[meteorite-list] Iron Meteorites as the Not-So-Distant Cousins of Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Jul 25 23:29:08 2006
Message-ID: <200607240515.WAA05365_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/July06/asteroidGatecrashers.html

Iron Meteorites as the Not-So-Distant Cousins of Earth
Planetary Science Research Discoveries
July 21, 2006

--- Numerical simulations suggest that some iron meteorites are
fragments of the long lost precursor material that formed the Earth and
other terrestrial planets.

Written by William F. Bottke (Southwest Research Institute, Boulder, CO)
and Linda M. V. Martel (Hawai'i Institute of Geophysics and Planetology)

Iron meteorites are fragments from the cores of small differentiated
asteroids (20-200 kilometers in diameter) that formed very early in Solar
System history. They are commonly assumed to have originated in the same
region as most stony meteorite parent bodies, namely the main asteroid belt
located between Mars and Jupiter. A new paper in the journal Nature by William
Bottke, David Nesvorn??, and Robert Grimm (Southwest Research Institute,
Boulder, Colorado) along with Alessandro Morbidelli and David O'Brien
(Observatoire de la Cote d'Azure, Nice, France), however, finds that the
iron meteorites may have come from a different and possibly much more
intriguing place. According to their numerical simulations that tracked
the dynamical evolution of Moon- to Mars-sized planetary embryos
interacting with tens of thousands of test bodies during the first 10
million years of Solar System evolution, many iron meteorite parent
bodies formed and fragmented in the same region where Mercury, Venus,
Earth and Mars are found today. The fast accretion times of
planetesimals in this zone allowed heat produced by the decay of
short-lived radioactive isotopes like
26Al to melt and differentiate many of these objects into core, mantle,
and crust. At the same time, gravitational interactions with planetary
embryos increased their mutual impact velocities, enough that these
planetesimals broke apart when they struck one another. The net result
was the production of millions of fragments continually jostled about by
planetary embryos. Over millions of years, a small fraction of this
differentiated debris was scattered into the innermost region of the
main belt, where it then stayed for billions of years until chance
collisional and dynamical events sent it on a crash course to Earth.
Bottke and colleagues' prediction of these asteroid main belt
gatecrashers could mean that some of the iron meteorites we hold in our
hands today are pieces of the same precursor fabric that formed the
Earth and other terrestrial planets.

Reference:

    * Bottke, W. F., D. Nesvorn??, R. E. Grimm, A. Morbidelli, and D. P.
      O'Brien (2006) Iron Meteorites as Remnants of Planetesimals Formed
      in the Terrestrial Planet Region. Nature, v. 439, p. 821-824. [pdf
      link <http://www.boulder.swri.edu/~bottke/Reprints/Reprints.html>
      opens in a new window]

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

The Story Told by Iron Meteorites and Main Belt Asteroids

Numerical simulations have demonstrated that most of the stony and iron
meteorites found today in worldwide collections came from asteroids in
the main belt. For this reason, it is often taken as a given that their
parent bodies formed there as well. This assumption, while reasonable
for many meteorite types, does not do a very satisfying job of
explaining the origin of the iron meteorites.

Most irons are pieces of the cores of distinct, small (20 to 200
kilometer-diameter) differentiated asteroids. We know this from
chemical, petrographic (mineralogical and textural), and cooling rate
studies of iron meteorites performed in laboratories. Very few irons are
thought to be impact melts or fragments from a few, larger (>500
kilometer-diameter) differentiated bodies.

[showing differentiation <http://www.mnh.si.edu/earth/text/5_1_4_0.html>]

This series of graphics from the Smithsonian depicts the process of
differentiation. Pictured left to right: Dust and grains clump together
until a small body forms. The growing body heats up inside and begins to
melt. Dense molten metal pools and sinks towards the center core of the
body. Less dense silicate liquid, or magma, rises towards the surface,
leaving dense residues of solid minerals in the mantle. The result is a
differentiated body with a core, mantle, and crust. [Click image to view
the source page.]

