MSU News – MSU researchers publish paper on the optics of Yellowstone thermal springs

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

A paper authored by Joe Shaw, professor at Montana State University and director of the university’s Optical Technology Center, along with his doctoral student Paul Nugent and visiting German colleague Michael Vollmer, details the new model and showcases images of the springs. The paper appeared recently in the journal Applied Optics, which is published by the Optical Society (OSA).

“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. “Researchers at MSU have explored Yellowstone’s thermal pools for decades, bringing us historic scientific discoveries and some of the most important lines of inquiry MSU has ever undertaken. Meanwhile, MSU’s optical science and engineering researchers have pushed the envelope of how we can measure of our world with laser and thermal imaging technology. It is exciting to see the two disciplines overlap.”

Yellowstone National Park is a geothermal wonderland, with Grand Prismatic Spring and its neighbors acting as envoys, steaming in front of visitors’ cameras and often gracing the internet with their ethereal beauty. While the basic physical phenomena that render these colorful delights 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 in the unique, stunning fashion that they do.

“What we were able to show is that you really don’t have to get terribly complex – you can explain some very beautiful things with relatively simple models,” Shaw said.

Using a relatively simple one-dimensional model for light propagation, the group was able to reproduce the brilliant colors and optical characteristics of Yellowstone National 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.

“When we started the study, it was clear we were just doing it for fun,” Vollmer said. But they quickly discovered there was very little in the scientific literature on the subject. That’s when things got interesting.

In the summer of 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 were able to create a simple model whose renderings of the pools were strikingly similar to actual photographs.

In the case of Morning Glory Pool, they were even able to simulate what the pool once looked like between the 1880’s and 1940’s, 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.

A general relationship between shallow water temperature (hence microbial mat composition) and observed colors was confirmed in this study. However, color patterns observed in deeper segments of the pool are caused more by absorption and scattering of light in the water. These characteristics – mats having greater effect on color in shallow water, and absorption and scattering winning out in the deeper areas – are consistent across all the measured pools.

“Our paper describes a very simple, one-dimensional model, that gives the first clue if you really want to do more,” Vollmer said.

“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.”

“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.”

Nugent, Vollmer and Shaw are continuing their research, delving further into infrared imaging at Yellowstone National Park.

Contact: Joe Shaw, (406) 994-7261, jshaw@ece.montana.edu.