Superglue of planet formation: sticky ice
Pacific Northwest National Lab experiments point to clingy grains of ice
to solve age-old mystery of how primordial dust pulled together to form
planets
RICHLAND, Wash.-How dust specks in the early solar systems came together
to become planets has vexed astronomers for years.
Gravity, always an attractive candidate to explain how celestial matter
pulls together, was no match for stellar winds. The dust needed help
coming together fast, in kilometer-wide protoplanets, in the first few
million years after a star was born, or the stellar wind would blow it
all away.
Scientists at the Department of Energy’s Pacific Northwest National
Laboratory, reporting in the current issue of Astrophysical Journal,
offer a cool answer to the planet- formation riddle: Micron-wide dust
particles encrusted with molecularly gluey ice enabled planets to bulk
up like dirty snowballs quickly enough to overcome the scattering force
of solar winds.
“People who had calculated the stickiness of dust grains found that the
grains didn’t stick,” said James Cowin, PNNL lab fellow who led the
research. “They bounce, like two billiard balls smacked together. The
attraction just wasn’t strong enough.”
Cowin’s team has spent years studying, among other things, the chemical
and physical properties atmospheric dust and water ice, using an array
of instruments suited to the task at the PNNL-based W.R. Wiley
Environmental Molecular Sciences Laboratory.
Much of the pre-planetary dust grains were either covered by or largely
composed of water ice, having condensed at temperatures close to
absolute zero, at 5 to 100 Kelvin. Evidence of this icy solar system can
be seen in comets, and planets and moons a Jupiter’s distance from its
star and beyond are icy.
“This ice is very different from the stuff we chip off our windows in
winter,” Cowin said. “For example, we saw that at extreme cold
temperatures vapor-deposited ice spontaneously becomes electrically
polarized. This makes electric forces that could stick icy grains
together like little bar magnets.”
PNNL staff scientist Martin Iedema, a member of Cowin’s group with an
astronomy undergraduate degree, surveyed the astrophysics literature and
found that the planet growth mystery resided in the same cold
temperatures of the lab ices.
Iedema found that the high background radiation in the early solar
system would have neutralized a polarized, micron-sized ice grain in
days to weeks-or hundreds of thousands of years before it could accrete
a critical mass of material and grow to the size of a medicine ball,
enabling it to get over the critical size hurdle in planet formation.
But, Iedema said, ice grains colliding into each other would have
chipped and broken in two to upset electrical equilibrium and, in
essence, recharging the ice grains and restoring their clinginess. Then
he discovered an additional feature that gave the sticky ice theory a
new bounce.
“More of an anti-bounce,” Cowin emended, “from the cushioning, or
fluffiness, of this ice. The more technical phrase is ‘mechanical
inelasticity.’ We knew that ice, when grown so cold, isn’t able to
arrange its molecules in a well-ordered fashion; it becomes fluffy on a
molecular scale.”
Cowin conjured an image of “billiard balls made of Rice Krispies.” Such
balls would barely bounce. “Colliding fluffy ice grains would have
enough residual electrical forces to make them stick, and survive
subsequent collisions to grow into large lumps.”
To test this, PNNL postdocs Rich Bell and Hanfu Wang grew ice from the
vapor in a chamber that reproduced primordial temperatures and vacuum.
They measured bounce by dropping hard, 1/16th- inch hard ceramic balls
on it. With a high-speed camera, they observed the balls consistently
rebound about 8 percent of their dropped height from fluffy ice grown at
40 Kelvin, whereas on the hard, warmer and much more compact ice that
forms naturally on Earth, the ice rebound was as high as 80 percent.
“This huge inelasticity provides an ideal way for fluffy icy grains to
stick and grow eventually to protoplanets,” Cowin said.
Cowin and colleagues further speculate that similar electrical forces,
minus the fluffy cushioning, were at work during the infancy of hotter
inner planets like Earth, involving silicate dust grains instead of ice.
* * *
PNNL ( www.pnl.gov ) is a DOE Office of Science laboratory that solves
complex problems in energy, national security, the environment and life
sciences by advancing the understanding of physics, chemistry, biology
and computation. PNNL employs 3,900, has a $650 million annual budget,
and has been managed by Ohio-based Battelle since the lab’s inception in
1965.
