If you’ve ever been on a cave tour where the guide shines a black light, you’ve seen the walls and formations glow. The stunning fluorescence is often the result of impurities trapped in calcite rocks that illuminate under UV light that is otherwise invisible to the human eye.
Now, researchers at the University of Northern Iowa are using these UV spectral signatures to assess the chemical makeup of these cave features.
“It’s a much better way of getting that first look at the cave” before deciding which parts of the rock to break off and bring into the lab, says Joshua Sebree, an astrochemist at the University of Northern Iowa leading the research. “Many of these formations took 10,000 years to form; they're not going to grow back again in the human lifetime.”
His team is studying this fluorescence in the extreme conditions of underground cave systems to help identify the ingredients and chemistry to support life in extraterrestrial environments.
“It’s important to have terrestrial comparisons so we know what to look for on the icy moons of Saturn and Jupiter,” which may harbor organic compounds, says Jacqueline Heggen, an undergraduate biochemistry student at the University of Northern Iowa who presented the work at a talk on Tuesday at a Division of Analytical Chemistry and Division of Geochemistry session at the American Chemical Society Spring 2025 meeting.
Studying a wide variety of caves, such as Wind Cave in South Dakota, the Mystery Cave in Minnesota, and the Coldwater Cave in Iowa, Sebree and his students noted a range of fluorescent hues. A bright pink glow seemed to suggest manganese, and the researchers suspect that a brilliant blue to soft green fluorescence can indicate the presence of organic compounds such as fulvic acid and humic acid.
But they found that the UV spectral signature varies from cave to cave. “The fluorescence is based on the [cave’s] water source—more specifically, the organics in the water source,” Heggen says.
When leaves and roots decompose, they break down and leach organic compounds into water. This groundwater percolates through cracks and crevices and gets trapped in cave formations. “Where we’ve got a lot of vegetation on the surface, we get really, really rich fluorescence,” Sebree says. “So, we actually try to establish if there is a fully formed nutrient trail from the surface of the cave down to the bottom.”
Sharon Bone, a biogeochemist at the SLAC National Accelerator Laboratory at Stanford University, who organized the session but wasn’t involved in the research, thought it was “interesting” that the spectral signatures were changing “depending on the cave and depending on the chemistry of the solution.”
The research team is now developing a database of fluorescence fingerprints they’ve found in different caves. They’re also trying to figure out how organic compounds fluoresce in subfreezing conditions like those on icy moons thought to harbor habitable environments and the building blocks for life.
For instance, Heggen has been collecting water from underground caves and freezing it using liquid nitrogen. She’s comparing the fluorescence fingerprints in the cryogenic ice samples to those obtained from the rocks while also figuring out what molecules this fluorescence technique fails to detect.