These yellow crystals were discovered after NASA’s Curiosity rover happened to pass through a rock and pop it open on May 30. Using an instrument on the rover’s arm, scientists later determined that these crystals were elemental sulfur—and 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 Curiosity rover traversed cracked open to reveal 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 forms when water evaporates. But where past discoveries have been of sulfur-based minerals—in other words, a mixture of sulfur and other materials—the rock Curiosity recently opened up is made of elemental (pure) sulfur. It is not clear what relationship, if any, elemental sulfur has with other sulfur-based minerals in the area.
While people associate sulfur with the smell of rotten eggs (a result of hydrogen sulfide gas), elemental sulfur is odorless. It formed only in a narrow range of conditions that scientists do not associate with the history of this location. And Curiosity found a lot of it — a whole field of bright rocks that look similar to the ones the rover crushed.
NASA’s Curiosity captured this close-up image of a rock called Snow Lake on June 8, 2024, the 4,209th Martian day or salt of the mission. Nine days earlier, the rover had broken open a similar rock and revealed crystalline textures — and elemental sulfur — inside. Credit: NASA/JPL-Caltech/MSSS
“Finding 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 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 while off-roading in the Gediz Vallis channel, a groove that curves down part of the 3-mile (5-kilometer) high Mount Sharp, at the base of which the rover has been climbing since 2014. Each layer from the mountain represents a different period in the history of Mars. Curiosity’s mission is to investigate where and when the planet’s ancient terrain could have provided the nutrients needed for microbial life, if it 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 canal was carved by flows of liquid water and debris that left a ridge of rock and sediment extending 2 miles down the mountainside below the canal. The aim is to develop a better understanding of how this landscape changed billions of years ago, and although 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 investigated whether ancient floods or landslides built the large mounds of debris that rise from the canal floor here. Recent clues from Curiosity suggest that both played a role: some piles were likely left behind by turbulent flows of water and debris, while others appear to be the result of more localized landslides.
NASA/JPL-Caltech/MSSS NASA’s Curiosity rover captured this view of the Gediz Vallis channel on March 31. This area was probably formed by large floods of water and debris that piled stones into mounds within the channel. Credit: NASA/JPL-Caltech/MSSS
These conclusions are based on rocks found in the debris mounds: while stones carried by water currents become rounded like river rocks, some of the debris mounds are perforated with more angular rocks that may have been deposited by dry avalanches.
Finally, the water soaks up all the material that settles here. Chemical reactions caused by the water bleached the white “halo” forms in some of the rocks. Erosion from wind and sand revealed these halo shapes over time.
“This was not 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 was an exciting amount of activity here. We are looking at multiple flows down the channel, including vigorous floods and boulder-rich flows.”
While exploring the Gediz Vallis channel in May, NASA’s Curiosity captured this image of rocks that show a pale color near their edges. These rings, also called halos, resemble markings seen on Earth when groundwater seeps into rocks through cracks, causing 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 the team’s early expectations, and they’ve been eager to take a rock sample from the channel to learn more. On June 18, they got their chance.
While the sulfur rocks were too small and fragile to be sampled with the drill, a large rock called “Mammoth Lakes” was spotted nearby. Rover engineers had to search for a section of rock that would allow safe drilling and find a place to park on the loose, 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 pushed the powdery rock into instruments in its belly for further analysis so scientists could determine what materials the rock is made of.
Since then, Curiosity has moved away from Mammoth Lakes and is now on its way to see what other surprises are waiting to be discovered in the canal.
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