Morning Glory thermal spring, in Yellowstone National Park (photo taken in August 2012). Credit: Joseph Shaw, Montana State University

Science Friday’s Picture of the Week is of the Morning Glory thermal spring, in Yellowstone National Park.  It hasn’t always had the colors you see today.  Scientists used biological, chemical, and optical data to construct a mathematical model of how the spring has changed over time.  Tourists have been tossing coins, rocks, and other debris into it for decades.  That debris has partially blocked the underground heat source and lowered the temperature.  Now, the spring is home to photosynthetic microorganisms that probably couldn’t tolerate the conditions before the contamination.  The model was described in a paper in the Journal of Applied Optics.

Pigments produced by swaths of those microbes—called microbial mats—are responsible, at least in part, for the brilliant yellows, greens, and oranges that now tinge Morning Glory and other thermal pools in Yellowstone. Each species has a preferred temperature, according to Brent Peyton, director of the Thermal Biology Institute at Montana State University. Because the springs are “hottest at the center and typically coolest at the edges,” a color gradient appears, he says.

But the colors we see also depend on the way light interacts with the water at different depths, a phenomenon that intrigued Joseph Shaw, director of the Optical Technology Center at Montana State University, and his team. “We lived right up the road from Yellowstone and were always wondering about the optics [of the pools],” he says.