[meteorite-list] The Composition of Asteroid 433 Eros

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:02:09 2004
Message-ID: <200203010553.VAA23422_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Feb02/eros.html

The Composition of Asteroid 433 Eros

     --- X-rays and reflected light suggest that asteroid 433
     Eros is similar in composition to the most common type of
     meteorite--maybe.

Written by G. Jeffrey Taylor
Hawaii Institute of Geophysics and Planetology
February 26, 2002

The Near-Earth Asteroid Rendezvous (NEAR) mission spent about a year
orbiting the asteroid 433 Eros, a 33 x 13 x 13 km chunk of rock. The main
goal of the mission was to determine the chemical and mineral make up of the
asteroid and to try to settle an argument about the nature of S-asteroids.
S-asteroids are a somewhat diverse group of little planets with similar
characteristics in the spectra of light reflected from them. The consensus
was that they are mixtures of iron-magnesium silicates with some metallic
iron, but that is where the agreement ended. Some scientists argued that
S-asteroids are like ordinary chondrite meteorites, which are unmelted rocks
left over from when the solar system formed. Others argued just as
vigorously that S-asteroids are differentiated objects, little worlds that
were melted soon after they formed. Using measurements of x-rays emitted
from the asteroid and light reflected off it, the consensus is that Eros is
more like an ordinary chondrite than any other type, though a little bit of
melting cannot be ruled out. This measurement of one S-asteroid, however,
has not settled the argument. More asteroids need to be visited and samples
returned from them.

     References:

     Clark, Beth E. and 11 others (2001) Space weathering on Eros:
     Constraints from albedo and spectral measurements of Psyche crater.
     Meteoritics and Planetary Science, vol. 36, p. 1617-1637.

     McCoy, Timothy J. and 16 others (2001) The composition of 433 Eros: A
     mineralogical-chemical synthesis. Meteoritics and Planetary Science,
     vol. 36, p. 1661-1672.

     Nittler, Larry R. and 14 others (2001) X-ray fluorescence measurements
     of the surface elemental composition of asteroid 433 Eros. Meteoritics
     and Planetary Science, vol. 36, p. 1673-1695.

     McFadden, Lucy A. and 7 others (2001) Mineralogical interpretation of
     reflectance spectra of Eros from NEAR near-infrared spectrometer low
     phase flyby. Meteoritics and Planetary Science, vol. 36, p. 1711-1726.

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

The importance of asteroids

Asteroids are planets whose growth was stunted when the solar system formed.
This probably happened because of the rapid assembly of Jupiter, the largest
planet. Jupiter's huge gravity field would have distorted the orbits of
objects in the asteroid belt between Mars and Jupiter, preventing formation
of a large planet. Instead, we ended up with a plethora of tiny planets.
This turns out to be a lucky thing because asteroids preserve a record of
the processes that operated as the solar system was forming from a cloud of
gas and dust. They are also a natural laboratory for studying what happens
when small bodies melted, sometimes forming cores and mantles like the
Earth. Asteroids give us clues to the nature of the planetesimals that
accumulated to form the planets. Besides providing this scientific treasure,
asteroids might someday provide other kinds of treasure-- resources for
space settlers throughout the solar system.

We have a huge number of samples of asteroids in the form of meteorites.
These fascinating stones that fall from space contain lots of great
information about the formation of the solar system and how small bodies
melted. The only problem is that we do not know which asteroid each type of
meteorite comes from, with one exception. The exception is a group of
meteorites called howardites, eucrites, and diogenites (nicknamed HED
meteorites). The HED meteorites apparently come from asteroid 4 Vesta. All
the other types of meteorites come from somewhere out there, but we do not
know where.

Asteroid specialists have tried to establish a link between some types of
meteorites and specific asteroids or group of asteroids. Asteroids can be
classified astronomically by the nature of the light reflected from them.
One group of asteroids, the S-asteroids, has been the subject of spirited
debate during the past two decades. One school of thought identified them as
related to ordinary chondrites, the most common type of meteorite observed
to fall on Earth. Chondrites were not melted after their components accreted
to form asteroids and, thus, contain a wealth of information about the gas
cloud surrounding the primitive Sun. The spectra of light reflected from
S-asteroids does not match the spectra of chondrites measured in the
laboratory, but scientists who advocate that S-asteroids are chondrites
appeal to space weathering. Cynics might say that there is nothing like a
mysterious process to help save your idea from the harsh reality of data,
but we know from studies of the Moon that the spectral properties of
planetary materials are changed by exposure to solar wind and micrometeorite
impacts. We do not know much about space weathering on asteroids.
Understanding it better was a goal of the NEAR mission.

