Saturn in a Jug

Two scientists recreate the earliest stages of planet formation in a washing machine-sized container.

Illustration by Scott Landry

GO BACK IN TIME five billion years, to the region of space where our solar system is now. No sun, no Earth, no planets--instead, a vast and seething ocean of gas and dust, shot through with bolts of lightning and warped into rings and whirlpools by great gravitational tides. In this cold, turbulent world, frozen gasses--hydrogen, oxygen, water, ammonia--floated molecule by molecule next to tiny rocky particles of iron and silica. All the matter that would later form our Earth--this page, our air, your body--jostled as billions of tiny grains in that primal nebula. The planets were born as particles in this planetary nursery, orbiting the young sun, swept and stuck together. The same process may be going on today, around other young stars throughout the galaxy.

The problem is, how exactly do you make a planet out of billions of particles smaller than a pinhead? Something must have held the little particles together long enough for them to form larger ones. Scientists lined up the usual cosmic force suspects. Gravity? No-- gravity depends on mass, and is too weak to keep very small masses together after they collide. The electrostatic force, the same one that binds socks to sweaters in your dryer? No--unlike gravity, it does bind tiny masses, but it can't hold together anything larger than a dust grain. Big chunks, about a kilometer across, generate a lot of heat when they collide. They melt and fuse together, and gravity begins to bind them into proto-planets. But, until recently, astronomers had no good idea what force might bind the tiny chunks of primordial dust long enough for them to form those big ones.

A clue might come from the rings of Saturn. To astronomers, Saturn's rings look a lot like our early solar system. Since Earth and its sister planets orbit the sun in the same flat plane, astronomers believe that the matter that formed them was originally spread out in one vast flat ring.

Two scientists at U.C. Santa Cruz, astrophysicist Douglas Lin and physicist Frank Bridges, are exploring the problem of planetary formation as an outgrowth of their interest in the rings of Saturn. "One of the long-term motivations for solar system exploration is to understand our roots," says Lin. "Until recently, we only had the relic of a nursery--we had the planets. Now, the best analogy to this is Saturn's rings." The particles in Saturn's rings are mostly water ice, and they range from pea-sized to bigger than a house. While the larger particles are still too small to melt and fuse, they are still far bigger than scientists expect. In the push and pull of gravity tides around the giant planet, ice balls ought to break up before they grow past grapefruit size--at least in theory. The Santa Cruz scientists think the mysterious sticking force that helped the planets form could still be operating today, letting ice balls grow like proto-planets in the rings of Saturn.

In a unique series of experiments, Lin and Bridges have been colliding ice balls in a vacuum at -175 C. While physicists at Germany's JENA facility have modeled planetary formation by colliding tiny, rocky dust balls shot from cannons, the Santa Cruz experiments are the only ones in the world designed to replicate the speed, temperature--and iciness-- of particles in Saturn's rings. They have found that particles stick only when both surfaces are coated with frost. Ordinary frost, just frozen water vapor. At collision speeds expected within Saturn's rings, frosted ice sticks at least a hundred times more strongly than smooth ice colliding at the same velocity.

"We call it the Velcro model," says Bridges' graduate student Kimberley Supulver. "Frost is fluffy, and the little peaks and valleys hook together like Velcro when they collide." Bridges thinks frosty "Velcro" might have been the missing sticking force that let the primordial dust form large chunks. "You need some glue to hold them together till they reach kilometer size," says Bridges. "We propose that frost is the original solar-system glue."

The experiments that revealed the glue started with Lin's desire to study short, glancing collisions, the kind that jostle all the bits in Saturn's rings into a myriad of ringlets. Since those ringlets are hard to see at 800 million miles away--Saturn's closest approach to Earth--Lin teamed up with Bridges to re-create ring conditions a little closer to home. Bridges designed a kind of Saturn-in-a- bottle for the laboratory, an experimental apparatus composed of a circular pendulum inside a steel barrel. The whole thing would fit in the drum of a washing machine--"we call it 'the Can'," says Supulver. While its insides look nothing like the majestic planet's ring fields, conditions in "the Can" are a fair imitation of those you'd find in space next to Saturn. Its chamber can be cooled to -175 C with liquid nitrogen and pumped out to create a vacuum. To simulate colliding ring particles, the apparatus sends a polished ice sphere the size of a tennis ball bumping into a stationary ice brick. The ice ball, attached like a frozen lollipop to the side of the pendulum wheel, rises when the wheel is pulled backwards by magnets. When the magnets are released, the wheel swings down, and the ice ball collides with the brick.

