hawaii.edu/Nov07/PSRD-HEDs-Vesta.ppt> (with accompanying notes).

Cosmochemistry of Vesta from Orbit

photo of GRaND instrument

How do you take the grand idea of sending a spacecraft out to explore
the asteroids and make it even better? Add a GRaND science instrument.
(Sorry. I love the fact that creating acronyms is a favorite diversion
for certain scientists.) The Gamma Ray and Neutron Detector instrument
(GRaND) is one of three science payload instruments on NASA's Dawn
Mission, which launched on September 27, 2007. GRaND will measure
elemental compositions of the surface of asteroid Vesta (beginning in
2011) and dwarf planet Ceres (in 2015).

GRaND uses 21 sensors to measure the energy from gamma rays and neutrons
that are reflected or emitted by the different elements on the target
body's surface and down to a depth of one meter. Scientists will make a
global elemental map of Vesta to show where elements exist and in what
abundances. They are particularly interested in mapping major elements
(Si, Fe, Ti, Mg, Al, Ca) and minor elements (K, Th, U) on Vesta. The
beauty of the minor elements is that they track the melting that led to
the formation of the crust of the body during differentiation.

The GRaND instrument onboard NASA's Dawn spacecraft is 26 x 19 x 26
centimeters, see image on the left. This device was developed and is run
by scientists from the Los Alamos National Laboratory. It consists of 21
sensors for high resolution gamma ray spectroscopy and neutron
detection. It also carries analog and digital signal processing
electronics, and low- and high-voltage power supplies.

Our Knowledge about Vesta's Rocks

What we know about Vesta, let's call it pre-Dawn knowledge, is assembled
from ground-based and Earth-orbiting telescopic spectra of its surface.
Vesta is a dry, differentiated
body that resembles the rocky bodies of the inner solar system,
including Earth. Vesta's surface is basaltic, rich in the minerals pyroxene
and plagioclase (see the reflectance spectrum below, right).

Hubble map of Vesta. Click for more details.
reflectance spectrum of Vesta from Earth-based telescope
On the LEFT is a shaded image of Vesta created from Hubble
Space Telescope topographic data. The gold circle superimposed on the
image shows the best spatial resolution (~300 kilometers in diameter)
GRaND will achieve during its low-altitude mapping orbit. On the RIGHT
is a spectrum of Vesta obtained at the NASA Infrared Telescope Facility
on Mauna Kea, Hawaii by scientists in France using remote control
networks from the Observatoire de Paris-Meudon. In particular, it is the
presence of the 0.9 and 1.9 micrometer absorption bands for pyroxene in
the spectra of Vesta that match spectra of the HED meteorites.

Based on Vesta's pyroxene mineralogy and what they know about achondrite
meteorites, scientists expect Vesta's
surface to be a mixture of three rock types: diogenites, basaltic
eucrites, and cumulate eucrites. First a little word about these types
and how they form. They are rocks, rich in pyroxene, that crystallized
from magmas. Different minerals crystallize at different times during
cooling in a magma chamber. Mineral crystals that form early and are
more dense than the surrounding magma can sink and form piles on the
floor of a magma chamber. Rocks formed from these piles of accumulated
crystals are called cumulates. So when you see a cumulate rock you know
it's a product of an over abundance of a particular mineral, such as
pyroxene. Diogenites are such a rock; they are cumulates that formed at
depth, made mostly of magnesium-rich, calcium-poor orthopyroxene.
Eucrites have basaltic compositions with iron-rich pyroxene and
sodium-poor plagioclase and may have formed from surface or near-surface
lavas or as cumulates. Scientists think some basaltic eucrites were
later metamorphosed by heat. Together
with howardites, which are breccias of fragments of eucrites and
diogenites, these rock types are known collectively as the
howardite-eucrite-diogenite meteorite class or HEDs.

Once the Dawn spacecraft reaches Vesta, GRaND data will be used to make
global maps of elemental abundances. The best spatial resolution will be
about 300 kilometers per pixel (see Vesta image above). This is larger
than the ~50-kilometer resolution Hubble data that already showed
variability in composition of Vesta's surface. And it is far larger than
the centimeter-size compositional variations in HED meteorites. Because
of GRaND's coarse spatial resolution, Usui and McSween expect, and are
prepared to interpret, data that mimic the mixing of HED meteorites.