A second school of thought interprets S-asteroids as being small planets
that were chemically processed to varying extents. Some might have melted so
much that they formed cores and mantles. Others might have melted partially
to form surfaces covered with lava flows. Others might have melted only a
tiny amount, causing modest redistribution of rock and metallic iron and
iron sulfide. Some S-asteroids have spectra similar to some partially melted
meteorite groups, but not all do.

So, we have a dilemma: Two schools of thought arguing over ambiguous data.
The spectra of reflected light cannot be interpreted in any straightforward
way. (The record of the debate about S-asteroids and meteorites suggests,
however, that the spectral observations can be over-interpreted in
imaginative ways!) What we need is chemical compositions of the surface
materials and new ways of determining the identities and abundances of the
minerals present on the surface. NEAR-Shoemaker gave us chemical information
and improved spectral observations.

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

The NEAR-Shoemaker Mission

NEAR is one of the better acronyms in NASA's acronym-rich history. It stands
for Near-Earth Asteroid Rendezvous. In March 2000 the spacecraft was named
after Eugene Shoemaker, a brilliant, insightful planetary geologist and
great man. Gene Shoemaker showed us that asteroid impact is an important
process that shaped the planets. He also established the lunar geological
time scale, which allowed us to date the relative ages of features on the
Moon. The mission became known as the NEAR-Shoemaker mission.

The spacecraft was sent to asteroid 433 Eros, which is one of numerous
asteroids whose orbits cross that of Earth. These are relatively easy to get
to, therefore requiring less fuel than do asteroids way out in the main belt
between Mars and Jupiter.

NEAR-Shoemaker carried several instruments, but two are central to
determining the mineralogical and chemical composition of the asteroid: the
x-ray gamma-ray spectrometer (XGRS for short) and the NEAR infrared
spectrometer (NIS). The XGRS is really two instruments, one that detects
x-rays generated by high-energy x-rays from the Sun, and another that
detects gamma rays generated in the asteroids by cosmic rays. Both give
elemental abundances, but during the mission the gamma ray spectrometer had
some difficulties and did not return useful data from orbit. The x-ray
spectrometer returned the abundances of magnesium, aluminum, silicon, iron,
calcium, and sulfur.

The NIS was a spectrograph that used a diffraction grating to disperse the
infrared light reflected from the asteroid onto two electronic detector
arrays. The instrument recorded the intensity of light reflected at 64
different wavelengths between 804 and 2732 nanometers. It was able to
measure rectangular regions on the surface of about 650 x 1300 meters, or
square regions 1300 x 1300 meters. Because different minerals have specific
spectral characteristics, the NEAR team hoped that they could identify the
types of minerals present on the surface.

NEAR-Shoemaker also carried a color camera called the multispectral imager
(MIS). The MIS provides visible and near-infrared images of the asteroid
surface, with a spatial resolution of 10x16 meters from 100 kilometers away.
It has an eight-position filter wheel covering the spectral range from 450
to 1100 nm (visible to near infrared).

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

Chemical composition of Eros

Data from the x-ray spectrometer consists of counts of x-rays with different
energies. It takes a heroic effort to convert these to percentages of
elements (see article by Nittler and others). In fact, it is almost
impossible to obtain reliable estimates of elemental abundances. Instead,
the NEAR team used ratios of elements. These work just as well for most
purposes.

The diagrams below show the some of the NEAR data (aluminum/silicon vs.
calcium/silicon, and magnesium/silicon vs. iron/silicon) as circles with
symbols for numerous types of meteorites. The circles represent variations
in the measurements of different regions on Eros (large circles) or the
uncertainty in the mean of the data (small circles). Meteorites that formed
by melting on their parent asteroids do not match the composition determined
for Eros. The HED meteorites (howardites, eucrites, and diogenites) are
basalt lava flows (eucrites), coarse-grained igneous rocks (diogenites), or
mixtures of the two (howardites). They are very different in composition
from Eros. Angrites are basaltic meteorites with substantial amounts of
calcium, making them clearly different from Eros. Pallasites and
mesosiderites are mixtures of rock and metallic iron, and their compositions
plot far from the Eros analyses. Ureilites are complicated carbon-bearing
rocks that appear to be the residues from partial melting of an asteroid.
Their compositions, especially their high Mg/Si, rule them out as candidates
for Eros. So, all those meteorites can be ruled out on compositional
grounds. This means that Eros did not undergo a global melting event.

