[meteorite-list] Hit-and-Run as Planets Formed

[Ron Baalke]

http://www.psrd.hawaii.edu/Nov06/hit-and-run.html  

Hit-and-Run as Planets Formed
Planetary Science Research Discoveries
November 27, 2006

--- Collisions between large protoplanets as the planets formed may have
ripped some of them to shreds, producing molten asteroid-sized bodies,
driving off water and other volatiles, and scrambling partially molten
protoplanets.

Written by G. Jeffrey Taylor 
Hawai'i Institute of Geophysics and Planetology

Planet formation involved collisions between Moon-sized or larger
protoplanets to make even bigger ones. However, planet growth is not the
only result of the collisions. Erik Asphaug, Craig Agnor, and Quentin
Williams (University of California, Santa Cruz) point out that many
protoplanet interactions were what they call "hit-and-run" collisions,
causing substantial effects on the bodies, particularly on the smaller
one. The effects might have included widespread melting, disruption, and
formation of an assortment of metal-rich objects that might be found
among asteroids and meteorites. Their ideas give cosmochemists a whole
new way of looking at asteroid formation and planetary differentiation.

Reference:

    * Asphaug, E., C. B. Agnor, and Q. Williams (2006) Hit-and-run
      planetary collisions. Nature, v. 439, p. 155-160.

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

Giant Impacts as Planets Formed

Planet formation was rough, messy, and complicated. Dust grains did not
settle gently onto slowly-growing rocky bodies. Instead, bodies a few
hundred kilometers across grew fast and then accreted into larger
objects that ranged in size from 1000 km across to the size of Mars.
These protoplanets smashed into each other over a period of about 50
million years to form the terrestrial planets and one such collision, a
slightly off-centered one, resulted in formation of Earth's Moon.
Computer simulations of the process of planet formation generally assume
that most of the collisions result in accretion. That is, the smaller
protoplanet becomes part of the larger one, and the now larger object
has taken another step towards planethood.

[painting of two body collision]
The current view of formation of the terrestrial planets involves
collisions between growing protoplanets. In this painting by James Garry
an object larger than the Moon is hit by a smaller one, resulting in
growth of the larger protoplanet.

Erik Asphaug and his colleagues point out that not all protoplanetary
encounters result in accretion. In many cases the smaller object barely
hits the bigger one, but ends up greatly affected by the close
encounter. It may even be ripped apart. Part of the reason for this
destruction is that the gravity field of the larger protoplanet extends
well beyond the surface of the object. There is a zone of interaction
from the center out to about 2.5 times the radius of the target that is
strong enough to exert tidal forces on solid or molten objects, even if
the smaller protoplanet does not make physical contact with the larger
one. The outer limit of this zone is called the Roche limit, named after 
Edouard Albert Roche, a French mathematician. (The Roche limit is the 
smallest distance at which a planetary object that has no internal 
strength can orbit another body without being torn apart by the larger 
body's gravitational force.)

The UC Santa Cruz researchers used computer models of planetary
interactions to examine what happens during close encounters that do not
lead to accretion. In the calculations, both the impactor and the larger
target protoplanet are differentiated into metallic core and silicate
(rocky) mantle, with the core making up 30 weight percent of the volume.

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

Sudden Pressure Decrease

Large protoplanets would have been compressed because gravity pulls
everything towards the center of the body. There is considerable energy
tied up in this compression. For a Mars-sized protoplanet, Asphaug and
coworkers calculate that decompression of the planet's mantle releases
about the same amount of energy per gram as does TNT. This energy is
available for heating the entire body, including considerable melting of
the rocky mantle. This happens because the melting temperature of rock
increases with increasing pressure. If the pressure drops without
cooling, melting ensues. The hit-or-miss team calculates that the
pressure inside the smaller protoplanet could decrease 30 to 50% for
about an hour during a non-impact close approach (see graph below),
resulting in a permanent decrease of 20% because of mass loss and
increase in the protoplanet's rate of rotation. The pressure drop could
cause widespread melting or, if the body is already partly molten,
widespread increase in the percentage of the interior that is molten.

[graph of pressures vs. time]
This graph depicts how a non-impacting close approach of a Moon-sized
protoplanet with a body the size of Mars would result in a dramatic,
though short-lived (about an hour) decrease in the pressure inside the
smaller body. This could lead to widespread heating and melting of the
interior.

If the bodies experience a grazing collision, shearing and mechanical
stresses are considerable, and add to the effects of pressure release.
Collisions like this could lead to partial, or complete, disruption of
the smaller protoplanet, spewing its rocky and metallic guts into a
strung out collection of protoplanets.