Mixing Model

Usui and McSween developed a way to evaluate the contribution of
different HED rock types --diogenites, basaltic eucrites, and cumulate
eucrites-- to the GRaND spectra. The table below provides microscope
images, at the same scale, taken in cross-polarized light of
thin-sections of these rock types. Each image links to additional
information from the Meteoritical Bulletin Database
<http://tin.er.usgs.gov/meteor/metbull.php> maintained by the
Meteoritical Society <http://www.meteoriticalsociety.org/>.

Thin section of Johnstown meteorite. Click for more information.
Type: Diogenite Name: Johnstown Source: Observed fall in United States,
1924. Description: Johnstown is a typical diogenite (cumulate rock
formed at depth) composed almost entirely of coarse-grained
orthopyroxene. Thin section of Pasamonte. Click for more information.
Type: Basaltic Eucrite Name: Pasamonte Source: Observed fall in United
States, 1933. Description: Pasamonte is a basaltic eucrite. Its
fine-grained texture indicates the rock cooled quickly on the surface.
The needle-shaped minerals are plagioclase. The larger crystals are
pyroxene. Thin section of Serra de Mage meteorite. Click for more
Type: Cumulate Eucrite Name: Serra de Mag?? Source: Observed fall in
Brazil, 1923. Description: Serra de Mag?? is a cumulate eucrite
consisting almost entirely of pyroxene, which changed into a mixture of
two different compositions as the rock cooled. You can see this as
colored needles within the grey pyroxene grains. The other crystals in
this image are plagioclase.

The research team decided to start with two mixing models using a
diogenite named Shalka, the cumulate eucrite named Serra de Mag??, and
either one of two different basaltic ecurites named Stannern or Nuevo
Laredo. Stannern and Nuevo Laredo, though basaltic ecurites, plot
separately on graphs of Mg# (Mg/(Mg+Fe) ratio) versus TiO2 which leads
cosmochemists to think these rocks formed in different ways. One simple
explanation (there are others) is Stannern formed by the primary,
partial melting of the interior of Vesta, and Nuevo Laredo by fractional
crystallization of a cooling magma body. They expect the chemical
compositions on the surface of Vesta to be mixtures of the components of
these rocks.

An interesting aspect of this approach is that the mixing relations of
the three end-member rock types can be plotted in an appropriate
two-dimensional diagram. The mixing model devised by Usui and McSween is
based on molar ratios of the elements, magnesium, iron, aluminum, and
silicon. They plot (Mg+Fe)/Si abbreviated as [M/Si]mol and Al/Si
abbreviated as [Al/Si]mol. This plot, below, shows how the three rock
types are well discriminated compositionally. Diogenites are plotted in
orange, cumulate eucrites in green, and basaltic eucrites in purple.

plot of compositional differences between HED meteorites
Graph showing the compositional differences between HED meteorites based
on molar ratios of magnesium, iron, aluminum, and silicon. Arrows point
to the specific data points for Shalka, Serra de Mag??, Nuevo Laredo, and
Stannern. Also plotted are polymict eucrites (labeled P-Eucrite) and
Howardites, which are rocks that are breccias or mixtures of eucrite and
diogenite fragments.

The figure, above, helps to show that the end member rocks define a
space wherein all the HEDs plot. Using the three end members, Usui and
McSween delineated mixing lines in the plot to create a mixing model for
the upcoming GRaND data. Next, to show the accuracy of their model, Usui
and McSween compared their modeled compositions with the actual, mean
compositions for the meteorites based on concentrations of oxides of Si,
Fe, Ti, Mg, Al, Ca, Cr, Mn, Na, and K. The next plot shows the excellent
agreement between these compositions. The mixing model can accurately
estimate the abundances of all the major elements that can be measured
by GRaND as well as predict abundances of minor elements (Na, Cr, and
Mn) not analyzed by the instrument.

