The yellow crystals were discovered after NASA’s Curiosity rover accidentally bumped into a rock and shattered it on May 30. Using an instrument on the rover’s arm, scientists later determined that the crystals were elemental sulfur. This is the first time this type of sulfur has been found on the Red Planet. Credit: NASA/JPL-Caltech/MSSS
Scientists were stunned on May 30 when a rock that NASA’s Mars rover Curiosity drove over cracked, revealing something never before seen on the Red Planet: yellow sulfur crystals.
Since October 2023, the rover has been exploring a region of Mars rich in sulfates, a type of sulfur-containing salt that forms when water evaporates. But where previous detections have been of sulfur-based minerals (in other words, a mixture of sulfur and other materials), the rock recently cracked by Curiosity is made of elemental (pure) sulfur. It’s unclear what, if any, connection elemental sulfur has to other sulfur-based minerals in the region.
Although people associate sulfur with the smell of rotten eggs (the result of hydrogen sulfide gas), elemental sulfur is odorless. It forms only under a narrow range of conditions that scientists haven’t linked to the history of this place. And Curiosity found plenty of them: an entire field of shiny rocks that look like the one crushed by the rover.
NASA’s Curiosity rover captured this close-up image of a rock nicknamed “Snow Lake” on June 8, 2024, the 4,209th Martian day, or sol, of the mission. Nine days earlier, the rover had crushed a similar-looking rock and revealed crystalline textures and elemental sulfur inside. Credit: NASA/JPL-Caltech/MSSS
“Discovering a field of rocks made of pure sulfur is like finding an oasis in the desert,” said Curiosity project scientist Ashwin Vasavada of NASA’s Jet Propulsion Laboratory in Southern California. “It shouldn’t be there, so we have to explain it. Discovering strange and unexpected things is what makes planetary exploration so exciting.”
It’s one of several discoveries Curiosity has made as it travels through the Gediz Vallis channel, a gorge that winds along part of the 3-mile-high Mount Sharp, whose base the rover has been climbing since 2014. Each layer of the mountain represents a different period in Mars’ history. Curiosity’s mission is to study where and when the planet’s ancient terrain might have provided the nutrients needed for microbial life, if microbial life ever formed on Mars.
Floods and avalanches
Spotted from space years before Curiosity launched, the Gediz Vallis channel is one of the main reasons the science team wanted to visit this part of Mars. Scientists believe the channel was carved by flows of liquid water and debris that left a ridge of rock and sediment stretching 2 miles (3 km) down the mountainside below the channel. The goal was to better understand how this landscape changed billions of years ago, and while recent clues have helped, there’s still much to learn about this spectacular landscape.
Since Curiosity arrived in the channel earlier this year, scientists have been investigating whether ancient floodwaters or landslides formed the large mounds of debris that rise from the channel floor at this location. Curiosity’s latest clues suggest that both played a role: Some mounds were likely left by violent flows of water and debris, while others appear to be the result of more local landslides.
NASA’s Curiosity rover captured this view of the Gediz Vallis channel on March 31. This area was likely formed when large floods of water and debris piled up piles of boulders into mounds in the channel. Credit: NASA/JPL-Caltech/MSSS
These conclusions are based on the rocks found in the debris mounds: while stones carried by flowing water become rounded like river rocks, some debris mounds are riddled with more angular rocks that may have been deposited by dry avalanches.
Eventually, water penetrated all the materials that were deposited here. Chemical reactions caused by the water bleached some of the rocks into “halo” shapes. Erosion by wind and sand revealed these halo shapes over time.
“This was not a quiet time on Mars,” said Becky Williams, a scientist at the Planetary Science Institute in Tucson, Arizona, and deputy principal investigator for Curiosity’s Mast Camera, or Mastcam. “There was some exciting activity here. We’re seeing multiple flows along the channel, including energetic floods and boulder-rich flows.”
While exploring the Gediz Vallis channel in May, NASA’s Curiosity telescope captured this image of rocks that have a faint color near their edges. These rings, also called halos, resemble the markings seen on Earth when groundwater seeps into rocks along fractures, causing chemical reactions that change the color. Credit: NASA/JPL-Caltech/MSSS
A hole in 41
All of this water evidence continues to tell a more complex story than the team had initially anticipated, and they were eager to take a rock sample from the canal to learn more. On June 18, they got their chance.
Although the sulfurous rocks were too small and brittle to be sampled with the drill, a large rock nicknamed “Mammoth Lakes” was spotted nearby. Rover engineers had to search for a section of rock that would allow safe drilling and find a parking spot on the soft, sloping surface.
After Curiosity drilled its 41st hole using the powerful drill at the end of the rover’s 7-foot (2-meter) robotic arm, the six-wheeled scientist ran the powdered rock through instruments inside its belly for further analysis so scientists could determine what materials the rock is made of.
Curiosity has since moved away from Mammoth Lakes and is now heading out to see what other surprises await in the canal.
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