Visitors from around the world view with awe the stunning colors of thermal pools in Yellowstone National Park, situated primarily in Wyoming and extending into Montana and Idaho. The brilliant hues of these natural wonders – Morning Glory Pool, Sapphire Pool, Grand Prismatic Spring and others – have engaged the curiosity of Dr. Joseph Shaw, professor at Montana State University and director of the university’s Optical Technology Center. With the goal to understand the pools’ colors from an optical perspective, Shaw and his team developed a simple mathematical model based on optical measurements to help explain the physical source of the hues. Shaw, along with MSU doctoral student Paul Nugent and Dr. Michael Vollmer from Brandenburg University of Applied Sciences in Germany, published the work in Applied Optics (doi: 10.1364/ao.54.00b128). “Much of the science explaining the pool colors was already known, but different pieces were known in different scientific communities,” Shaw said. When the team reviewed the scientific literature, it found, for example, that biologists understood the colors were related to the different species of microbes living in mats that coat the rock beneath the water. The biologists didn’t focus on the optical effects of water and air, which, conversely, were the main focus in the ocean optics community literature. A new mathematical model based on optical measurements helps explain the colors of Morning Glory Pool and other thermal pools at Yellowstone National Park. Courtesy of Joseph Shaw, Montana State University. Scientists who understood the microbial mat biology – how the mat colors are related to water temperature – concluded that different color regions of the pools have different water temperatures, Shaw explained. The new model presents a simpler possible explanation. “I think the most important aspect of our work was putting these pieces together and explaining them in a fairly simple, physically intuitive way, with a focus on the important role of water optical effects in deep water (and clarifying that the role of reflected skylight is quite small).” With their model, the researchers reproduced the colors and optical characteristics of the hot springs by accounting for each pool’s spectral reflection due to microbial mats, optical absorption and scattering of water, and the incident solar and diffuse skylight conditions when the measurements were taken. At each pool the scientists visited during their 2012 study, they simultaneously photographed the pool, recorded spectra of the reflectance at multiple points in and around the pool, and recorded thermal imagery of the pool and its surroundings. They then consulted scientific literature to identify gaps in knowledge or explanations. This thermal image of Yellowstone National Park’s Grand Prismatic Spring shows apparent temperature in degrees Celsius. Courtesy of Paul Nugent, Joseph Shaw and Michael Vollmer. They also programmed a “simple radiative transfer” computer model to simulate what they had seen. Here, “radiative transfer” refers to the mathematical calculation of how light transmits from one point to another. “This was a purely optical model that doesn’t explicitly include biology or chemistry,” said Shaw. The model, initiated with the team’s spectral measurements, indirectly incorporates the appropriate biology and/or chemistry of the microbial mats. In the process, researchers used their visible photographs and thermal images to verify the previously published relationship between microbial mat color and water temperature, he added. The team plans to encourage the National Park Service to incorporate missing optical-related information about the pool colors into its park brochures and signs, which at this time focus primarily on the microbe aspects of the pools. To help people understand the physics of the colors they observe, Shaw’s team is creating intuitive teaching modules for easy online access. The new mathematical model is also of interest to biologists, as it may help them monitor and understand changes in pool colors. “The model enables ‘what-if’ studies that ask what would happen if one parameter were changed,” said Shaw.