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MSU researchers study optics of Yellowstone thermal springs

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BOZEMAN – Researchers with Montana State University’s College of Engineering have used optical technology to create a simple mathematical model explaining how temperature and chemical composition in Yellowstone’s thermal springs combine to create their amazing colors.

The model can be used to visually recreate how the springs appeared years ago, before decades of contamination from make-a-wish coins and other man-made detritus.

The paper, which details the new model and showcases images of the spring, was recently published in the journal “Applied Optics” and authored by Joe Shaw, an MSU professor and director of the university’s Optical Technology Center, along with doctoral student Paul Nugent and visiting German colleague Michael Vollmer.

“This is a paper that showcases MSU’s strength in optical science with the locally interesting application of better understanding Yellowstone’s hot springs,” Shaw said. “MSU’s optical science and engineering researchers have pushed the envelope of how we can measure our world with laser and thermal imaging technology.”

While the basic physical phenomena that render the bright colors of Yellowstone’s geothermal features have long been scientifically understood – they arise because of a complicated interplay of underwater vents and lawns of bacteria – no mathematical model existed that showed empirically how the physical and chemical variables of a pool relate to their optical factors and coalesce.

Using a relatively simple one-dimensional model for light propagation, the group reproduced the brilliant colors and optical characteristics of the park’s hot springs by accounting for each pool’s spectral reflection due to microbial mats, their optical absorption and scattering of water and the incident solar and diffuse skylight conditions present when measurements were taken.

“We didn’t start this project as experts on thermal pools,” Shaw said. “We started this project as experts on optical phenomena and imaging, and so we had a lot to learn.”

In summer 2012, Vollmer, on sabbatical from the Brandenburg University of Applied Sciences, travelled with Shaw and Nugent to the park. Using handheld spectrometers, digital SLR cameras for visible images and infrared thermal imaging cameras for non-contact measurement of the water temperatures, the group took measurements at a number of pools in Yellowstone, including Morning Glory Pool, Sapphire Pool and Grand Prismatic Spring.

Using these data, along with previously available information about the physical dimensions of the pools, they created a simple model whose renderings of the pools were strikingly similar to actual photographs.

In the case of Morning Glory Pool, they were able to simulate what the pool looked like between the 1880s and 1940s, when its temperatures were significantly higher. During this time, its waters appeared a uniform, deep blue.

An accumulation of coins, trash and rocks over the intervening decades has partially obscured the underwater vent, lowering the pool’s overall temperature and shifting its appearance to a terrace of orange-yellow-green. This change from blue was demonstrated to result from the change in composition of the microbial mats, as a result of the lower water temperature.

“There are people at my university who are world experts in the biological side of what’s going on in the pools,” Shaw said. “They’re looking for ways to monitor changes in the biology – when the biology changes, that causes color changes – so we’re actually looking at possibilities of collaborating in the future.”

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