[non-accretion collision model]
Two examples of a non-accretion collision between a protoplanet with the
mass of Mars and a smaller protoplanet. In a (top sequence), the
impactor has a mass equal to half that of the target; in b (bottom
sequence), the impactor has a mass of only one-tenth that of the target.
Red indicates the metallic cores and blue indicates the rocky mantles of
the bodies. When the impacting protoplanet is close to the mass of the
target protoplanet (top sequence), it experiences loss of rocky mantle,
but the residue stays intact. Its ratio of metal to silicate has
increased, however. The rocky debris could accrete to form an object
with a smaller than normal amount of metallic iron. When the impacting
protoplanet is small (bottom sequence), it is destroyed and a chain of
metal-rich protoplanets results, possibly leading to formation of
metal-rich asteroids.

The two movies, below, show collisions between a Mars-mass protoplanet
and a Moon-mass protoplanet (1:10 mass ratio collision), where the
encounter occurs at twice the mutual impact velocity, at an impact angle
of 45 degrees. Neither protoplanet is rotating initially.

The movie on the left shows the frame of reference we are used to
examining: what happens to the bigger protoplanet when it is struck. It
loses some of its mantle, is spun up, and undergoes free gravitational
oscillations. (Movie courtesy of Erik Asphaug and Craig Agnor.) The
movie on the right shows the frame of reference we are not accustomed to
examining: what happens to the smaller protoplanet, the impactor, which
is usually not accreted in giant impacts. In a hit-and-run collision,
the impactor is shredded into a chain of debris, with the largest bodies
in the chain greatly increased in iron percentage. This is because less
dense mantle rock is more easily pulled away by the potent tidal forces.
A one-particle-thick veneer of rock remains about each iron-rich body,
for numerical reasons inherent to the modeling method. That means the
blue objects you see in the center of the chain of debris are
mostly-iron protoplanets.(Movie courtesy of Erik Asphaug and Craig Agnor.)

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

Fizzing Protoplanets

Water can be dissolved in silicates, including in magma. How much
dissolves, however, depends on pressure: the higher the pressure, the
more H2O can be dissolved. If protoplanets had H2O inside them
(reasonable, but not a guarantee), the pressure release associated with
a close encounter or a grazing impact could cause the H2O to bubble out
as vapor. The huge volume change associated with the gas loss could
cause widespread, dramatic eruptions and even loss of silicate from
smaller objects. The escaping H2O gas might also react with the rocky
materials or metallic iron, to change the chemical composition and
oxidation state of some regions. Asphaug and his colleagues have not yet
studied the chemical effects, but their results show that there is
interesting work to be done.

[graph of pressures at which degassing begins]
This graph shows the rock density versus the pressure at the base of the
mantle of a Moon-sized impactor. The curves represent the amount of H2O
that can be dissolved in the rock. Note that for any concentration, the
amount that can be dissolved increases with pressure. The higher the H2O
content, the lower the density. If a Moon-sized protoplanet were
disrupted so that the pressure decreased suddenly, the pressure would
move to the left. This is illustrated, by the black arrow, for the case
of 5% H2O at a pressure of 60 kilobars (60,000 times the pressure at
Earth's surface). If the pressure drops to about 20 kilobars, the
solubility of H2O drops to only 1%. The rest escapes rapidly as gas,
causing the planet to fizz like a shaken carbonated drink.

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

Ripping Asunder

If the smaller projectile were solid, it can still be disrupted by the
shear forces on it as it passes near the larger protoplanet. A 500
kilometer body would crack into fragments about 200 meters across. A
solid object double that size would break into smaller fragments, only
70 meters across, because the disassembly releases more gravitational
energy. However, because many (perhaps most) protoplanets would have
been heated by the decay of short-lived radioactive isotopes
such as aluminum-26 (26Al) or by impacts,
the usual case is interaction between partially molten objects. In that
case, disruption occurs more readily.

The bottom line is that close encounters and grazing impacts during
planet formation lead to planet growth, but it is accompanied by
creation of a lot of protoplanet debris.

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

Answering Asteroid Mysteries

The idea of hit-and-run collisions provides an answer for some meteorite
and asteroid mysteries. A classic idea of asteroid history is that they
formed cool, heated up by the decay of 26Al, and differentiated into a
core and mantle, with subsequent melting of the mantle to create a
crust. Curiously, we have lots of samples of chemically distinct iron
meteorites and a few basaltic crusts (such as asteroid 4 Vesta), but few
of the mantle. We do not see much of it among the meteorites or from
spectral observations of asteroids. Planetary scientists have wondered
for a long time where the missing mantle is hidden. The most common
explanation is that the mantle rock is weaker than the iron cores, which
are basically steel, so that impact disruption over the eons has
destroyed it. Nobody really likes that idea because some objects have
basaltic crusts. Why, skeptics ask, would impacts strip away so many
mantles completely, while leaving some asteroids with their basaltic
crusts intact?