The fact that we have iron core fragments attests to the powerful
collisions that repeatedly shattered their parent bodies during the
early history of our Solar System. In our meteorite collections today,
irons represent over two-thirds of the unique parent bodies sampled
among all meteorites. This large percentage would suggest that
differentiated parent bodies and their fragments are common in the main
asteroid belt. The problem is there is little observational evidence to
support this idea, and it's not for lack of looking.

Despite intense searches with telescopes equipped with cameras and
spectrographs, the only clearly intact differentiated asteroid
identified so far is (4) Vesta, the second largest asteroid in the main
belt at 530 km diameter [see images <
http://neo.jpl.nasa.gov/images/vesta.html> from NASA's Hubble Space
Telescope; link opens in a new window]. Spectroscopic observations of
asteroid families (clusters of asteroid fragments with similar orbits
produced by catastrophic collisions over the last several billion years)
show few signs that their parent bodies had a distinct iron core nor a
mantle/crust derived from melted rock. Instead, we see the opposite;
most of the asteroid families investigated to date are made up of
members with remarkably similar spectroscopic signatures.

While some main belt asteroids do look like fragments from
differentiated bodies, the total number is relatively small when
compared to our expectations based on the iron meteorite record. For
example, only one asteroid is known to sample the crust of a Vesta-like
but non-Vesta differentiated asteroid. It is called (1459) Magnya, and
is a 20-30 km asteroid located in the outer main belt. Note that this
body could also be an intact differentiated body. Similarly, main belt
spectroscopic surveys have only identified 22 A-type asteroids, which
many believe are mantle fragments from Vesta-like bodies, out of a
sample of 950 objects. This material, which is likely composed of
olivine-rich metal-free silicates, is mostly missing from our meteorite
collection; this deficiency is often referred to as the "great dunite
shortage." There have also been spectroscopic searches for the exposed
cores of differentiated asteroids. The majority of the bodies with the
right spectroscopic signatures to be iron cores (a set with diameter >
60 km), however, also show evidence for hydrated minerals, low
densities, and/or radar signatures inconsistent with iron-rich material.

The story is further complicated by the very early formation age of iron
meteorites. Researchers using hafnium-tungsten and aluminum-magnesium
isotoptic dating techniques have reported the very interesting (and
perplexing) result that core formation in iron meteorite parent bodies
was nearly contemporaneous with the formation of the Ca-Al inclusions,
some of the first solids to form in the Solar System. Iron meteorite
core formation also predates the formation of those chondrules
found in ordinary and carbonaceous
meteorites by one to several million years. (See, for example, PSRD
article: Dating the Earliest Solids in the Solar System
<http://www.psrd.hawaii.edu/Sept02/isotopicAges.html>.) Bottke and colleagues argue that if
small asteroids differentiated in the main belt at such early times, it
is reasonable to expect that larger bodies forming nearby would also
differentiate, leaving the inner main belt literally teeming with such
bodies. According to their numerical results, there is simply no
reasonable way to get rid of all of this evidence.

To sum up, the iron meteorites tell us that small differentiated
asteroids were once common and they formed very early, while asteroid
observations suggest that little differentiation ever occurred in the
main belt region. Somehow, we have to reconcile these different stories.

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

Formation Location Closer to the Sun than the Main Asteroid Belt

Bill Bottke and his colleagues are tackling the puzzle of iron
meteorites using their expertise in modeling the formation and evolution
of asteroids and meteoroids. By considering the ways that asteroid
parent bodies formed, heated, collided, fragmented, and scattered, they
make the case that the formation location of most iron meteorite parent
bodies was outside the asteroid main belt, and most likely in the
terrestrial planet region. It is an idea that was first postulated in
1979 by John Wasson (UCLA) and George Wetherill (Carnegie Institution)
based on meteorite evidence and has now been updated using models of the
collsional, dynamical, and thermal evolution of asteroids.