The data imply that Eros is somewhat primitive in composition. Chondritic
meteorites are good candidates, and the ordinary chondrites (H, L, and LL
types) consistently fall inside the Eros bull's eye on the diagrams. The
compositions of enstatite (E) chondrites and two types of carbonaceous
chondrites (CV and CO) are not as consistent and can probably be ruled out.
R chondrites (another type of carbonaceous chondrite) fall within or close
to the Eros ranges and remain viable candidates. There are other types of
fairly primitive meteorites and their compositions are generally consistent
with that of Eros. These include acapulcoites and lodranites, which come
from the same asteroid, winonaites, and brachinites. All have been melted to
varying amounts, but not so much that their compositions are drastically
altered from that of chondrites.

One measurement surprised the NEAR team. The sulfur/silicon ratio is far
below that of ordinary chondrites (see diagram below). Does this rule out
ordinary chondrites? Certainly not--we can always appeal to space
weathering. The surface of airless bodies like asteroids, the Moon, and
Mercury are bombarded with solar wind ions and micrometeorites. Nittler
(Carnegie Institution of Washington) and others calculate that the upper 5
cm of Eros could have lost all its sulfur in 10 million years. Since the
x-rays used for chemical analysis come from the upper 100 micrometers, we
would expect a low sulfur/silicon ratio.

          [asteroid Eros]
          Although most chemical parameters provide good fits
          between ordinary chondrites and Eros, the x-ray
          spectrometer measurements indicate that sulfur is much
          lower than in ordinary chondrites. This might be due to
          space weathering effects involving the volatilization
          of sulfur, or partial melting that removed sulfur-rich
          melts from the body.

There are other explanations for the low sulfur on Eros, and Nittler and
colleagues review them. If it is not just a surface effect, and the entire
body is depleted in sulfur, perhaps it simply formed that way. However, we
have no primitive extraterrestrial material that contains so little sulfur.
This explanation is unlikely. Alternatively, the sulfur might have been lost
through partial melting. As mixtures of iron, nickel, and sulfur are heated,
the first melt to form is very rich in sulfur, containing about 30 wt%.
Lodranites, which are the residues of partial melting, have very low
sulfur/silicon ratios. They lost their sulfur by migration of the first,
sulfur-rich liquid to form. Nittler argues that it might not be easy to lose
the sulfur-rich melt without melting the silicate, too. However,
calculations by my colleagues Lionel Wilson and Klaus Keil show that
sulfur-rich melts might be driven to the surface so fast that they would
escape an asteroid. For now, the NEAR team favors the space weathering
explanation.

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

Mineralogical composition of Eros

Most minerals reflect visible and infrared light in unique ways. In
principle, this allows us to identify which minerals are present, a key aid
to rock identification. Too bad it is not easy to do. The way light reflects
depends on the grain sizes of the minerals, the temperature, the sun angle,
and the extent of space weathering, besides what minerals are present.
Spectroscopists must also calibrate the camera system. It is all quite
tricky (and explains why I work with other types of data).

An important and striking characteristic of the spectra of light reflected
from Eros is that it is virtually the same everywhere, except for some areas
being brighter than others are. Compositional variations are no more than a
few percent. This is what one would expect from an unmelted asteroid or from
one melted only very slightly. Lucy McFadden (University of Maryland) and
co-authors made a detailed study of the spectra taken of Eros and conclude
that the spectral properties are consistent with that of an ordinary
chondrite.

Beth Clark (Ithaca College) and her colleagues studied spectra of Psyche
crater to try to understand the process of space weathering. Even in the low
gravity of a small asteroid, rock and dirt can slide downhill, leaving
behind patches of fresher, lighter materials. The lighter materials may have
experienced less space weathering, so by comparing light and dark areas it
may be possible to understand the process.

          [Psyche crater]
          Psyche crater (5.3 kilometers in diameter) dominates
          one hemisphere of Eros (left). Peeking inside the
          crater (right) we see lighter and darker regions formed
          when materials moved down the steep slopes. Note the
          boulders (several meters across) collected in the
          bottom of the crater.

