[meteorite-list] Using Chondrites to Understand the Inside of Asteroid 433 Eros

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:00:04 2004
Message-ID: <200206302308.QAA21845_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/June02/ErosPorosity.html

Using Chondrites to Understand the Inside of Asteroid 433 Eros
Planetary Science Research Discoveries
June 28, 2002

     --- Data from ordinary chondrite meteorites and
     from the NEAR mission suggest that asteroid 433
     Eros is heavily fractured.

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

Asteroid 433 Eros is one of the most closely scrutinized chunks of rocky
debris in our solar system. We know about its bulk properties, internal mass
distribution, and the shape, composition, and mineralogy of the surface from
instruments on the Near Earth Asteroid Rendezvous (NEAR) Shoemaker
spacecraft. Using mass and volume measurements scientists determined the
bulk density of this asteroid for the first time. An interdisciplinary
research team with expertise in cosmochemistry, planetary geology, remote
sensing, and orbital dynamics compared this orbital information with density
and porosity data from meteorite samples to estimate the porosity of the
asteroid. Sarah Wilkison and Mark Robinson (Northwestern University), Peter
Thomas and Joseph Veverka (Cornell University), Tim McCoy (Smithsonian
Institution), Scott Murchie and Louise Prockter (Applied Physics Lab), and
Donald Yeomans (Jet Propulsion Lab) report a macro (structural) porosity for
Eros of approximately 20%. They compared this estimate with features seen on
the surface of Eros and with previously proposed models for the formation of
asteroids to conclude that Eros has been heavily fractured by impact
collisions but was not demolished to the extent that it is now a rubble
pile.

     Reference:

     Wilkison, S. L., Robinson, M. S., Thomas, P. C., Veverka, J., McCoy, T.
     J., Murchie, S L., Prockter, L. M., and Yeomans, D. K. (2002) An
     estimate of Eros's porosity and implications for internal structure.
     Icarus, v. 155, p. 94-103.

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

Density of Eros

The NEAR Shoemaker spacecraft thoroughly photographed Eros during its
year-long mission. Using countless images, Joe Veverka and Peter Thomas and
their colleagues painstakingly determined the total volume of this elongated
object. Don Yeomans and colleagues made a careful analysis of the
spacecraft's orbit around Eros by analyzing radio signals. This allowed them
to determine the total mass of the asteroid. With the volume and mass known,
it is easy to calculate the density. The bulk density of Eros is 2.67 ± 0.03
g/cm3. This value seems to be nearly uniform throughout the asteroid based
on NEAR Shoemaker gravity data and implies little variation in its global
composition. That composition most closely resembles ordinary chondrite (OC)
meteorites based on the chemical, mineralogical, and magnetometer data from
NEAR Shoemaker.

                                     Thin section of
                                     chondrules and
                                     chondrule fragments in
 [cross-polarized thin section of OC]a dark matrix under
                                     cross-polarized light.
                                     Antarctic meteorite
                                     GRO 95596, an LL3
                                     ordinary chondrite.

These meteorites, composed of tiny metal and silicate grains and melted
silicate particles called chondrules, represent primitive asteroids that
never melted. A little bit of melting on Eros, however, cannot be ruled out
completely, but available data suggests that surface-altered OCs exposed to
space weathering best match the properties of Eros. [See PSRD article: The
Composition of Asteroid 433 Eros.] Despite this correspondence with OC
meteorites, it surprised no one that the bulk density of Eros did not match
the average value for OCs (3.40 g/cm3). Eros's density is lower. It's
assumed that cracks and voids exist inside the asteroid due to innumerable
impact collisions. Some early predictions placed Eros in the "reassembled
rubble-pile" model wherein the asteroid is a pile of gravitationally bound
fragments from earlier catastrophic smashes. For a review of this model see
PSRD article: Honeycombed Asteroids. Just how much empty space is there
inside Eros? The answer is based on porosity data from ordinary chondrites.

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

Porosity of Eros

Wilkison and team used two sets of density and porosity data for ordinary
chondrite meteorites from Consolmagno and Britt (1998) and Flynn and others
(1999) as their starting point to estimate the porosity of Eros. The 42
ordinary chondrite meteorites in the datasets range in porosity from 0 to
23% and have a median porosity of 6%. The team eliminated two meteorites
with textures rarely seen in OCs and chose an OC porosity range of 0 to 15%
(a range of overlap in the two datasets.) This porosity is defined as
microporosity because it refers to the voids and cracks between mineral
grains in the meteorite rock samples. Using the average bulk density of OCs,
3.40 g/cm3 and microporosities between 0 and 15%, the team calculated grain
densities of OCs from 3.4 to 4.0 g/cm3. They compared these meteorite grain
densities with Eros's bulk density of 2.67 g/cm3 to arrive at Eros's total
porosity: 21 to 33%. Using the median value of 6% microporosity of OCs, the
average bulk density of OCs, and the bulk density of Eros, the team
estimated the total porosity of Eros as 26%. If you assume that
microporosity is too small-scaled to affect an asteroid's cohesive strength,
then the 6% microporosity can be subtracted from the total porosity of Eros
to result in a macroporosity (all fractures and voids larger than the
mineral-grain sizes) for Eros of 20%. The macroporosity is the key to
understanding the impact history of Eros.

