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Butterfly Wing Colors Come From Space-Age Structures

Posted: 14 Jun 2010 12:00 PM PDT

Some butterflies get their fabulous colors from light refracted through membrane shapes that were first discovered by mathematicians and applied in space-age material science.

Using microscopes with three-dimensional nanoscale resolution, Yale University researchers found that shades of green in the wings of five butterfly species are produced by crystalline structures called gyroids.

The gyroid shape was conceived in 1970 by NASA physicist Alan Schoen in his theoretical search for ultra-light, ultra-strong materials for use in space. The new study describing the shape in butterflies is in the June 15 Proceedings of the National Academy of Sciences.

Gyroids have what's known as an "infinitely connected triply periodic minimal surface": For a given set of boundaries, they have the smallest possible surface area. The principle can be illustrated in soap film on a wireframe (see image at right). Unlike soap film, however, the planes of a gyroid's surface never intersect. As mathematicians showed in the decades following Schoen's discovery, gyroids also contain no straight lines, and can never be divided into symmetrical parts.

Yet even as mathematicians speculated on the nature of gyroids, entomologists found them in nature, at least in two dimensions. Microscopic images of butterfly wings showed that the surface of some scales, and how those scales reflected light, matched the predictions of gyroid math.

Those analyses looked only at scale surfaces. In the new study, the researchers look at three dimensions using a microscopy technique called synchrotron small angle X-ray scattering. Something like a combination of an electron microscope and X-ray machine, it revealed butterfly gyroids in structural high-definition.

The gyroids are made of chitin, a polymer used in insect exoskeletons, secreted by wing cells that fold naturally into gyroid shape. After cells die and decompose, the chitin shells remain. Light refracts through them, with subtle variations in gyroid shape and proportion producing different hues.

While the gyroids studied by the researchers were only responsible for green wavelengths, the basic principles — chitin shells in mathematically complex shapes — are likely used by butterflies to produce other colors, said study co-author Richard Prum, a Yale University biologist.

"By varying the kinds of proteins included in the membranes, butterflies may be able to develop strikingly different structures," he said.

Material scientists now use synthetic gyroids to make photonic devices, such as solar cells and communication systems, that manipulate the flow of light.

"Nature and the evolution of structures that create colors can be an excellent guide to how we might assemble and manufacture photonic materials," said Prum. "Organisms have already been there."

Images: 1. Wing scale photonic nanostructure, from electron micorscope to model./PNAS. 2. Soap film around a wire frame./Wikimedia Commons. 3. A gyroid model built by Alan Schoen./NASA. 4. Flickr/Claudio Gennari.

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Citation: "Structure, function, and self-assembly of single network gyroid (I4132) photonic crystals in butterfly wing scales," by Vinodkumar Saranathan, Chinedum Osuji, Simon Mochrie, Heeso Noh, Suresh Narayanan, Alec Sandy, Eric Dufresne, and Richard Prum. Proceedings of the National Academy of Sciences, Vol. 107 No. 24, June 15, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.

Japanese Spacecraft Returns From Asteroid Sampling Mission

Posted: 14 Jun 2010 08:55 AM PDT

The Japanese spacecraft Hayabusa returned from its seven-year trek to the asteroid Itokawa on Sunday, and a capsule that hopefully contains pieces of the asteroid was recovered in the Australian outback Monday morning. The mission was the first round trip to a planetary body beyond the moon, and — if the capsule isn't empty — the fourth space-sample return mission ever.

The capsule detached from the spacecraft at 7:51 pm Japanese time on June 13. The rest of the spacecraft burnt up in a dramatic, meteor-like blaze at 10:51 p.m., but a heat shield protected the capsule as it reentered the atmosphere. The capsule landed in the Australian Woomera Prohibited Area, 300 miles northwest of Adelaide, and was located around 11:56 p.m. It will be flown to Tokyo tomorrow, where scientists will open the capsule.

Hayabusa launched in 2003 with the goal of landing on Itokawa, a potato-shaped, 1,640-foot long asteroid several million miles from Earth, and coming back with a cartridge full of asteroid dust. Studying what the dust is made of could provide clues about the origins of rocky planets like Earth and Mars and the composition of the solar wind. The mission could also pave the way for future sample return missions and for landing humans on an asteroid.

The spacecraft landed on Itokawa twice in November 2005, but scientists were uncertain whether the metal bullet that was supposed to collect samples actually fired. Haybusa (Japanese for peregrine falcon) was supposed to return to Earth in 2007, but a series of setbacks, including broken control wheels, deterioration of its ion engine and battery malfunctions caused it to miss the window to return to Earth orbit until this year.

The capsule needs to be thoroughly cleaned and tested before its contents are known, which could take several months. If it has succeeded, Hayabusa will be the fourth sample return mission ever, after Apollo, the Stardust mission that collected samples from the comet Wild 2, and the Genesis mission that collected solar matter.

Image: NASA, JAXA

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