17 MAR 2005
From dust unto ... planets?
Picture a young sun, surrounded by gas 'n dust. Somehow, out of this swirling miasma, planets arose. Don't take our word for it. You can see the pictures of young stars cloaked in clouds of dust -- if you have a cool telescope.
Photo: Wide Field and Planetary Camera 2 Hubble Space Telescope.
But there's a problem: Gravity alone is too weak to bind the dust together. Otherwise, those motes of dust you see in the beam of a movie projector would bind together and drop to the floor.
This matters. If dust in the nebula does not congeal, the solar wind -- the charged particles zooming outward from the sun -- would blow it all away before the planets could form.
Within a million years, the solar wind would strip away all the dust, and we would not have planets. No planets means no microwaves. No strip malls. Not even any Why Files. The horror. The horror! It's the ultimate existential question: Why didn't the solar wind just blow it all away?
Science -- flash frozen
Now we hear from Martin Iedema and James Cowin of Pacific Northwest National Laboratory, who say the dust was slathered with a thin coat of flash-frozen ice. And their experiments show that ice forming at around 30° Kelvin is sticky enough to congeal into gunk-balls that eventually grow large enough for gravity to take over and cause further accumulation.
To understand what Cowin and Iedema are saying, slam your time machine into reverse, and imagine conditions in our planetary nebula, at the distance, say, of Jupiter, about 4.5 billion years back. The dust is so thick you can't even see the sun, which has recently begun "burning." Although it's dark, it's also windy. A solar wind of charged helium and hydrogen atoms is streaming away from the sun, eroding away the dust.
You would expect the wind to strip the dust away in, say, a million years. Presto-chango -- no planets! Gravity is too puny to congeal the dust particles, says Iedema. "Gravity alone is insufficient, it doesn't have any effect when you are talking about objects smaller than a meter in size, it won't cause little dust grains to gather together. You need some other mechanism."
Don't count on the electrical attraction called van deer Waals forces. Again, too flimsy.
The benefits of being unbalanced
A different mechanism emerged from lab experiments at Pacific Northwest, where the researchers grew ice on a super-cold plate in a vacuum chamber. The ice turned out to have a peculiar electrical property: Like tiny electric versions of bar magnets, the ice molecules had a slight preference to align themselves.
At these frigid temperatures, Iedema explains, "as ice grows from a gas onto a solid, it's so cold that the ice doesn't have a chance to reorient, it gets frozen into position." At somewhat warmer temperatures, he says, ice particles "would be free to jostle, to find a better configuration, where the charges would cancel each other out. But it's so cold that they don't have that freedom to explore. They get stuck, and wind up having a slight alignment."
Even if an impact breaks the ice, the electrical imbalance remains, he says. "If they are hit and disintegrate, there is some attraction to fall back together, and recombine, rather than bounce away."
But there's more. The flash-frozen ice has a different crystal structure than regular, hard ice. It's less dense, making the surface less "elastic." And when the researchers tested this "squishy" ice in the lab, stuff didn't bounce off the way it does when a solid object whangs into hard ice.
It's a nice explanation for something that needs explaining: how planets started forming. "People have had the hypothesis that this needs to happen in a certain time frame, or the material gets ejected from the forming solar system," says Iedema.
Eventually, when the electrically-attracted iceball grows to somewhere between a meter and a kilometer in diameter, gravity starts to play a role, he says, and the agglomeration accelerates.
But here's a caveat: The experiment was done at temps found at Jupiter's distance from the sun. Earth arose in warmer conditions, where ice would have been more "normal."
Yet Iedema suspects a similar electrical imbalance could have affected the heavier silicate dust -- the stuff that formed Earth in the early years of the Solar System and gave, for better or worse, an orbital platform for Jackass (the movie), Eminem (the rapper) and The Why Files (the nerd nuts).
At the distance of Earth, Iedema says, "some of the same electrical and mechanical properties we observed for water ice may be important."
-- David Tenenbaum
Sticky Ice Grains Aid Planet Formation: Unusual Properties Of Cryogenic Water Ice, H. Wang et al, The Astrophysical Journal, 620:1027-1032, 2005 February 20.
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