Those brilliant rainbow hues of Yellowstone National Park’s famous geothermal pools were created by people -- and decades of our make-a-wish coins and other tourist detritus. Before us, the pools were a deep, uniform blue, according to the results of a mathematical model published in Applied Optics last week.

We now know that the vibrant colors of Sapphire Pool, Grand Prismatic Spring, and their delightful neighbors are the result of underwater vents interacting with communities of bacteria. To better understand how these physical and chemical variables relate to the optical factors that produce the observed hues, an international trio of researchers led by Joseph Shaw from Montana State University visually recreated the hot springs with a relatively simple model for light propagation.

Using handheld spectrometers, they took measurements of spectral reflection caused by the microbial "mats" and the absorption or scattering of light in several Yellowstone pools. They also used infrared thermal imaging cameras to measure water temperatures. Here you can see a photo of Grand Prismatic Spring followed by a thermal image with its temperatures:

Combining these with images and existing physical data, the team was able to reproduce the colors and optical characteristics of each of the pools. "What we were able to show is that you really don't have to get terribly complex," Shaw says in a news release. "You can explain some very beautiful things with relatively simple models." The researchers found a relationship between the colors we see and temperature of shallow water.

Different communities of microbes lend different colors to the pool, Los Angeles Times explains, and since they also prefer different temperatures, their arrangement is what creates the concentric yellows, greens, and oranges we see. But microbial mats only affect color in shallow water. It turns out, the blues in the deeper parts of the pool are the result of the absorption and scattering of light in the water.

The team simulated what Morning Glory Pool looked like between the 1880s and 1940s, when its temperatures were significantly higher (and less hospitable). Before Yellowstone became a national park in 1872, its waters appeared to be a uniform deep blue. However, the accumulation of coins, trash, and rocks has partially obscured the underwater vent, lowering the pool's temperature overall. This changed the composition of the microbial mats and shifted its appearance to the series of orange, yellow, and green we see today. Here are historic (left) and current (right) simulations of Morning Glory Pool:

Images: Joseph Shaw, Montana State University (photographs), Nugent, Shaw, and Vollmer (thermal image, simulations)