Log-Log plot of measured vs. mixing concentrations to show accuracy of
mixing model
This is a plot of the concentrations of oxides of Si, Fe, Ti, Mg, Al,
Ca, Cr, Mn, Na, and K measured in HED meteorites versus their
concentrations calculated from the mixing model developed by Usui and
McSween. The dashed line represents perfect agreement between the two.
In general the agreement is excellent. The diagram is plotted on a
logarithmic scale to easily show all the elements on one diagram (K is
lowest and Si is highest). Elements are not labeled to avoid cluttering
the diagram.

If Vesta's surface chemistry is analogous to HED meteorites as expected,
then the Usui and McSween HED-based mixing model will help determine the
proportion of different rock types represented in the GRaND spectral
data. The researchers say combining GRaND data with topographic data
would yield further information on stratigraphic chemical variations.
For example, finding evidence for the mineral olivine in the huge impact
crater (460-kilimeters diameter) near Vesta's south pole would give
information about the asteroid's mantle. Knowing this, Usui and McSween
created another mixing model to show that it is possible to estimate the
abundance of olivine from elemental data obtained by GRaND. Data from
the visible and infrared mapping spectrometer onboard the Dawn
spacecraft will further help to interpret the mineralogy on Vesta. As
Usui and McSween point out, the new orbital data could help constrain
the geological context for HED meteorites and provide new insight into
the geologic history of the asteroid. If GRaND sends back spectra that
do not mimic HED rock compositions, then that could mean we are seeing a
rock type on the asteroid that is not yet in our meteorite collections.

No matter what, new cosmochemical data will help us get to know Vesta
better. The use of the vast amount of data on HED meteorites to devise a
mixing model to understand data from a spacecraft orbiting Vesta shows
the value of an integrated approach to solar system exploration. Both
laboratory analysis of samples like that funded by the Cosmochemistry
Program and remote sensing data like that to be radioed back by the Dawn
mission are crucial bits of information about the igneous histories of
the building blocks of the planets.



    * PSRDpresents: Getting to Know Vesta --Short Slide Summary
      <http://www.psrd.hawaii.edu/Nov07/PSRD-HEDs-Vesta.ppt> (with accompanying notes).

    * Dawn Mission homepage <http://dawn.jpl.nasa.gov/index.asp>
    * Keil, K. (2002) Geological History of Asteroid 4 Vesta: The
      Smallest Terrestrial Planet. In Asteroids III, (eds. W. F. Bottke,
      Jr., A. Cellino, P. Paolicchi, and R. P. Binzel.) University of
      Arizona Press, Tucson, AZ, p. 573-589.
    * Krot, A. N., Keil, K., Scott, E. R. D., Goodrich, C. A., and
      Weisberg, M. K. (2007) Classification of Meteorites. In Treatise
      on Geochemistry, v. 1.05 (eds. H. D. Holland and K. K. Turekian.)
      Elsevier Ltd., 52 p.
    * Prettyman, T. H. and 16 others (2006) Gamma Ray and Neutron
      Spectrometer for Dawn. 37th Lunar and Planetary Science Conference
      abstract 2231
    * Treiman, A. H., Lanzirotti, A., Xirouchakis, D. (2004) Ancient
      Water on Asteroid 4 Vesta: Evidence from a Quarz Veinlet in the
      Serra de Mag?? Eucrite Meteorite. Earth and Planetary Science
      Letters, v. 219, p. 189-199.
    * Usui, T. and McSween, Jr., H. Y. (2007) Geochemistry of 4 Vesta
      based on HED meteorites: Prospective study for interpretation of
      gamma ray and neutron spectra for the Dawn mission. Meteoritics
      and Planetary Science, v. 41, p. 255-269.
    * Vernazza, P., Moth?? -Diniz, T., Barucci, M. A., Birlan, M.,
      Carvano, J. M., Strazzulla, G., Fulchignoni, M., and Migliorini,
      A. (2005) Analysis of near-IR spectra of 1 Ceres and 4 Vesta,
      targets of the Dawn Mission. Astronomy and Astrophysics, v. 436,
      p. 1113-1121.
Received on Wed 28 Nov 2007 12:18:23 PM PST

Help support this free mailing list:

Yahoo MyWeb