The ball-and-brick model lets Lin and Bridges study the way ice behaves when it collides at different speeds and with different surface textures. The pendulum can be raised or lowered to create faster or slower collisions. The ice ball and brick can be left smooth and glassy, or frosted with water vapor to create rougher surfaces. When the ball hits the ice brick, the amount of energy lost is measured by how high the ball bounces as it returns. If the ball doesn't bounce, but sticks to the ice brick, the collision is called completely inelastic. The researchers measure how firmly the ice sticks by recording the amount of force it takes to pull the ball up off the brick.

And this sticking is a force to be reckoned with when considering how the planets formed. Water vapor could have precipitated on the surface of pinhead-sized rocky particles in the planetary nursery, just as it does on the iceballs. "The better theories of solar-system formation contain a number, a sticking parameter," Bridges says, "but they don't explain how that (sticking) can happen. What we're trying to do is provide a viable mechanism for this step in the evolution of the planets."

Lin and Bridges first described the Velcro model in 1991. Since then, they have concentrated on measuring the energy lost when the ice ball is set for a glancing collision. In a paper appearing in the March 1995 issue of Icarus, the American Astronomical Society's journal of solar system studies, Supulver reports that surprisingly little energy is lost when glancing collisions are frost-free.

Some scientists doubt that frost is the only thing sticking small particles together, Bridges concedes, but other models, like ones proposing that electrostatic forces are responsible, also "have problems", he said. "They work fine for micrometer-sized particles, but not for bigger particles. In thousands of collisions, we observed no sticking unless there was frost."

In a field where debate over theory is sometimes heated, the pioneering Santa Cruz ice experiments have drawn little criticism. " Nothing is worth doing without a critic, right? Maybe our work isn't worth doing," jokes Lin. "Actually, we have no strong competitor...essentially, because we are so far ahead of everyone else."

And hardly anyone else is running a laboratory experiment, when it comes to planetary formation. In a field where mathematical models and computer simulations are the rule, does her job description ever draw a double-take from colleagues? Kim Supulver laughs. "Yeah! People are always impressed. They say, 'you do experimental astronomy? Wow! How do you do that?' Usually you have to look out (into space), or do computer models. "

The ice ball experiments have gained Santa Cruz an international reputation among solar-system scientists. "It's unique work, and results of that work have been used by a whole lot of theoretical modelers around the world, who've been trying to understand the local structure of planetary rings," said Dr. Jeff Cuzzi, a leading space scientist at NASA Ames, who also studies solar system formation. "It's been very useful to a whole lot of people trying to understand these things to have a well-parametered experimental model of these inelastic collisions."

The work is far from over. Next, Bridges will study the dynamics of glancing collisions with frost. And for Supulver's thesis, she plans to explore the sticking properties of frosts other than water ice. "We'll try carbon dioxide first, try to discover the difference it makes, compared with water vapor," she says. "Also methane, and ammonia. They were probably very prevalent in the solar nebula at that stage."

In a closet-sized laboratory on the fourth floor of the building that houses UCSC's astronomy program, Supulver's brow wrinkles thoughtfully. She is repairing "the Can," now in pieces on a bench, surrounded by wrenches and snippets of cable. Its heating mechanism is refusing to bring it back to room temperature. Hair-fine copper wires connect delicate sensors to the big brass body of the vacuum chamber, hulking like an oversized spaghetti pot on the bench. The next phase of the voyage back in time to the start of our solar system is still in the planning stage, and there are a hundred new scenarios to test. "We probably should mix gasses together, and see if that has any effect," she says. "Because that's probably how it was, in the early solar system. We're finally starting to get some real data that we can put into the theoretical models."

Science Notes / Summer 1995 / Science Communication Program
University of California, Santa Cruz