To address the formation the iron meteorite parent bodies, we first need
to understand why small bodies differentiate and where this is most
likely to take place. According to various studies, planetesimals are
predominantly heated by the decay of short-lived radionuclides
<../PSRDglossary.html#radioactivity> like 26Al (and 60Fe.) (See PSRD
article: Asteroid Heating: A Shocking View
<../April04/asteroidHeating.html> for more about heating with 26Al.)
Because 26Al decays rapidly (half-life <../PSRDglossary.html#half-life>
= 0.73 million years) and small bodies lose heat quickly, only the
fastest-growing bodies have a chance to melt. According to planetesimal
formation models, growth is a function of distance from the Sun and
swarm density. This means that until we reach the snowline (the orbital
distance beyond which water ice is stable), the fastest-growing bodies
are closer to the Sun than farther away from the Sun. Combining these
ideas with the very early core formation times of the iron meteorite
parent bodies, Bottke and colleagues deduced that the iron meteorite
parent bodies may very well have formed in the terrestrial planet region.

forming, melting parent bodies
<http://www.windows.ucar.edu/tour/link=/cool_stuff/tour_Archean_1.html>

During early Solar System formation, iron meteorite parent bodies may
have formed in a zone where melting and differentiation were likely to
take place; a zone closer to the Sun than the asteroid main belt zone.


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

Evolution, Delivery to Main Belt, and Survival

The formation of the terrestrial planets started with the accretion
<../PSRDglossary.html#accretion> of roughly kilometer-sized
planetesimals out of the solar nebula
<../PSRDglossary.html#solarnebula>. Some of these bodies experienced
"runaway growth," a phase where the largest body in a given feeding zone
takes advantage of its larger gravitational cross section to rapidly
agglomerate all of the remaining nearby planetesimals. Over time, this
leads to a period where the inner Solar System, which includes the main
belt region, is shared by both Moon- to Mars-sized planetary embryos and
planetesimals whose size distribution has roughly the same shape as that
found in the main asteroid belt.

The dynamics of how these bodies evolved and where they went in the
early phases of Solar System history can be modeled using computer
simulations. Bottke and coauthors tracked tens of thousands of bodies
that evolved amid a swarm of protoplanets spread between 0.5-3.0 AU
<../PSRDglossary.html#au> (where 1 AU is defined as the average distance
between Earth and Sun). Some of the intact bodies, and certainly their
fragments, are shown to have scattered to stable regions in the asteroid
main belt (located ~2.1 to 3.4 AU.) See the animation shown below.

[animation of scattering bodies]

The plots on the left show 10 million years of evolution for 1,000 test
bodies at a time when Jupiter had not yet formed. [The animation repeats.]

The test bodies were initially placed in three zones away from the Sun,
indicated on the x-axis as Semimajor axis a (AU): 0.5-1.0 AU, 1.0-1.5
AU, and 1.5-2.0 AU. In addition, planetary embryos (colored pink) were
initially placed between 2.0-3.2 AU. The y-axis describes the
eccentricity and inclination of the bodies. Eccentricity e defines the
orbit's shape, and can be thought of as the ellipticity of the orbit
where the Sun is placed at one focus. Circular orbits have e = 0, while
parabolic orbits have e = 1. Inclination is defined as the angular
distance of the body's orbital plane from the reference plane of the
Solar System (often considered the ecliptic plane) in degrees. The
asteroid main belt zone is outlined in bright purple.

Watch how the test bodies move over time due to their gravitational
interactions with the embryos. The test bodies are assumed to have no
mass. Encounters with embryos scatter the test bodies away from their
initial zones and spread their semimajor axis distribution.

Almost immediately, we see test bodies from the 1.5-2.0 AU region enter
the stable, main belt zone through gravitational interactions with
planetary embryos. Bodies from the 0.5-1.0 AU and 1.0-1.5 AU regions
also reach the main belt zone, but not until several millions of years
have passed.

Note that many of the gatecrashers go to the inner main belt (2-2.5 AU),
the region where dynamical models indicate the vast majority of
meteorites come from.

