These yellow crystals were discovered after NASA’s Curiosity rover accidentally ran over a rock and shattered it on May 30. Using an instrument on the rover’s arm, scientists later determined that these crystals were elemental sulfur—the first time this type of sulfur had been discovered. found on the Red Planet. Credit: NASA/JPL-Caltech/MSSS
Scientists were stunned on May 30 when a rock passed over by NASA’s Curiosity Mars rover cracked, revealing something never before seen on the red planet: yellow sulfur crystals.
Starting in October 2023, the rover will explore a region of Mars rich in sulfates, a type of salt that contains sulfur and is formed when water evaporates. But where sulfur-based minerals have been detected in the past—in other words, a mixture of sulfur and other materials—Curiosity’s recently opened rock is made of elemental (pure) sulfur. It is not clear what relationship, if any, elemental sulfur has to other sulfur-based minerals in the area.
While people associate sulfur with the smell of rotten eggs (the result of hydrogen sulfide gas), elemental sulfur is odorless. It is formed only in a narrow range of conditions, which scientists have not connected with the history of this place. And Curiosity found a lot—an entire field of bright rocks that look similar to the one the rover crushed.
NASA’s Curiosity captured this close-up image of the rock nicknamed “Snow Lake” on June 8, 2024, the 4,209th Martian day, or solu, of the mission. Nine days ago, the rover crushed a similar-looking rock, revealing crystalline structures — and elemental sulfur — inside. Credit: NASA/JPL-Caltech/MSSS
“Finding a field of pure sulfur rocks 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 now we have to explain it. Discovering strange and unexpected things is what makes planetary exploration so exciting.”
It’s one of several discoveries Curiosity made during its field drive in Gediz Vallis, a trough that winds down a 3-mile (5-kilometer) section of Mount Sharp, the base of which the rover has been climbing since 2014. Each layer of the mountain represents a different period of Martian history. Curiosity’s mission is to investigate where and when the planet’s ancient terrain might have provided the nutrients needed for microbial life, if any had formed on Mars.
Floods and avalanches
The Gediz Vallis channel, spotted from space years before Curiosity’s launch, 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 boulders and sediment stretching 3 miles 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 is still much to learn from the dramatic landscape.
Since Curiosity’s arrival in the canal earlier this year, scientists have been investigating whether ancient floods or landslides created the large piles of debris that rise from the canal floor here. The latest tracks from Curiosity suggest that both played a role: Some of the piles were likely left behind by violent flows of water and debris, while others appear to be the result of more localized landslides.
NASA/JPL-Caltech/MSSS NASA’s Curiosity Mars rover captured this view of the Gediz Vallis channel on March 31. This area was probably created by large floods of water and debris that piled up rock piles into embankments in the channel. Credit: NASA/JPL-Caltech/MSSS
These conclusions are based on the rocks found in the debris piles: While stream-borne rocks are rounded like river rocks, some of the debris mounds are littered with more angular rocks that may have been deposited by dry avalanches.
Eventually, the water seeped into all the material that settled there. Chemical reactions caused by water have bleached white “halo” shapes into some rocks. Erosion from wind and sand revealed these halo shapes over time.
“This has not been a quiet period 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’s been some exciting activity here. We’re tracking several flows through the channel, including energy floods and boulder-rich flows.”
While exploring the Gediz Vallis channel in May, NASA’s Curiosity probe captured this image of rocks that have a pale color around the edges. Also called halos, these rings resemble the marks on Earth when groundwater seeps into rocks along faults and causes chemical reactions that change color. Credit: NASA/JPL-Caltech/MSSS
Hole in 41
All this evidence of water continues to tell a more complex story than originally expected, and the team was eager to take a rock sample from the channel to learn more. On June 18, they got their chance.
While the sulfur stones were too small and fragile to be removed by a drill, a large stone nicknamed the “Mammoth of Lakes” was spotted nearby. Rover engineers had to search for a section of rock that would allow safe drilling and find a parking space on a loose, sloping surface.
After Curiosity drilled its 41st hole using a powerful drill at the end of the rover’s 7-foot (2-meter) robotic arm, a scientist on six wheels poured dusty rock into instruments inside its belly for further analysis so scientists could determine what materials the rock is made of made.
Curiosity has since left Mammoth Lakes and is now heading out to see what other surprises await discovery in the canal.
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