Now we have an alternative explanation. If the mantle rock is not
molten, a close encounter with another protoplanet could cause it to
break into thousands of little pieces tens to a couple of hundred meters
across. These small objects would not survive the collision-dominated
early Solar System. Or, if partially molten and water-rich, larger
protoplanets could have fizzed out lots of water and droplets of molten
asteroid. If the water reacted with the silicates from which it was
evolving, the resulting product might not even be recognizable as a
chunk of a protoplanet mantle. Mixing of iron and silicate during this
process would also create a large volume of stony-iron objects, not just
rocky ones. This view is complementary to one proposed by Bill Bottke
(Southwest Research Institute, Boulder) and colleagues in which some
iron meteorites are fragments of the long lost precursor material that
formed the Earth and other terrestrial planets. (See PSRD article Iron
Meteorites and the Not-So-Distant Cousins of Earth
<http://www.psrd.hawaii.edu/July06/asteroidGatecrashers.html>.)

Meteorite studies show that there are at least 100 chemically distinct
types of iron meteorites. That's a lot of protoplanet core material to
reveal by impacts chipping away at small, differentiated asteroids. The
problem is at least partly solved if many of those iron cores formed by
a hit-and-run collision, as depicted above.

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

More Than One Way to Cook an Asteroid

Asphaug and his coworkers give us a whole new way of looking at how
asteroids might have formed and melted, but other ideas are certainly in
the running. In particular, asteroid heating by the decay of short-lived
26Al and other isotopes (e.g., (60Fe) also explains the widespread
melting of asteroids. There might be problems with getting rid of rocky
mantles, but it makes a simpler explanation for the history of asteroid
4 Vesta, ancestral home of the eucrite, howardite, and diogenite
meteorites. As shown by Phonsie Hevey and Ian Sanders (Trinity College,
Dublin, Ireland), even asteroids as small as 20 kilometers in radius
would have melted enough to differentiate into core and mantle, if they
formed with the 26Al to 27Al ratio measured in calcium-aluminum rich
inclusions (CAIs) in primitive (unmelted)
meteorites. Short-lived isotopes also provide a good explanation for the
heating and thermal metamorphism (without melting) of chondritic
meteorites.

[calculations of asteroid heating/melting]
Calculations of asteroid heating indicate that even small asteroids will
melt inside if they form while 26Al is still present. The longer it
takes for an asteroid to form, the less 26Al and heating from it, hence
a body must be larger to heat to its melting temperature. Curves in the
shaded region indicate how much of the body is heated above the melting
temperature (assumed to be 1450 oC.
Dashed contours outside the melting zone indicate the maximum
temperature reached.

The next step is to reconcile these two likely mechanisms of asteroid
heating. Erik Asphaug, his colleagues, and other experts in modeling
planet formation are working on more elaborate models and investigating
collisions between protoplanets with a wide range in size.
Meteoriticists are searching for evidence to prove or disprove the
theoretical models. We are at the beginning of a fascinating
intellectual adventure.

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

Effects on Final Planets

Hit-and-run collisions between protoplanets might drastically disrupt
the course of crystallization of their magma oceans. Consider, for
example, the Martian magma ocean. Linda Elkins-Tanton and her colleagues
at Brown University have constructed geochemically and geophysically
reasonable models of the crystallization of the Martian magma ocean,
including its overturn triggered by an unstable density gradient
resulting from initial crystallization (see PSRD article A Primordial
and Complicated Ocean of Magma on Mars
<http://www.psrd.hawaii.edu/Mar06/mars_magmaOcean.html>
A hit-and-run collision during magma ocean crystallization would
scramble the pile of crystals deposited at its base up to that time,
perhaps resulting in a temporarily stable configuration of the
cumulates. Continued crystallization would produce a density gradient
atop that rearranged pile. Subsequent overturn would be substantially
more complicated than the simpler models predict, perhaps leading to a
mantle even more heterogeneous than predicted and a complicated primary
crust.

The theoretical studies of Erik Asphaug and his colleagues present
challenges to cosmochemists, the scientists who are testing the ideas
through chemical and isotopic analyses of meteorites and samples
returned from the Moon, and eventually, asteroids and Mars.

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



ADDITIONAL RESOURCES   

    * Asphaug, E., C. B. Agnor, and Q. Williams (2006) Hit-and-run
      planetary collisions. Nature, v. 439, p. 155-160.
    * Bottke, W. F. and L. M. V. Martel (2006) Iron meteorites as the
      not-so-distant cousins of Earth. Planetary Science Research
      Discoveries.
      <http://www.psrd.hawaii.edu/July06/asteroidGatecrashers.html>.
    * Hevey, P. J. and I. S. Sanders (2006) A model for protoplanet
      meltdown by 26Al and its implications for meteorite parent bodies.
      Meteoritics and Planetary Science, v. 41, p. 95-106.
    * Taylor, G. J. (2006) A primordial and complicated ocean of magma
      on Mars. Planetary Science Research Discoveries.
      <http://www.psrd.hawaii.edu/Mar06/mars_magmaOcean.html>.