The figure below shows the fractions of test bodies, from different
initial zones in the inner Solar System, that reach the main belt by
gravitational interactions with planetary embryos. Bottke and colleagues
conclude from this that it is plausible that the current main belt
contains samples from the feeding zones of Mercury, Venus, Earth, and
Mars. Moreover, most of these bodies are injected into the inner main
belt, the same region that, according to dynamical models, produces the
most meteorites. This fact that meteorite delivery mechanisms are biased
in favor of material from this region may help explain the unusual
diversity of iron meteorites in worldwide collections.

[graph of % reaching main belt]

The curves in the plot, above, were generated by tracking 17,000 test
bodies for 10 million years. Ten percent of the bodies from the 1.5-2.0
AU zone (red curve) reached the main belt after one million years. About
one percent of the bodies from the 1.0-1.5 AU zone (yellow curve) reach
the main belt after a longer delay of two million years. And only
0.01-0.1% of bodies from the 0.5-1.0 AU zone reach the main belt after
six million years.

We know that collision probabilities and impact velocities of the bodies
striking one another (see graphic below, left) increase closer to the
Sun. Bottke and colleagues found that most 20-kilometer-diameter bodies
at <1.5 AU break up quickly enough (graphic, right, lower two curves)
that few reach the main belt zone. This helps explain the apparent
absence of small, intact differentiated bodies in the main belt today.
Because each break-up produces millions of fragments, however, it is
statistically likely that some portion of the debris made its way into
the main belt region. The 20 kilometer bodies scattered in from 1.5-2.0
AU have a better chance of reaching the main belt, but their distance
from the Sun implies longer formation times and hence the increased
likelihood that they never differentiated.

Finally, these results show that if small differentiated bodies had once
formed in the main asteroid belt, a large fraction would still be around
today. So far, none have been observed. This raises the bar for models
that would create iron meteorite parent bodies in the main belt and then
eliminate the evidence by collisional and/or dynamical processes.

[artist's illustration of collision sequence graph of fraction surviving]

What would a catastrophic collision between two bodies look like?
Pictured above is an artist's rendition of a collision sequence, from
left to right, between two rocky bodies that produces millions of
fragments.

The plot on the right focuses on 20-kilometer-diameter bodies. Between
0.5-1.5 AU they disrupt quickly; only their fragments reach the main
belt. Bottke and colleagues say these results help explain the paucity
of small intact differentiated bodies in the main belt today.

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

Gatecrashers: From Here to There and Back Again

Bottke and coauthors have shown using numerical simulations how the iron
meteorite parent bodies and their fragments could have started here in
the inner Solar System before being scattered out to the main belt.
Still, the results of this model beg the following question: if it works
for irons, shouldn't it also work for the stony component of the
differentiated asteroids? In other words, if fragments of crust, mantle,
and core were indeed main-belt gatecrashers, it is logical to ask why so
few olivine and basaltic meteorites (mantle and crust material,
respectively) from sources other than asteroid (4) Vesta are found in
the meteorite collection.

To address this issue, Bottke and colleagues used yet another computer
model to track the evolution of a hypothetical population of olivine
(A-type) asteroids in the inner main belt. They concluded there are
simply not enough A-type asteroids in the inner main belt to keep a
healthy population of olivine meteoroids replenished by a collisional
cascade over 4.5 billion years. Instead, they report these asteroids
were steadily eroded over time by collisions and dynamical depletion
processes, such that the current population, while non-negligible, is
statistically unlikely to produce a significant number of present-day
meteorites.

For iron meteoroids, which represent our core fragments, the main belt
survival and delivery situation is substantially different. For example:
(1) irons are roughly 10 times stronger than stones, meaning they are
less susceptible to disruption and are more likely to survive
atmospheric entry to Earth, and (2) irons drift roughly 10 times more
slowly than stones in space by thermal radiation forces (i.e., the
Yarkovsky Effect <../PSRDglossary.html#Yarkovsky>, a thermal thrust
produced when small bodies orbiting the Sun absorb sunlight, heat up,
and reradiate the thermal energy after a short delay produced by thermal
inertia. Because iron conducts heat better than stones -- think of how
your iron skillet retains heat so well -- the temperature variation
across the surface of an iron meteoroid is smaller, which in turn
reduces the efficiency of the Yarkovsky effect at moving these objects
in space.) Taken together, the researchers surmise that the population
of small iron asteroids in the inner main belt has probably not changed
much over the last four billion years.

