Nasa's 'strangest find': pure sulphur on Mars

Nasa's Mars rover has made its "most unexpected" discovery since it landed on the red planet in 2012: rocks made of sulphur.

On 30 May, the Curiosity rover "happened to drive over a rock and crack it open, revealing yellow-ish green crystals", said CNN. Nasa used the robot's instruments to analyse the rock, receiving data that indicated it was pure sulphur. Scientists were "stunned", said Nasa when it revealed the discovery last week. 

"Finding a field of stones made of pure sulphur is like finding an oasis in the desert," said Ashwin Vasavada, Curiosity's project scientist at Nasa's Jet Propulsion Laboratory in Southern California. It's the "strangest find" and "most unexpected" of Curiosity's 12 years on Mars. 

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Previous research has suggested that pure sulphur "may have played a key role in the origin of life on Earth", said the Daily Mail. Now scientists are trying to establish what its presence can tell us about the red planet's past. But the discovery adds to the "growing evidence of other life-sustaining elements identified on Mars".

Curiosity's Mars mission

In November 2011, Nasa launched the Mars Science Laboratory (MSL) mission from Florida with the Curiosity rover aboard – the fourth Nasa robot sent to Mars since 1996, and at the time the largest and most capable. Its main goal was to determine whether the planet could have once supported life.

Weighing almost a tonne and approximately the size of a car, Curiosity landed successfully on Mars's Gale Crater the following August. The six-wheeled scientist transmitted its first images on 6 August 2012, showing its shadow on the red planet's surface. 

Since 2014, Curiosity has been ascending Mount Sharp, a three-mile-high central peak in the crater. Each layer of the mountain "represents a different period of Martian history", said Nasa. 

Curiosity has already found organic deposits trapped in mudstone that once contained organic molecules. That suggests that life could have existed on ancient Mars – although it doesn't prove it. 

This year, the rover arrived at the Gediz Vallis channel, a winding groove carved into a steep side of the mountain and long observed from Earth. Scientists believe the channel was carved by liquid water and debris 3 billion years ago. It is "one of the primary reasons the science team wanted to visit this part of Mars", said Nasa. There's "much to learn from the dramatic landscape".

In February, the rover took photos of "waves" carved into "an ancient lakebed by Martian water millions of years ago", said LiveScience. And in May it found rocks containing manganese oxide – "the best evidence yet that the Red Planet once had an oxygen-rich, Earth-like atmosphere".

Pure sulphur 'shouldn't be there'

Curiosity has already found sulphur-based minerals – a mix of sulphur and other elements in compounds known as sulphates. But this newly discovered rock is made of elemental, or pure sulphur. "No one had pure sulphur on their bingo card," said Vasavada.

Pure sulphur rocks are typically found in hydrothermal vents on Earth. They form in a "narrow range" of conditions, such as volcanic activity or springs, said Nasa. 

"Think Yellowstone!" said Briony Horgan, co-investigator on the Perseverance rover mission and professor of planetary science at Purdue University in West Lafayette, Indiana. That makes pure elemental sulphur a "very weird finding" on Mars, she told CNN. 

"My jaw dropped when I saw the image of the sulphur," she said. It's a "big mystery to me as to how this rock formed in Mount Sharp".

Curiosity also "found a lot of it", said Nasa: "an entire field of bright rocks that look similar to the one the rover crushed".  

Scientists aren't sure how it formed, or whether it has "any connection to other sulphur-based minerals previously discovered in the region", said Space.com.

Pure sulphur "shouldn't be there, so now we have to explain it", said Vasavada. "Discovering strange and unexpected things is what makes planetary exploration so exciting."