Dust Demon

Physicist David Peak has big plans for small dust balls.

By Bennett DavissMar 1, 1992 6:00 AM


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He’s not out to create a big new world. His lab, a windowless cubicle at Union College in Schenectady, New York, couldn’t accommodate anything much larger than a sandbox. It’s actually just the principle he’s after: the fundamental process by which real estate evolves from cosmic debris.

Researchers have long assumed that the solar system’s inner planets--the rocky ones--grew from collisions of giant, moonlike planetesimals. The planetesimals, of course, grew from similar collisions--bigger rocks from smaller rocks all the way back to primordial grit. The problem is that no one ever bothered to check this idea in the lab.

You have to understand that planetary people are more concerned with the question of what happens when two planetesimals collide, Peak says. People spend a lot of time doing computer simulations of collisions between really large objects. The study of the very beginnings of the solar system hasn’t received a lot of attention yet.

Except from a handful of researchers including Peak himself. For three years now the 50-year-old physics professor and his undergraduate students have been trying out a relatively new, rival concept: the idea that planets coalesced not from space rocks but directly from icy clouds of cosmic dust.

In making planets, pebbles don’t work, Peak declares. If you shoot pebbles at each other at Kepler-like velocities, they don’t stick together. They shatter or fly apart. So unless there’s some magical interaction between pebbles that existed billions of years ago and doesn’t exist now, there’s something wrong with that idea.

Peak, an expert in fractals and nonlinear dynamics, was introduced to the alternative idea in 1986 by Bertram Donn and Joseph Nuth, astrophysicists at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Bert had pioneered the idea, Peak says, that contrary to popular opinion, protoplanets were very low density aggregates of dust and ice instead of large condensed objects that had evolved from smaller condensed objects.

Low density is the key. Rocks fly apart after they collide because they’re too dense to be compressed further by the force of impact. Fields of fluff, on the other hand, can easily be compressed, and they can also intertwine. Their growth pattern, in fact, is comparable to that of homegrown dust wuzzies, which, significantly, are fractals: like clouds or coastlines, these fuzzy objects are irregular shapes that look the same no matter how large or how small the scale on which they are examined.

In any fractal shape, the structure of the large object results from the structure of the original small object; in this case the structure of the dust balls is determined by that of the dust grains. They’re not round beads, Peak explains. They form strings. The strings grow branches, the branches intertwine, and soon wuzzies drift out from their hatcheries under sofas and beds and even, it seems, outer space. If they managed to collide and stick together, says Peak, they’d aggregate into something that had a very complex, fractal filamentary structure.

As a dust ball grows, the structure reproduces not just its strings and branches but also the voids that separate them. One result of this, says Peak, is that the surface area remains large while the density remains low--and so cosmic dust bodies would tend to float like dandelion seeds in the wind currents of a condensing cloud. Solid objects, by contrast, would behave more like lead shot in this faint breeze: they would quickly pick up speed as stellar gravity won out over wind pressure, and they would tend to get locked into independent orbits as tiny asteroids. The fluff balls, meanwhile, would drift lazily with the swirling gas, bump gently against neighboring fluff balls, and form ever larger aggregates.

It’s a true application of fractal theory, Peak beams. The two neat things these fractals get you are compressibility and slow motion, and that’s Bert Donn’s great contribution.

In theory, at least, it’s a short step from a miles-wide dust bunny to an embryonic planet. A dust aggregate could eventually become so large that it would compress into a reasonably high density under its own gravitation, Peak surmises. Radioactivity also could be a catalyst in amalgamating rocklike densities. In the early days of the solar system, dust collections would have had a lot of radioactive isotopes in them. Really large dust balls might have formed with interiors that became so hot from radioactive decay that there could have been localized melting, with material recrystallizing as rocks.

Donn’s experimental work was less elegant than his theory, though. Peak calls some of his early tests embarrassingly homey. In one, Donn threw snowballs into a snowdrift to see how the flakes would compact.

We thought we could help him out with some concepts that were a bit more sophisticated, says Peak. Funded by NASA, Peak and his students set out to discover the mechanical properties of space dust. They constructed vacuum chambers and filled the bottoms with powdered quartz, which is one-twentieth as dense as the mineral in rock form. It’s about the lowest-density stuff you can find that’s still stuff, Peak notes. According to calculations and computer simulations, it’s roughly analogous to the density of the particles in those early solar system dust bodies.

Left to itself, though, dust in a vacuum doesn’t do much. How could such bits of detritus self-assemble? Peak and his crew started out by dropping projectiles into their airless fluff. After plopping pellets of various densities, from styrofoam to lead shot, into both the powdered quartz and into talcum powder (magnesium silicate, a legitimate planetary ingredient), they examined the resulting craters to see how the materials compacted. What they’ve seen suggests that, in addition to condensing on their own, dust wuzzies might have been aided by asteroids or planetesimals on their way to planethood.

When a dense object strikes another dense object, as when asteroids struck the moon, material is sprayed in all directions, Peak points out. But when we impact these low-density dust balls, the energy remains focused in a forward direction. In other words, cosmic rocks entering a primordial dust body might have acted like a snowplow, pressing the loose material into denser wads.

Peak and his team have also learned that powders in a vacuum trap heat. You can imagine that these putative collisions between dust bodies would convert the kinetic energy of motion into heat, he says, and that the heat would hang around for a very long time. If the collision was big enough, it might well have melted a region of the dust ball, which would gradually crystallize and form a rock.

Still, Peak knows that there’s more to a planet than quartz dust and heat. He recently put the finishing touches on a four-foot Pyrex cylinder in his lab for the next round of tests. Because of Pyrex’s insulating properties, we were able to cool our samples to very low temperatures, he explains. We wanted to work next with powder-and-ice mixtures--stuff that’s much closer to what comets are made of and probably the combination of materials that the earliest condensed objects needed to get the planetary process going. Just as he expected, Peak found that the adhesive properties of ice are important in making big condensed objects.

A bigger question confronts the theory: Could cosmic dust balls have condensed into planets within 5 billion to 10 billion years? That’s half the age of the universe, from the Big Bang to the beginning of geologic time. If dust balls can’t shape up in such a generous allotment of time, then Peak’s theory might apply to future planets but not to the one we’re standing on, or any that we’ve observed so far.

It’s an interesting challenge to this whole concept, Peak admits. Pebbles colliding at velocities of kilometers per second give you one time scale; fluffy fractals coming together at a few meters per second give you an enormously different one. There are some back-of-the-envelope calculations you can do, but we really need a systematic study of the question.

Peak will soon publish his results; he has already presented them informally at conferences and has discussed the tests with astronomers and other physicists. The reviews so far are mixed, but Peak’s view is firm. The fundamental question here is whether very low density dust aggregates, colliding at low velocities, have enough cohesive power and mechanical integrity to grow into a larger, solidlike object. The critical take-home lesson from what we’ve been doing is that they do.

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