When NASA’s Curiosity rover arrived at an unusual outcrop on Mars this month, it found a pristine crisscross of pure, elemental sulfur crystals. It was the first time a Martian deposit of such exotic sulfur had been found, and science quickly got the hint. Channeled by the flowing water of long ago, this sulfur needed an explanation, one that could illuminate the geologic history of Mars.

Curiosity stumbled upon them in late May 2024 by driving over and cracking open a rock – and uncovering such bright yellow crystals inside that scientists could immediately confirm, with the help of an instrument aboard the rover’s arm, that they were pure elemental sulfur.

In spite of sulfates being a common type of salt on Mars, the crystals appear to be pure sulfur, possibly formed when water evaporated, leaving the sulfur behind.

‘This really is a serendipitous find,’ said mission scientist Ashwin Vasavada from NASA’s Jet Propulsion Laboratory in Pasadena. ‘We didn’t expect to find elemental sulfur on Mars, and this greatly expands our research possibilities.’

Just as importantly, finding elemental sulfur further demonstrates that Mars was a much wetter and, perhaps, habitable planet. Sulfur is an essential building block for life, and its discovery indicates that Mars may have been an environment hospitable for microbial life.

Curiosity has been creeping its way across the sulfate-rich region of Mars since October 2023 and has turned up other compelling signs of wet, warmer pasts, such as evidence of ancient riverbeds and lakes. It has continued to investigate the region to date, and is expected to share with scientists further hints of the planet’s wet past.

The identification of elemental sulfur crystals represents one more piece in the giant puzzle that is Mars; one that continues to hold a few surprises in store as we extend our understanding of the probability for life there, and of an intricate and often subtle geological history.

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