In the movies, for example "At the Earth's Core" (1976) and "The Core"
(2003), humans often go to extraordinary (and mostly implausible)
efforts to reach Earth's core. It is interesting to consider the
possibility that our iron meteorite collection could save us the trip,
in that they may actually be samples of the same kinds of materials now
contained thousands of kilometers below our feet.

[poster poster Earth's interior]
Movie posters of imaginary worlds and a diagram of the real world.

Accordingly, if we can place the known iron meteorites into the
appropriate Solar System context, it may eventually be possible to use
them as probes into the unknown and perhaps forever unreachable
materials located within the deep interiors of Mercury, Venus, Earth,
the Moon, and Mars. Studies like these by Bottke and colleagues show
clearly the extraordinary benefits of combining laboratory analyses of
meteorites, astronomical observations of asteroids, and numerical
modeling of asteroid formation and evolution.

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

ADDITIONAL RESOURCES

    * Bottke, W. F., D. Nesvorn??, R. E. Grimm, A. Morbidelli, and D. P.
      O'Brien (2006) Iron Meteorites as Remnants of Planetesimals Formed
      in the Terrestrial Planet Region. Nature, v. 439, p. 821-824. [pdf
      link <http://www.boulder.swri.edu/~bottke/Reprints/Reprints.html>]

    * Bottke, W. F., D. Vokrouhlicky, D. P. Rubincam, and D. Nesvorn??
      (2006) The Yarkovsky and YORP effects: Implications for asteroid
      dynamics. Ann. Rev. Earth Planet. Sci., v. 34, p. 157-191.

    * Bottke, W.F., D. D. Durda, D. Nesvorn??, R. Jedicke, A. Morbidelli,
      D. Vokrouhlicky, and H. Levison (2005) Linking the collisional
      history of the main asteroid belt to its dynamical excitation and
      depletion. Icarus, v. 179, p. 63-94.

    * Burbine, T.H., T. J. McCoy, A. Meibom, B. Gladman, and K. Keil
      (2002) Meteoritic parent bodies: Their number and identification.
      In Asteroids III (eds. Bottke, W. F. et. al.). University of
      Arizona Press, Tucson, p. 653-667.

    * Cellino, A., S. J. Bus, A. Doressoundiram, D. Lazzaro (2002)
      Spectroscopic properties of asteroid families. In Asteroids III
      (eds. Bottke, W. F. et. al.). University of Arizona Press, Tucson,
      p. 633-643.

    * Chabot, N. L. H. and Haack (2006) Evolution of asteroid cores. In
      Meteorites and the Early Solar System II (eds. Lauretta, D.S. and
      McSween, H.Y.). University of Arizona Press, Tucson, p. 747-771.

    * Halliday, A. N. and T. Kleine (2006) Meteorites and the timing,
      mechanisms, and conditions of terrestrial planet accretion and
      early differentiation. In Meteorites and the Early Solar System II
      (eds. Lauretta, D.S. and McSween, H.Y.). University of Arizona
      Press, Tucson, p. 775-801.

    * McSween, H. Y., A. Ghosh, R. E. Grimm, L. Wilson, and E. D. Young
      (2002) Thermal evolution models of asteroids. In Asteroids III
      (eds. Bottke, W. F. et. al.). University of Arizona Press, Tucson,
      p. 559-571.

    * Parent Bodies: A Meteorite Family Tree. American Museum of Natural
      History.
      http://www.amnh.org/exhibitions/permanent/meteorites/origins/parent.php

    * Wasson, J. T. and G. W. Wetherill (1979) Dynamical chemical and
      isotopic evidence regarding the formation locations of asteroids
      and meteorites. In Asteroids. University of Arizona Press, p. 926-974.
Received on Mon 24 Jul 2006 01:15:17 AM PDT