PLANETS and comets might owe their existence to the unusual stickiness of ultra-cold ice. This could explain how the ice-covered dust particles that swirl around a newborn star clump together.
How planets and comets begin to form is a problem that has long bugged astronomers. Gravity is clearly important. But it is only when clumps of dust and ice grow to a few kilometres across that their gravity becomes strong enough to drag in new material. Smaller lumps can only grow by sticking to each other.
What鈥檚 more, there is a window of only a few million years in which this can happen before the particles of dust gradually spiral into the mother star and solar winds blast away the gases. This is too brief for random collisions to be responsible. Something must make the particles sticky.
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Now a team of chemists at the Pacific Northwest National Laboratory in Richland, Washington, led by Jim Cowin say that the important ingredient may be ice. The team simulated the way ice forms in space by cooling a small ceramic ball to 鈭�230 掳C and blowing water over it until the ball became covered with a layer of ice a few thousandths of a millimetre thick. Unlike ordinary ice, which has a tightly bonded crystalline structure, ice that forms at this temperature has a fluffy, amorphous structure.
The team then measured the height of the bounce when the ball was dropped from a height of 12 centimetres and compared it with the bounce of another ball covered in ordinary ice. They found that the ball covered in ordinary ice bounced to a height of 8 centimetres, while the ball covered in amorphous ice bounced only 1 centimetre. Their paper is to be published in The Astrophysical Journal.
The colder, more disordered ice absorbs more of the energy of the impact, says team member Martin Iedema, because the molecules rearrange themselves during the collision. The team speculates that dust particles coated with this amorphous ice could stick together rather than bounce apart.
Now the team hopes that observations of the icy rings of Saturn or data on the composition of comets could provide more evidence to support this mechanism. 鈥淲e鈥檙e chemists, not astrophysicists. What we want to do is get the response of the community,鈥� says Iedema.