Bubbles formed in water can generate electric sparks powerful enough to ignite methane.

In marshy areas, observers sometimes notice a mysterious flash of blue light hovering above the surface. A plausible explanation for these “Will-o’-the-wisps,” or Ignis Fatuus, is that the flashes originate from the combustion of gases such as methane and phosphine, produced by decomposing organic matter in the murky waters below. However, the ignition source for these gases remains uncertain, with theories suggesting causes ranging from static electricity to lightning strikes.

Recently, Richard Zare from Stanford University and his team have observed spontaneous electrical sparks occurring between methane bubbles and air submerged underwater, dubbing them microlites. They assert that these occurrences have sufficient energy to ignite methane gas.

“We are continually uncovering new phenomena about water. They seem obvious once we grasp them, but prior to that, they appear entirely bizarre,” Zare remarks. “People typically associate water with extinguishing fires, not igniting them. Instead, it can generate sparks that light a fire. This concept is revolutionary.”

Zare and his team previously documented water droplets, comparable to the size of salt grains, generating sparks spontaneously. This led them to hypothesize that similar effects could arise from methane bubbles in water. They employed nozzles to inject microbubbles of methane combined with air into water and utilized a high-speed camera, photon counter, and spectrometer to monitor bubble interactions.

As the bubbles ascended through the water, they altered their shape and merged. Upon collision, the contrast in charge between the bubbles sparked flashes, which Zare’s team successfully recorded using both the camera counter and the photon counter.

They also analyzed the light frequency from the flashes, finding it consistent with signatures of specific chemically excited compounds. This indicates that the sparks are potent enough to ignite gases like methane.

“[The Italian physicist Alessandro] Volta was the first to assert that these ignis fatuus exist and speculated on their nature. In some ways, he was correct, but not for the reasons he thought. They actually originate from the droplets.”

“This is incredibly intriguing,” states Detlef Lohse from the University of Twente in the Netherlands. While this does not definitively resolve the mystery of Will-o’-the-wisps, it presents a credible explanation, and the findings are likely to stimulate further research.