The spectra taken of lighter and darker areas in Psyche differ by several
percent. Several factors affect the spectra of reflected light: the sizes of
the grains making up the surface, the amounts of olivine and pyroxene
(iron-magnesium silicates), and the effects of space weathering. These
effects can be modeled mathematically. Clark's modeling calculations
indicate that the spectral variations in Psyche cannot be caused solely by
variations in grain size, olivine, pyroxene, or a moon-like weathering
process. Other factors must be operating.

Beth Clark concludes that the spectral variations might be caused by an
enhanced concentration of iron sulfide (the mineral troilite), which darkens
the material, combined with lunar-like space weathering. This would seem to
be contradictory to the XGRS data, which indicate that sulfur is depleted on
the surface of Eros. However, it takes only a few percent of a dark mineral
to affect the brightness of a surface deposit. Overall, the spectral
properties of even the freshest surfaces do not make perfect matches for
ordinary chondrites or primitive achondrites, the best candidates for
matching the composition of Eros. Chondrites are a good match at wavelengths
up to 1.5 microns, but depart significantly at longer wavelengths. Primitive
achondrites such as acapulcoites do not fit as well even at wavelengths
shorter than 1.5 microns. The best bet is that space weathering has affected
the surface materials and altered the spectral properties.

     [Psyche crater plot]
     This is a plot of the relative amount of light reflected in
     wavelengths from 0.8 to 2.5 microns on Eros. The Eros data are
     compared to LL ordinary chondrites and a primitive type of
     achondrite called acapulcoites. Clark and co-workers scaled the
     reflectance to be exactly 1 at 1.4 microns so the shapes of the
     spectra could be compared easily. Chondrites are similar at
     wavelengths less than 1.5 microns, but differ at longer
     wavelengths.

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

Putting it all together

Tim McCoy (Smithsonian Institution) and others synthesized all the
observations to derive the best guess as to what type of meteorite is most
similar to Eros. They examined the data in detail and tried to devise a
unique solution. Unfortunately, they conclude that a unique solution is not
possible. Nevertheless, they do show that the possibilities are reasonably
narrow. Using a Venn diagram, they infer that only ordinary chondrites (if
space weathered) and primitive achondrites match both the XGRS chemical data
and the MSI, NIS mineralogical data. Venn diagrams are used in mathematics
to show the relationships between sets, but they have many other uses as
well.

              [venn diagram]
              In this diagram, only those types of meteorites
              that satisfy both chemical and spectral data are
              likely candidates for Eros.

In the Venn diagram, the types of meteorites outside any circle do not have
chemical or spectral properties like those of Eros. Some meteorite groups
(acapulcoites, lodranites, winonaites, brachinites, and R chondrites) have
chemical compositions consistent with that of Eros, but do not match in
spectral properties. Conversely, ureilites and CO chondrites are similar to
Eros spectrally, but do not match in chemical composition. Only
surface-altered chondrites and a type of as-yet-undiscovered primitive
achondrite are likely candidates.

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

What next?

The NEAR science team did excellent, creative work in trying to determine
which, if any, type of meteorite matches the properties of Eros. No type
fits exactly, though some candidates are clearly better than others. We need
more precise mineralogical and chemical data of both surface materials and
bedrock. As McCoy and his colleagues note, the way to get the highest
fidelity information is to return samples from asteroids to Earth for
intense study in laboratories. Scientists studying the materials would be
able to see what space weathering processes operate on asteroids, the
relation of bedrock to altered materials, and the detailed nature of the
rock making up each asteroid from which we grab samples. Such a mission
would also carry remote sensing instruments to help select sampling sites.

Additional Resources

     Clark, Beth E. and 11 others (2001) Space weathering on Eros:
     Constraints from albedo and spectral measurements of Psyche crater.
     Meteoritics and Planetary Science, vol. 36, p. 1617-1637.

     McCoy, Timothy J. and 16 others (2001) The composition of 433 Eros: A
     mineralogical-chemical synthesis. Meteoritics and Planetary Science,
     vol. 36, p. 1661-1672.

     Nittler, Larry R. and 14 others (2001) X-ray fluorescence measurements
     of the surface elemental composition of asteroid 433 Eros. Meteoritics
     and Planetary Science, vol. 36, p. 1673-1695.

     McFadden, Lucy A. and 7 others (2001) Mineralogical interpretation of
     reflectance spectra of Eros from NEAR near-infrared spectrometer low
     phase flyby. Meteoritics and Planetary Science, vol. 36, p. 1711-1726.

     NEAR-Shoemaker homepage

     Proposed Hera mission
Received on Fri 01 Mar 2002 12:53:52 AM PST