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

Asteroid Structural Changes through Time

The porosity of an asteroid is the result of a long, complicated history. It
depends on the initial composition and physical state of the asteroid and on
the intensity of the largest impacts that it suffered. The impact history
probably dominates the final porosity. Wilkison and her coworkers identify
three main categories of internal structures of asteroids, as summarized in
the table below. In the most mild case the asteroid is fractured, but still
coherent. Impacts have jarred its surface and seismic waves from the point
of impact cause fracturing, but the body is still largely a single solid
mass. No big chunks of the interior have moved relative to adjacent regions.
Its porosity ought to be similar to individual pieces of asteroids, as
sampled by meteorites. If the asteroid were made of rock like ordinary
chondrites, the porosity would be 0 to 15%.

                         Asteroid Structural Models

         Model Coherent but Heavily Rubble Pile
                      Fractured Fractured

                   Asteroid is Asteroid is The asteroid's
                   mildly broken into original body was
                   fractured but pieces by completely
                   is still a significant, disrupted and the
                   coherent body. multiple different-sized
                   If fractures fractures. bits and pieces
      Description break through Fragments have have reassembled
                   the asteroid, moved or into a
                   no fragments rotated into gravitationally
                   have moved or different bound body.
                   rotated from places creating
                   their original more void
                   positions. spaces.

                        0-15% >30%
                       based on 15-30% based on
        Tentative comparisons based on comparisons with
        Porosity with ordinary comparisons unconsolidated
         Range chondrite with lunar and terrestrial
                      meteorite terrestrial sediments, lunar
                       samples impact samples regolith, and
                                                     reassembly models

Stronger impacts can cause an asteroid to be pervasively fractured. Wilkison
and colleagues call this heavily fractured. The fracturing is so intense
that multiple cracks crisscross the body, and fragments have slipped or
rotated into different places. This creates void spaces in the asteroid,
increasing its porosity. Comparing to samples from lunar and terrestrial
impact craters, Wilkison and colleagues estimate that the porosity would be
15 to 30%.

Extremely strong impacts can blast an asteroid to smithereens, sending the
fragments far from each other. However, there is a range of impact potencies
that do not catastrophically disrupt an asteroid. Instead, an impact will
bust an asteroid apart, but not impart enough energy to the fragments to
cause them to lose gravitational track of each other. The fragments, or at
least a large percentage of them, fall back to the center of mass of the
system, creating a reassembled pile of rubble. The disorganized pile would
be highly porous. Wilkison and team suspect it would be as porous as
unconsolidated terrestrial sediments (like sand) or the lunar regolith, so
porosity would be greater than 30%.

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

Rubble Piles

The rubble pile concept has been quite popular among asteroid experts,
though not universally accepted. Theoretical simulations of asteroid impact
histories suggest that rubble piles should be fairly common. Data on the
cooling rates of ordinary chondrites argue strongly that the process
happens: Some chondrites are called "regolith breccias." These are rocks
reworked by impacts on the surfaces of asteroids. They contain grains of
metallic iron-nickel. It is possible to determine how fast these metallic
particles cooled by measuring their compositions and sizes.

Numerous analyses indicate that in a given regolith breccia, the metallic
particles cooled at rates ranging from 1 to 1000 oC per million years. Using
the laws of heat conductivity, we can calculate how deep a rock must be
buried to cool at a given rate. The range of cooling rates of the particles
in regolith breccias indicates original burial depths of a few kilometers
(those cooling at 1000 degrees per million years) to 100 kilometers (those
cooling at 1 degree per million years). Clearly, the asteroids on which
these breccias formed are jumbled.

At first glance, one would think that craters of different sizes could dig
up rocks from a large range of depths and deposit them onto the surface
where they could be incorporated into regolith breccias. It is not so easy,
however. Jeff Taylor and his colleagues calculated that craters large enough
to excavate rock from a depth of 60 kilometers would demolish asteroids
smaller than 500 kilometers in diameter [See PSRD article: Honeycombed
Asteroids.] The easier way to deposit rocks from great depths and to mix
them with rocks from shallow depths is to bust up the asteroid and
reassemble it into a rubble pile.

This does not mean that any given asteroid, such as Eros, is a rubble pile,
however. In fact, we do not know how common rubble piles might be. Wilkison
and colleagues tested whether Eros is a rubble pile or a less fractured
asteroid.

