[meteorite-list] New Study Highlights Role of Hit-and-Run Collision in the Formation of Planets, Asteroids, and Meteorites

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jan 12 15:19:51 2006
Message-ID: <200601122018.k0CKIFV15322_at_zagami.jpl.nasa.gov>

http://www.ucsc.edu/news_events/press_releases/text.asp?pid=799

January 11, 2006
UC Santa Cruz Press Release
Contact: Tim Stephens (831) 459-2495; stephens_at_ucsc.edu

New study highlights role of hit-and-run collisions in the formation
of planets, asteroids, and meteorites

Hit-and-run collisions between embryonic planets during a critical
period in the early history of the Solar System may account for some
previously unexplained properties of planets, asteroids, and meteorites,
according to researchers at the University of California, Santa Cruz,
who describe their findings in a paper to appear in the January 12 issue
of the journal Nature.

The four "terrestrial" or rocky planets (Earth, Mars, Venus, and
Mercury) are the products of an initial period, lasting tens of millions
of years, of violent collisions between planetary bodies of various
sizes. Scientists have mostly considered these events in terms of the
accretion of new material and other effects on the impacted planet,
while little attention has been given to the impactor. (By definition,
the impactor is the smaller of the two colliding bodies.)

But when planets collide, they don't always stick together. About half
the time, a planet-sized impactor hitting another planet-sized body will
bounce off, and these hit-and-run collisions have drastic consequences
for the impactor, said Erik Asphaug, associate professor of Earth
sciences at UCSC and first author of the Nature paper.

"You end up with planets that leave the scene of the crime looking very
different from when they came in--they can lose their atmosphere, crust,
even the mantle, or they can be ripped apart into a family of smaller
objects," Asphaug said.

The remnants of these disrupted impactors can be found throughout the
asteroid belt and among meteorites, which are fragments of other
planetary bodies that have landed on Earth, he said. Even the planet
Mercury may have been a hit-and-run impactor that had much of its outer
layers stripped away, leaving it with a relatively large core and thin
crust and mantle, Asphaug said. That scenario remains speculative,
however, and requires additional study, he said.

Asphaug and postdoctoral researcher Craig Agnor used powerful computers
to run simulations of a range of scenarios, from grazing encounters to
direct hits between planets of comparable sizes. Coauthor Quentin
Williams, professor of Earth sciences at UCSC, analyzed the outcomes of
these simulations in terms of their effects on the composition and final
state of the remnant objects.

The researchers found that even close encounters in which the two
objects do not actually collide can severely affect the smaller object.

"As two massive objects pass near each other, gravitational forces
induce dramatic physical changes--decompressing, melting, stripping
material away, and even annihilating the smaller object," Williams said.
"You can do a lot of physics and chemistry on objects in the Solar
System without even touching them."

A planet exerts enormous pressure on itself through self-gravity, but
the gravitational pull of a larger object passing close by can cause
that pressure to drop precipitously. The effects of this
depressurization can be explosive, Williams said.

"It's like uncorking the world's most carbonated beverage," he said.
"What happens when a planet gets decompressed by 50 percent is something
we don't understand very well at this stage, but it can shift the
chemistry and physics all over the place, producing a complexity of
materials that could very well account for the heterogeneity we see in
meteorites."

The formation of the terrestrial planets is thought to have begun with a
phase of gentle accretion within a disk of gas and dust around the Sun.
Embryonic planets gobbled up much of the material around them until the
inner Solar System hosted around 100 Moon-sized to Mars-sized planets,
Asphaug said. Gravitational interactions with each other and with
Jupiter then tossed these protoplanets out of their circular orbits,
setting off an era of giant impacts that probably lasted 30 to 50
million years, he said.

Scientists have used computers to simulate the formation of the
terrestrial planets from hundreds of smaller bodies, but most of those
simulations have assumed that when planets collide they stick, Asphaug
said.

"We've always known that's an approximation, but it's actually not easy
for planets to merge," he said. "Our calculations show that they have to
be moving fairly slowly and hit almost head-on in order to accrete."

It is easy for a planet to attract and accrete a much smaller object
than itself. In giant impacts between planet-sized bodies, however, the
impactor is comparable in size to the target. In the case of a Mars-size
impactor hitting an Earth-size target, the impactor would be one-tenth
the mass but fully one-half the diameter of the Earth, Asphaug said.

"Imagine two planets colliding, one half as big as the other, at a
typical impact angle of 45 degrees. About half of the smaller planet
doesn't really intersect the larger planet, while the other half is
stopped dead in its tracks," Asphaug said. "So there is enormous
shearing going on, and then you've got incredibly powerful tidal forces
acting at close distances. The combination works to pull the smaller
planet apart even as it's leaving, so in the most severe cases the
impactor loses a large fraction of its mantle, not to mention its
atmosphere and crust."

According to Agnor, the whole problem of planet formation is highly
complex, and unraveling the role played by hit-and-run fragmenting
collisions will require further study. By examining planetary collisions
from the perspective of the impactor, however, the UCSC researchers have
identified physical mechanisms that can explain many puzzling features
of asteroids.

Hit-and-run collisions can produce a wide array of different kinds of
asteroids, Williams said. "Some asteroids look like small planets, not
very disturbed, and at the other end of the spectrum are ones that look
like iron-rich dog bones in space," he said. "This is a mechanism that
can strip off different amounts of the rocky material that composes the
crust and mantle. What's left behind can range from just the iron-rich
core through a whole suite of mixtures with different amounts of silicates."

One of the puzzles of the asteroid belt is the evidence of widespread
global melting of asteroids. Impact heating is inefficient because it
deposits heat locally. It is not clear what could turn an asteroid into
a big molten blob, but depressurization in a hit-and-run collision might
do the trick, Asphaug said.

"If the pressure drops by a factor of two, you can go from something
that is merely hot to something molten," he said.

Depressurization can also boil off water and release gases, which would
explain why many differentiated meteorites tend to be free of water and
other volatile substances. These and other processes involved in
hit-and-run collisions should be studied in more detail, Asphaug said.

"It's a new mechanism for planetary evolution and asteroid formation,
and it suggests a lot of interesting scenarios that warrant further
study," he said.

This research was funded by NASA's Planetary Geology and Geophysics Program.

______

Note to reporters: You may contact Asphaug at (831) 459-2260 or
asphaug_at_pmc.ucsc.edu; Agnor at (831) 459-2426 or cagnor@pmc.ucsc.edu;
and Williams at (831) 459-3132 or quentw_at_pmc.ucsc.edu.

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Received on Thu 12 Jan 2006 03:18:14 PM PST