Researchers at Montana State University and Brandenburg University of
Applied Sciences in Germany have created a simple mathematical model
based on optical measurements that explains the stunning colors of
Yellowstone National Park's hot springs and can visually recreate how
they appeared years ago, before decades of tourists contaminated the
pools with make-a-wish coins and other detritus.
The model, and stunning pictures of the springs, appear today in the journal Applied Optics, which is published by The Optical Society (OSA).
If Yellowstone National Park is a geothermal wonderland, Grand
Prismatic Spring and its neighbors are the ebullient envoys, steaming in
front of the camera and 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," said Joseph Shaw, a professor at Montana
State University and director of the university's Optical Technology
Center. Shaw, along with his Ph.D. student Paul Nugent and German
colleague Michael Vollmer, co-authored the new paper.
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.
Montana State University, in Bozeman, Mont., is a short drive away
from Yellowstone National Park. 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 long wave
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 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.
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."
Future work for Nugent, Vollmer and Shaw includes delving further into infrared imaging at Yellowstone National Park.
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