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

Putting It All Together: Asteroid Formation and Change Through Time

Asteroids may have had a variety of initial states soon after they formed.
The bodies were then modified by impacts as discussed above, leading to
different outcomes as shown in the diagram below.

               [asteroid formation models]

               This cartoon shows five models for Eros
               (labeled A through E) and how each body
               changes with increased fracturing. Shapes and
               sizes of fragments are not to be taken
               literally. Colors represent different
               chondritic material.

Case A depicts the seemingly simple case where an asteroid formed from
uniform material and was only very modestly heated. There is no layering or
internal structure. Such objects might be relatively weak, but if large
enough (bigger than about 50 kilometers in radius), their strength will be
governed by their gravity, not the strength of the rock. It is not clear how
porous such bodies might be initially. B is nicknamed the onion shell model.
This applies to asteroids that formed cold and were heated and
metamorphosed. This process reduced the initial porosity, especially in the
center where temperatures were highest. The degree of metamorphism decreases
from the center to the surface. C, the heterogeneously heated model, is like
case B except that the heating was not even throughout the body. The extent
of metamorphism is patchy. D is called the metamorphosed planetesimal model.
It depicts the heating as taking place in numerous relatively small bodies
(less than 10 kilometers in diameter). Each of the bodies has an onion-shell
structure, but they accrete randomly into the final asteroid. E is the case
where the asteroid was heated hot enough to melt. If melted to a high
temperature it could differentiate into a core, mantle, and crust. Less
melting would lead to a body with a crust and a mantle that still contained
at least some metallic iron. In either case, E is a coherent, strong
asteroid.

Asteroids in general, and Eros in particular, are heavily cratered and thus
fractured, but it is not easy to choose which parent body or structural
model shown in the figure above applies to Eros. That's why Wilkison and
colleagues used the additional parameter of porosity to help infer the
structure inside Eros.

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

How Broken Up is Eros?

A macroporosity of 20% for Eros is consistent with values obtained from
impact breccias found on the Earth and the Moon, leading Wilkison and
coauthors to conclude that Eros is a heavily fractured body. The estimated
porosity value does not seem low enough to match the coherent yet fractured
model nor high enough to justify calling Eros a reassembled rubble pile.
Ridges, troughs, and grooves on the surface of Eros seen from orbit suggest
a consolidated and extensively fractured interior but one with enough
structural strength to support such features. A prominent ridge system,
called Rahe Dorsum, in the northern hemisphere indicates major structural
buckling perhaps due to impact-induced compressional shock waves.

                [surface of Eros]

                These images show surface features on Eros
                which may be indicators of global internal
                strength. A: arrows point to Rahe Dorsum
                ridge system, B: arrows point to twisted
                ridge, C: square craters may have resulted
                from impacts into preexisting fracture
                patterns, and D: grooves and aligned pits.

It seems that Eros is not a rubble pile, unless it is composed of only a few
large fragments. Other asteroids might be, though. To find out how many it
will be necessary to send spacecraft to numerous asteroids to determine
their densities and chemical compositions, and derive their porosities.
Perhaps such measurements could lead to efficient ways of determining the
internal structure and strength of asteroids. This would be very useful in
assessing the danger of asteroid impact and in figuring out how to deflect
an asteroid menacing the Earth. The strategy used to deflect a rubble pile
might be drastically different from that used to deflect a fractured
asteroid.

Wilkison and colleagues' work shows the value of interdisciplinary research
on asteroids and asteroid samples (meteorites). It would not have been
possible without the database of laboratory measurements of meteorite
densities and porosities or without the spacecraft measurements of Eros.

[ADDITIONAL RESOURCES]

     NASA's NEAR mission homepage.

     Japan's Muses-C mission will land on an asteroid and bring back
     samples.

     Solar System Exploration: Missions to Asteroids

     Consolmagno, G. J. and Britt, D. T. (1998) The density and porosity of
     meteorites from the Vatican collection. Meteor. Planet. Sci. 33, p.
     1231-1241.

     Flynn, G J., Moore, L. B., and Klock W. (1999) Density and porosity of
     stone meteorites: Implications for the density, porosity, cratering,
     and collisional disruption of asteroids. Icarus 142, p. 97-105.

     Taylor, G. J. "The Composition of Asteroid 433 Eros" PSR Discoveries
     Feb. 2002 <http://www.psrd.hawaii.edu/Feb02/eros.html>.

     Taylor, G. J. "Honeycombed Asteroids" PSR Discoveries Aug. 1999
     <http://www.psrd.hawaii.edu/Aug99/asteroidDensity.html>.

     Wilkison, S. L., Robinson, M. S., Thomas, P. C., Veverka, J., McCoy, T.
     J., Murchie, S L., Prockter, L. M., and Yeomans, D. K. (2002) An
     estimate of Eros's porosity and implications for internal structure.
     Icarus, v. 155, p. 94-103.
Received on Sun 30 Jun 2002 07:08:23 PM PDT