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Wednesday, May 23, 2007

On an Icy Moon of Saturn, Gravity Causes Plenty of Moving and Shaking

When baking a potato, the smart cook pierces the skin in several places. That allows water vapor to escape as the potato heats, avoiding a potentially explosive situation.

Enceladus, an icy moon of Saturn that is about 300 miles across, is something like a baked potato. But instead of knife cuts, it has long parallel cracks in its icy shell, near its south pole. In 2005, the Cassini spacecraft spotted plumes of vapor escaping through these cracks, informally called “tiger stripes.”

Clearly something is heating the inside of Enceladus, and something is causing the cracks to open. Now studies by two research teams suggest what that something is: gravity.

Enceladus, a mere Tater Tot compared with Saturn, is hugely affected by the planet’s pull. The moon’s orbit is eccentric but its rotation is steady, so Saturn’s gravity affects it unevenly, creating tidal forces within it.

Those tidal forces cause lateral slip along the cracks, similar to the movement of a seismic fault on Earth, report Francis Nimmo of the University of California, Santa Cruz, and colleagues in the journal Nature. The shearing action generates enough heat, the authors say, to turn the water beneath the ice to vapor.

But how does the vapor escape? A second Nature paper, by Terry A. Hurford of NASA Goddard Space Flight Center and colleagues, suggests that the tidal forces periodically put the cracks in tension, forcing them open.

A Pathogen’s Movements Are Fairly Loopy And Quite Formulaic

The pathogenic bacteria Listeria monocytogenes glide through an infected cell with the greatest of ease. They take over the cell’s own protein-assembling mechanism to build a filamentous tail, bit by bit, on their surface. As the tail grows, the bacteria are pushed forward — hard enough to force them through a cell membrane and into a neighboring, healthy, cell.

Listeria have something of the grace and style of a figure skater. If you constrain the movements to two dimensions — squashed on a microscope slide, for instance — they trace spirals, figure eights, S curves and other patterns, as shown by the tails, which dissipate over time.

Vivek B. Shenoy, an engineer at Brown University who normally works on the mechanics of semiconductors, was introduced to Listeria by Julie A. Theriot, a Stanford University scientist who studies them, at a “boot camp” at the California Institute of Technology to introduce physical scientists to biology. “We were looking at all these bacteria tracks and it was pretty clear there was a pattern,” Dr. Shenoy said. “But we weren’t sure what it was.”

Applying his skills in mathematical analysis, Dr. Shenoy came up with a relatively simple model — a formula that, with only a few variables, can account for the various trajectories. He, Dr. Theriot and others published the findings in The Proceedings of the National Academy of Sciences.

Critical to understanding the movements, Dr. Shenoy said, is realizing that the proteins being linked together at the surface are not just pushing the bacteria but also spinning them. “It’s not just straight propulsion,” he said. “The proteins are able to impart other types of motion.”

The work should help scientists understand why the bacteria move as they do. “These bacteria don’t have much of a choice in where they’re going,” Dr. Shenoy said. “They’re basically just pushed by the tail.” Characterizing the movements, he added, “is the first step toward understanding the mechanics.”

A Fine-Art Cleaner for Those Places That Are Hard to Reach

Clogged pores. They’re the bane of teenagers, and of art conservators who work on frescoes, too. The tiny pits and pores in the plaster can become filled with contaminants, making them very hard to clean.

Piero Baglioni, a chemist at the University of Florence in Italy, has worked on alternative methods for cleaning frescoes and other works of art for years. The technique involves creating microemulsions, which consist of fine droplets of organic solvents and water, on the order of 50 to 100 nanometers wide.

In the journal Langmuir, Dr. Baglioni and colleagues report on their most efficient microemulsions yet, which they have used to remove acrylic polymers applied over frescoes during earlier, faulty conservation efforts.

Dr. Baglioni said determining the correct formula was difficult, but once it was obtained, the microemulsions formed spontaneously. They can be applied to a surface with cellulose fiber and washed off with water.

One advantage of the microemulsions, he said, is that the droplets are so small they can easily penetrate and remove the polymer inside the pores, “which pure solvents cannot do.”

Another advantage is that far less solvent can be used — as little as a gram or two in a liter of water — so the environmental impact is much less.

In the same paper, Dr. Baglioni’s team reported on using a different microemulsion to clean a heavy scum of hydrocarbons and salts from artwork damaged in the Florence flood of 1966. That’s another advantage of the technique — it can be customized for the particular restoration task.

Dr. Baglioni said art restorers tended to be a conservative bunch, but added, “In the next 10 years, probably everybody will use this approach.”

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