51³Ô¹ÏÍø

Tiny glassy particles discovered on a remote rock platform in Western Australia have been identified as the signature of lightning strikes, opening new possibilities for understanding Earth’s deep past and the potential for life on other planets.

Lightning spherules preserved in ancient rocks could reveal the role thunderstorms have played in shaping environments over time – and may help us recognise similar phenomena on celestial bodies including Mars. Mark Boyd PhD researcher, Department of Earth Science and Engineering, 51³Ô¹ÏÍø

Research led by 51³Ô¹ÏÍø found that these microscopic spheres, called spherules, form when lightning heats rock to thousands of degrees, spraying molten droplets that cool rapidly in mid-air. The finding gives geologists a way to spot lightning activity in the geological record on Earth, as well as across the solar system.

“At a time when our planet is rapidly changing, it is vital we understand the processes that have shaped it over its 4.54-billion-year history,” said first author , PhD researcher in the Department of Earth Science and Engineering (ESE) at 51³Ô¹ÏÍø.

“Lightning spherules preserved in ancient rocks could reveal the role thunderstorms have played in shaping environments over time – and may help us recognise similar phenomena on celestial bodies including Mars.”

A local origin

Spherules are tiny, perfectly round glassy particles that form on Earth only through extreme heat. Volcanoes, massive meteor impacts and even forest fires can create them, but perhaps the most common source is lightning, which strikes the planet millions of times each day, superheating rock to over 3,000°C in an instant.

Until now, scientists have lacked clear criteria for identifying lightning-made spherules from the past, leaving a key gap in our understanding of how thunderstorms have shaped Earth’s surface, and potentially the surfaces of other worlds.

The study, , looked to Australia’s Pilbara region as a potential site for micrometeorites (meteorites smaller than two millimetres across), since this locality has been previously hit by many lightning strikes. 

“Although ‘lightning doesn’t strike twice’ is a good general rule, when you are the highest point in the landscape for miles you are a magnet for lightning in every passing storm,” said co-author , Associate Professor in Earth and Planetary Science at ESE.  

 The team managed to uncover spherules trapped on a 20-metre-high exposure of granite gneiss.

Out of 135 particles recovered, almost all showed compositions and textures that were in fact inconsistent with micrometeorites, volcanic ash or industrial pollution (the usual sources of such particles).

Instead, the team concluded that the spherules formed when lightning strikes melted the local granite, ejecting molten droplets that rapidly cooled and solidified in mid-air. Computer modelling by the team suggests this happens in mere seconds.

“The compositions of these Pilbara spherules match the minerals in the underlying rock almost exactly, ruling out the most common explanations for their existence, such as cosmic or industrial,” said Dr Genge. “These particles were made right where we found them, accurately preserving local environmental conditions.”

Reading the geological record

Lightning spherules can be far more abundant than micrometeorites in some environments. This can flip the interpretation entirely when studying ancient sediments. Dr Matthew Genge Associate Professor in Earth and Planetary Science, Department of Earth Science and Engineering at 51³Ô¹ÏÍø

Just one spherule stood out from the rest: a genuine micrometeorite from space. Using this single particle as a natural benchmark, the team calculated that the Pilbara rock platform had accumulated spherules for around 700 years, yielding an estimated 26 lightning spherules on each square metre per year.

“This tells us that lightning spherules can be far more abundant than micrometeorites in some environments,” said Dr Genge. “This can flip the interpretation entirely when studying ancient sediments – you might think that every spherule came from space, when it could actually be the weather.”

Beyond Earth

The implications extend far beyond Western Australia. Microspherules have been found on the Moon and Mars, but their origins are not always clear. Understanding how lightning creates distinctive spherule signatures could help interpret these extraterrestrial finds.

Lightning spherules may give clues as to the origins of life on Earth. One of the leading hypotheses involves lightning providing the energy (heat) to drive prebiotic chemistry – essentially ‘putting the dough in the oven,’ as the researchers note. Understanding how lightning interacts with rock, and its prevalence across Earth’s history, could therefore help assess whether this process for kickstarting life is viable.

What’s more, since we know that lightning also occurs on Saturn, and possibly on exoplanets, perhaps some of the ingredients for life might be available on these worlds too.

The findings may also shed light on the formation of chondrules – tiny glassy droplets found inside meteorites whose origin remains a debated question in planetary science, and which also forms part of Dr Genge’s research.

One hypothesis suggests chondrules were formed by lightning in the bygone gaseous cloud of the solar nebula. The Pilbara spherules, formed by lightning today on Earth, provide a modern analogue for comparison which could reveal how frequent or intense lightning was in the solar nebula.

A new climate proxy

Perhaps most significantly, lightning spherules offer a way to reconstruct thunderstorm activity in Earth’s deep past since they are small enough to avoid mechanical breakage and may survive the alteration of sediments into sedimentary rock after deposition (diagenesis).

The team has developed a checklist for spotting lightning-made spherules in ancient rocks: they should have simple, mineral-like compositions; contain leftover fragments of the original rock; show uneven chemical patterns within their glass; and have a chemical makeup that matches local minerals, not space dust.

With a clear fingerprint to distinguish lightning-made spherules from other sources, we have a reliable proxy to map thunderstorm activity across geologic time – possibly even billions of years. Mark Boyd PhD researcher, Department of Earth Science and Engineering, 51³Ô¹ÏÍø

“With a clear fingerprint to distinguish lightning-made spherules from other sources, we have a reliable proxy to map thunderstorm activity across geologic time – possibly even billions of years,” said Mark.

“And when we find similar spherules on other planets or within meteorites, this may tell us that lightning was striking those worlds too.”