UNB researchers help confirm first evidence of elemental sulfur on Mars

Two researchers at the University of New Brunswick have helped confirm something that has never been directly observed on Mars before: pure elemental sulfur sitting in the Martian soil. The discovery, published this month in Science, emerged from data collected by the Curiosity Rover as it explored a canyon system called Gediz Vallis, which cuts through the layered rock of Gale Crater. Dr. Lucy Thompson and Dr. Catherine O'Connell-Cooper, who work at UNB's Planetary and Space Science Centre, operate the Alpha Particle X-Ray Spectrometer—a Canadian-built instrument mounted on the rover that analyzes rock composition by bombarding samples with alpha particles and measuring the X-rays that bounce back.

The finding matters because it rewrites what we thought we knew about Mars' chemical past. For decades, sulfur detected on the Red Planet had appeared only in compound forms—locked inside sulfates and sulfides, bound to other elements. When Curiosity's cameras spotted an unusual field of bright rocks in the Gediz Vallis region that looked different from the surrounding terrain, Thompson's team used the spectrometer to take a closer look. The analysis revealed something unexpected: sulfur in its pure, elemental state, not chemically bound to anything else. "All the sulfur that we'd found had pretty much been present as sulfate," Thompson said. "So it was really unusual and surprising and exciting to find elemental sulfur."

The discovery points to a chapter of Mars' history that scientists are still working to understand. The sulfur deposits suggest that billions of years ago, sulfur-rich fluids were moving through the subsurface of Mars, likely originating in underground ice. Over time, these fluids deposited elemental sulfur that accumulated in layers. The deposits were then buried and protected by debris flows and avalanche material—essentially sealed away from the Martian surface. Only recently, through erosion and weathering, have these ancient deposits been exposed again, allowing Curiosity to find them.

Thompson was one of the scientific leads for the Gediz Vallis campaign, which had been identified as a priority investigation long before Curiosity touched down on Mars in 2012. The canyon itself is scientifically valuable because it cuts through Mount Sharp, a five-kilometer-high mountain at the center of Gale Crater whose rock layers record a fundamental shift in Martian climate—from a wetter, more habitable environment in the planet's ancient past to the cold, dry desert it is today. By studying what Gediz Vallis reveals, researchers can read that transition in the geological record.

The paper involved 23 authors and researchers from NASA, the Canadian Space Agency, the University of Guelph, and Washington University in St. Louis. The lead author, Dr. Scott VanBommel of Washington University, is himself Canadian and a former University of Guelph graduate student who conducted the detailed analyses of the spectrometer data that confirmed the sulfur's elemental nature. Thompson and O'Connell-Cooper have been involved with the Curiosity mission for more than a decade, not only operating the spectrometer but also helping decide which geological targets the rover should investigate and collaborating with the broader scientific team to interpret what the data means.

The discovery underscores the continuing importance of Canadian technology and expertise in planetary exploration. The Alpha Particle X-Ray Spectrometer was built, funded, managed, and operated by Canadians—a fact that becomes more significant as space agencies around the world compete to understand Mars and plan future human missions. Each new piece of information about the planet's past chemistry and climate helps scientists assess whether Mars could have supported life and what conditions future explorers might encounter. The elemental sulfur deposit is one more clue in that larger puzzle, and it came from a tool and a team that have been asking the right questions for more than a decade.